Thanks very much Sophie, and good afternoon everyone, and listen to this webinar where we're gonna talk about the interpretation of, the sort of traditional biomarkers and also we're going to talk about this new renal biomarker STMA and when it may be warranted to measure that as well. So the learning objectives for this session, so hopefully by the end of this, lecture, you'll understand the causes of low and high serum urea and creatinine concentrations, and also know the limitations of these analytes as biomarkers of renal function. You should know how to interpret azotemia in a clinical patient and differentiate between the different causes of azotemia.
After this session, you should know how to interpret urine-specific gravity and the factors that influence urine-specific gravity. We'll go on to talk about, the circumstances in which measurements of serum SDMA concentration will be, may be helpful, and we'll finish off, by discussing the pathogenesis of proteinuria and its interpretation. So starting with the first learning objective then so to understand the causes of low and high so ureagrainine concentrations and the limitations of those biomarkers.
So before, we do any laboratory evaluation of renal function, you, usually would be suspecting renal disease based on consideration of factors like the signalment, because obviously some breeds are predisposed to renal disease, and these patients will often have clinical history that's compatible with renal disease, so things like polydipsy and polyuria or perhaps reduced appetite and weight loss. And also you may have picked up on some clinical exam examination findings that are suggestive of renal disease as well. For example, in cats, you may feel small, irregularly sized kidneys.
An accurate diagnosis of renal disease, usually requires, biochemistry analysis, but what's really important is to integrate the results of biochemistry with urinalysis. And that's often not done very well in first opinion practise, but often it's just the bloods that are performed, but it really is important to get urine, samples from these patients as well. And the serum urea and creatinine concentrations and the urine specific gravity are the most readily available and commonly used biomarkers renal functioning test practise at the moment.
So let's talk about urea first of all. So this is a, a major nitrogenous waste product within the body. It's formed from ammonia in the liver.
So the liver detoxifies ammonia to urea. And then urea is released into the bloodstream where it's freely filtered at the glomerulus, so it passes, freely across the glomerular barrier. However, some of it is reabsorbed in the tubules and collecting ducts, and this reabsorption will be increased when, an animal is dehydrated.
So, this diagram just shows how urea is made and, and, so urea is ultimately derived from protein, and the majority of protein within the body comes through the GI tract from, within the food. The protein is then, digested to amino acids and bacteria within. The GI tract will then metabolise those amino acids and produce ammonia as a, as a byproduct.
And that ammonia is toxic in the body, and it's taken up into the portal circulation from the gut and then picked up by the liver and detoxified to make the less toxic nitrogenous waste product urea. And then the urea is, freely filtered in the kidneys and excreted in the urine. Although the majority of ammonia in your ear and the body is derived from the from the dietary protein, there is, some ammonia in your ear that will be derived from amino acid turnover within the body itself, and that's usually secondary to increased muscle metabolism.
So urea handling in the kidney then, so I mentioned that urea is freely filtered across the, the glomerular barrier, but then some of that urea will be reabsorbed within the proximal tubule, and that's reabsorbed along with water. And also some of the urea will be reabsorbed with water within the collecting ducts under the action of anti-diuretic hormone or ADH. And then any urea that's, is filtered but not reabsorbed will then be excreted in the urine.
So, I mentioned that the urea reabsorption will be increased and dehydrated, and hypo-perfused animals. And the reason for that is that we have reduced renal perfusion and reduced filtration. This leads to a decreased tubular urine flow rate, and that gives greater opportunity for water and urea reabsorption in the proximal tubule.
So essentially whenever more water's being reabsorbed within the kidney, the urea will follow the water. And this is why when we have, elevations in, urea and creatinine concentrations or asotemia, secondary to dehydration, we see this, more marked increase in the serum urea concentration than the serum creatinine concentration of the so-called pre-renal, pattern. So CMU rea concentrations can be increased due to a variety of reasons, but the, the, the, the most common reason, perhaps, perhaps not the most common reason, but the, the most significant reason would be if we had renal dysfunction and reduced excretion of urea, from the, through the kidneys and out of the body.
But there are a variety of non-renal factors that can also increase the urea concentrations, and this is, one of the limitations of urea. So first of all, dehydration will increase the concentrations because we'll have this increased renal reabsorption of urea along with water in the proximal tubule and the collecting duct. We can have circumstances where if, if an animal has a high dietary protein content, and that's going to obviously provide more substrate for the, more amino acids for the bacteria to metabolise and produce more ammonia, which then means that more ammonia has to be detoxified to, to your ear and you increase the urea within the body as a result of that.
Along the same lines, if an animal has gastrointestinal bleeding, then this will increase the serum in your ear concentrations because blood contains lots of proteins, which again will provide more substrate for bacterial metabolism and production of more ammonia. And also, if we have conditions in which there's a high rate of endogenous protein metabolism, and therefore increased, endogenous, protein turnover, then that will also increase the serum urea concentrations. And we can see this in, hyperthyroid cats, but also in, animals with prolonged starvation and muscle metabolism secondary to that, or even with, pyrexia and fever.
And this, these non-renal factors are part of the reason why we suggest that you should starve animals for 12 hours prior to taking blood samples to assess renal function in order to try and minimise the effects of dietary protein intake on the, serum ear concentration. So that was the cause of increased ser in your ear, but serum in your ear can also be decreased in some circumstances, and this can have some clinical relevance as well. So we can see decreased serum your ear concentrations in animals with severely reduced heatic function because there would be less capacity to detoxify the ammonia.
To make your ear. However, we only tend to see this when the, degree of, of hepatic, damage is quite marked. So usually there has to be, a loss of greater than 70 to 80% of the functional hepatic mass before we would see a decrease in the serumUA concentrations.
We can also see low serum urea concentrations in animals with portosystemic shunts, and that's because the, in these cases, the ammonia that's produced by the bacteria in the gut will be taken up into the portal circulation, but then bypass around the liver through the portosystemic shunt and therefore avoid being detoxified to urea, so we get less urea production. We can sometimes see mildly reduced serumA concentrations in animals on low protein diets or with reduced food intake and prolonged anorexia as well. And then, we can also see low serum urea concentrations in circumstances where we have increased, renal excretion of the urea, and that usually is because we have increased water loss from the kidneys.
And if you remember the urea follows the water, so if we have if we have conditions that are causing, increased water loss, then that will cause increased urea loss as well. And the main causes of that would Be glucosuria, which will cause an osmotic diuresis and increased water loss through the kidneys, or if we have circumstances in which we have reduced ADH production or reduced ADH activity, so central or nephrogenic diabetes incipidus, and we can also see low serum concentrations in those circumstances as well. And an example of a condition that might cause that would be hyperadrenal corticism.
So that was the cause of low and high urea and the, and the, the, the background to your ear as a biomarker. So then what about creatinine? Well, creatinine is a bit more straightforward.
It's produced from skeletal muscle by the breakdown of creatine primarily. There's a small amount of creatine that will be taken in. Through the diet.
However, the contribution of this to the serum concentration of creatinine is relatively negligible. So creatinine is, in the serum is mostly, produced within the skeletal muscle, and it's produced at a constant rate by those skeletal muscle cells, and therefore it's mostly influenced by, changes in skeletal muscle mass. Creatinine is, a non-protein bound substance that is also freely filtered at the glomerulis, so will pass straight across the glomerular barrier into the bones capsule.
However, unlike urea, there's no significant tubular reabsorption or secretion, of creatinine in dogs and cats. And so it's less affected by dehydration than urea. Some creatinine is removed and metabolised by bacteria in the gut as well.
However, this is thought to just be a more prominent, have a more prominent effect in late stage renal disease and therefore is unlikely to have a significant clinical impact when you're trying to diagnose, kidney disease in in our. So, serum creatinine concentrations are generally only increased in animals with renal dysfunction because we're having reduced excretion of this creatinine from the body. However, occasionally you might, you might encounter an animal with a high muscle mass, and those animals may have a, a slightly higher creatinine than .
The normal reference interval. And that's because the reference intervals we have for creatinine are usually derived from a, a variety of species and ages, to be honest. And so, if we have animals with above average muscle mass, then those animals may have a slightly higher CM creatinine concentration, and we can see that in some breeds like greyhounds as well.
And we can also sometimes see decreased or, or low normal serum creatinine concentrations in animals with reduced muscle mass. And this is probably more clinically relevant because, you may encounter older animals, particularly older cats, which may have, may have reduced muscle mass. And in those cases, the serum creatinine concentrations may be, lower than they otherwise would be if the animal had a normal muscle mass.
And so, you do. Have to bear in mind that in animals with reduced muscle mass, the GFR may be overestimated by the serum creatinine concentration. So if you have an older animal, say an older cat, which has a high normal serum creatinine concentration, but it has very reduced body muscle mass, then you should consider that that cat may actually have some degree of renal disease, which if that muscle mass was normal, would, show up as an elevation in the serum creatinine concentration.
Oh So if we compare urea with creatinine as a, as a renal biomarker then, then urea is more influenced by non-renal factors, particularly dehydration than creatinine. And so creatinine is, is, is a better marker of renal function than urea in general. And actually, I think serum creatinine concentrations are a reasonable marker of glomerulla filtration rate, provided that the muscle mass of the animal is normal.
However, although serum creatinine is less influenced by hydration status than urea, it is influenced to some extent by hydration status anyway. And so it's really important to interpret any elevation in the serum creatinine concentration in conjunction with the urines specific gravity in order to avoid, false positive results. So how useful is creatinine for diagnosing chronic kidney disease?
Well, I think despite the limitations of creatinine associated with its, dependency on the, the skeletal muscle mass, I do still think it is a useful marker of renal function. One of the major issues that I've already alluded to with creatinine, I think, is that our reference intervals for creatine are quite wide. And that's, as I've said, because the reference intervals are generally derived from, a wide variety of, breeds and also ages.
And so this limits the sensitivity of serum creatinine for Diagnosing, diagnosing urinal disease, but it does mean that the specificity is excellent, particularly when it's interpreted with urines specific gravity. So if we have an animal with, an elevated serum creacine concentration and with poorly concentrated urine, then that is that's very specific test for renal disease in general. And probably if we were to derive age-specific reference intervals, for creatinine in, in, in our species, then that would also increase the sensitivity of creatinine as a biomarker of renal dysfunction.
One of the other limitations of creatinine is that there is a change in muscle mass with age, so animals tend to lose muscle mass as they get older, and therefore this will further decrease the sensitivity of creatinine as a biomarker of renal function. However, I do think that, essentially looking at the trend in serum creatinine concentrations can be very useful for looking for, changes in renal function and potentially detecting early renal disease. And any increase over time is likely to be significant, particularly in the context of this reduction in muscle mass that tends to occur with increasing age.
So I thought I'd just demonstrate this to you in a, a diagrammatic form. So here we have a graph on the, on the X-axis, we have the glomerular filtration rate, and on the Y axis, we have the CMU and creatinine concentration, and the black line represents the exponential relationship between the two parameters. And this top red line here shows the upper limit of the normal reference interval for let's say, creatinine in this case.
So if say you were to take a blood sample from an animal when it was, relatively young and had a good renal function with a high glomerunafiltration rate, you can see that the serum creatinine concentration here would fall well within the normal, reference interval. However, say you were to then retake a blood sample from that animal a year later, and, within that period of time, a renal insult had occurred and some renal disease had, was now present, which had corresponded, to a reduction in the glome. The filtration rate of say 2/3, you can see in this circumstance, actually the serum creatinine concentration would have increased, however, it would have remained well within the normal reference.
So if you were to only take a blood sample at this time point, you may not regard that animal as having any renal dysfunction. However, actually, if you compare the creatinine concentration here with the creatinine concentration you measured a year earlier, you would document an increase in the serum creatinine concentration that would correspond to a significant reduction in the glomera filtration rate. So I think looking at the trend in serum creatinine concentration is potentially a good way to detect early renal disease in our patients.
So there have been studies in which labs have looked at the amount of error in the measurement of the creatinine. And it appears that approximately a 25% increase for creatinine is probably likely to correlate with a significant reduction in glomerular filtration rate. However, this figure is derived from creatinine that's been measured by reference laboratories in which there is, usually greater quality control and, and greater repeatability and accuracy of our essays.
So it's likely that the threshold, for regarding, an increase in creatinine is significant would be higher if we were to use in-house machines. However, to date, no one's actually determined that what that value would have to be using in-house, analyzers. But as, as I said, I do think serial monitoring of serum creatinine may be useful for early detection of chronic kidney disease.
And certainly, if you are, routinely, taking, annual bloods from your older patients for, geriatric monitoring, then looking back at what the serum creatinine concentration has been in your animal over the previous years and looking for any, significant increase in that value would be a useful way to detect early chronic. Another factor to consider when interpreting our urea and creatinine biomarkers is to consider the effect of thyroid dysfunction as well, because thyroid function can have quite a significant impact on these biomarkers. This is most apparent in cats with hyperthyroidism.
And that's because we have in hyperthyroid cats, there's an increase in the glomerullar filtration rate, which will decrease the urea and creatinine concentrations. Also, these cats tend to have decreased muscle mass and therefore have lower creatinine concentrations than they otherwise would have. And the hyperthyroidism is also usually associated with increased muscle metabolism and increased protein intake secondary to the polyphagia, both of which will increase the, amount of urea being produced.
On the flip side in hypothyroidism, there is also, there's an artificially decreased glomerunafiltration rate, which will increase the urea and creatinine concentrations in those animals. So essentially, urea and creatinine are less accurate predictors of glomerular filtration rate and the presence of renal disease when thyroid dysfunction is present. So in hyperthyroid cats, we tend to underestimate the prevalence of chronic kidney disease using urea and creatinine when we have, when cats are have uncontrolled hyperthyroidism.
Whereas in, in, hypothyroid dogs, there may be a tendency to overestimate the prevalence of chronic kidney or of kidney disease. So it's important to re-evaluate renal function in hyperthyroid cats and hypothyroid dogs once you've, controlled the hyperthyroidism or, supplemented the hypothyroid dogs with thyroid hormone. So hopefully now you understand the causes of low and high serum urea and creatinine concentrations and the limitations of these biomarkers of renal function.
So just to summarise then, the serum ure concentrations are affected by many non-renal factors, including thyroid dysfunction. Serum creatinine concentrations are affected by reduced muscle mass, but also to some extent by thyroid dysfunction as well. And monitoring trends in the same creatinine concentrations, probably a greater than 25% increase over time can be a useful way to detect renal disease and chronic kidney disease tension.
So then the next part of the, the lecture is to talk about how to interpret azotemia in a clinical patient and to differentiate between the causes of azotemia. And also we'll talk about how to interpret your a specific gravity and a little bit about the factors that influence you in specific gravity. So when we document azotemia, so an increase in the urea and or the creatinine concentration, we can generally, group these, types of these aemic patients into one of three categories.
So asotemia can be caused by pre-renal factors or, what's called pre-renal aotemia. We can have aotemia secondary to intrinsic renal diseases or what we call renal aotemia, and we can have aotemia secondary to, post renal factors or what's called post renal azotemia. And the pre-renal factors, it causes the pre-renal nasalemia we've already talked about, to some extent.
So these are associated either with, disorders that cause decreased blood perfusion to the kidney. So that's most commonly dehydration, but we can also see this in animals that are hypovolemic due to blood loss and haemorrhage, for example, or in animals with congestive heart failure and forward failure, which is, also called cardio renal syndrome. And then the second cause of pre-renal azotemis can be associated with disorders that are associated with increased production of nitrogenous wastes, and this would tend to only increase the urea concentrations rather than creatinine as well.
And as we've already mentioned, this would include animals on high protein diets, animals with gastrointestinal haemorrhage, and animals with disorders that increase protein metabolism. So, for example, hyperthyroidism. Within the intrinsic renal disease category, we can, subdivide these asotenic patients into animals with with renal aotemia secondary to acute renal failure or what's now called acute kidney injury, or animals with renal asotemia secondary to a chronic kidney disease.
And then within the post renal factors or post renal teas, these are usually secondary to either urinary tract rupture and a leakage of urine into the abdomen and and the uroabdomen or urinary tract obstruction, such as in cats. So how can we distinguish these causes of aotenia? Well, when trying to distinguish between pre-renal and renal aotemia, then probably the most important distinguishing criterion is the urines specific gravity.
And so for this reason, it's important to try and obtain urine samples from animals prior to initiating fluid therapy. However, that's not always possible if the animal doesn't have a, a palpable bladder and it's obviously important to institute fluid therapy, as soon as possible when it's necessary. So in those circumstances, then other factors like the clinical history, and other clinical pathology data such as the pack cell volume and the serum to of protein, can be quite helpful as well.
The other thing to remember as well is that azotemia can be multifactorial, so we can have some circumstances where pre-renal azotemias or post-renal disorders can cause acute kidney injury or acute renal failure. And also, some animals with chronic kidney disease and renal aoenia can also develop hypovolemia if they fail to maintain their water intake and therefore can have concurrent pre-renal aotemia overlying the renal laser. So I said that urines specific gravity is the, probably the best distinguishing criteria for trying to differentiate between preren and renal laserenia.
So urines specific gravity is essentially using the refractive index of the urine to estimate the urine specific gravity, and this approximates the urine osmolarity. And the urine osmolarity in the urine specific gravity gives us an indication as to the tubular function and the ability of the tubules to concentrate the urine adequately. When assessing urines specific gravity, you should always use a refractometer and you should never use the dipstick urine-specific gravity, because that's very inaccurate in animals.
And also just bear in mind that urine specific gravity can be increased in animals with proteinuria and glucouria, although typically the degree of increase in the US USB is not more than 0.005 to 0.01 units.
So probably only of relevances in animals with borderline urine concentrating ability. So, this diagram just shows the change in urine osmoality as estimated by you in specific gravity through the normal nephron, first of all. So, when the, plasma is filtered and we get the, ultrafiltrate here in the bonus capsule, The osmolarity of the urine at this point is similar to the plasma.
And, the urine specific gravity is therefore within the isosuric range, which correlates with the, the plasma osmolarity of 1.07 to 1.013.
Then as the urine moves through the proximal tubule, we get reabsorption of solutes and water in approximately equal quantities and therefore, the osmolarity in the urine specific gravity of the urine doesn't change as it passes through the proximal tubule. Then as the urine moves down the descending limb of the leap of Henley, we get selective reabsorption of water alone, which will concentrate the urine and increase the urine osmolarity and urine gravity. So at the tip of the loop of Henley, the urine specific gravity is usually very concentrated at greater than 10:40.
Then as the urine moves up the ascending loop of Henley, we get selective reabsorption of only solutes, particularly sodium, potassium, and chloride, and therefore, the urine osmality falls and actually the urine osmolarity at the end of the assembly loop of henley is below that of the plasma. And so the eurospecific gravity is within this hyposanuric range which is 1.001 to 1.003.
And then as the urine moves through the collecting duct, this is where the urine becomes more concentrated. Water is reabsorbed, under the action of anti-diuretic hormone or ADH. And depending on the amount of water that's reabsorbed, this will ultimately, give us our final urine osmolarity or urines specific gravity, which can be anything over 1.001.
So the situation in animals, with dehydration is that we still have isosuric urine once it's been, once it's been initially filtered through the glomerulus. As the urine moves through the proximal tubule, we continue to get, solute and water reabsorption, which although enhanced, because the urea also follows the water, we get no overall change in the urine osmolarity as it moves through this, through the proximal tubule. The urine continues to be concentrated as it moves down the descending loop of Henley and then becomes less concentrated as it moves up the ascending leap of henley, so we still get the production of hypothenuric urine by the end of the ascending loop of Henley.
But then the critical difference in dehydrated animals is that we have significantly increased anti-diuretic hormone, which will massively increase our water reabsorption in collecting ducts and therefore, Give us a urine osmolarity and a urine specific gravity at the point of excretion, which is within our concentrated range, so in a dog, greater than 1.030, and in a cat greater than 1.035.
And then the situation in animals with renal disease is that we have, reduced renal, reabsorption of solutes and water in the proximal tubule due to tubular damage. We'll get reduced reabsorption of water in the, the Henley, reduced solute reabsorb. Option in the ascending leap pending and reduce water reabsorption in the collecting duct.
And so essentially our urines specific gravity will remain unchanged as it passes through the tubules and, the final urine-specific gravity will be, 1.007 to 1.013, reflecting a urine osmolarity that is the same as the plasma osmolarity.
So it's essentially unchanged as it passes through the kidney. Although in some animals with early renal disease, there may be some modification to the urine as a result of this digital tubular function, so the urine-specific gravity, the final urine specific gravity may be slightly above this range, but will be below the, urine-specific gravity that would be regarded as fully concentrated. So when we're trying to distinguish pre-renal versus renal laser temia, then we want to look at the urines specific gravity and if an animal has pre-renal laser temia, then the urines specific gravity should be greater than 1.030 in a dog and greater than 1.0 through 5 in a cat.
Whereas if an azotemia is secondary to intrinsic renal disease, then we would expect the urine to be isostenuric because the tube will not be able to, to concentrate that urine as they as, as we would want them to. And so the urine specific gravity will fall within this iso isostenuric range of 1.007 to 1.013.
I've, as I mentioned, some animals with early renal dysfunction may have, what I'd call suboptimally concentrated urine, so above the isostenuric range, but below the range regarded as concentrated, and this is often the case in cats with early renal disease. So then what's the situation in animals, with, reduced ADH, ADH activity? Well, the, the processes, that occur in the early nephron remain the same, so the urine ultrafiltrate will, continue to have an ulality that's equivalent to the plasma, so within the isotonic range, will continue to get solute and water reabsorption, the proximal tubule, the function of the leaf of Henley won't be affected, so we'll continue to get the production of high costsanuric urine.
But the difference here is that without ADH, we won't get water reabsorption within the collecting ducts. And so the urine, specific gravity will not be changed significantly from that of the urine at the end of the leap of Henry. So the urine that comes out of the animal will be within this high costs of uric range.
And so if we, because the production of hypothenuric urine requires functional tubular capacity, so we need to have functional lenly in order to get a urine specific gravity less than 1.007. So within this hypouric range.
If we have an animal with a USG in the high costsomuric range, then renal disease is very unlikely. However, if we do have an animal with hypothinuria, and the animal also has clinical signs of polydipsia and polyuria, then this is usually associated with, disorders that cause reduced ADH secretion or reduced ADH activity within the tubules. And common causes of this can include hypercalcemia because calcium will interfere with the action of ADH on the tubules, hyperadrenal corticism because steroid, will also interfere with ADH action.
We can see this in central diabetes incipidus, where we have reduced ADH secretion. And we can also see this in psychogenic polydipsia, where we have reduced ADH secretion, secondary to the reduced plasma osmolarity that will be associated with the increased water intake. So urine specific gravity is very helpful for distinguishing between pre-renal and renal lasotemia.
But what about post renal laserenia? Well, urines specific gravity is not going to be helpful for distinguishing post renal lasoenia from our other potential causes of aoenmia. And that's because the USG in these patients will be variable dependent on the hydration status at the time of the post-renal insult, and also the presence or absence of concurrent renal disease in that patient.
One of the most helpful factors to look for to distinguish post-creen laotenia from the other causes of aotenia is to consider the historical and clinical examination findings. But also the serum or plasma potassium concentration can be a useful marker because this is often increased in animals with post-renal aoenmia because the urine is the main, way that the body excretes potassium, So if we stop urine from leaving the body, as is the case with post-renal causes of aotemia, then we'll, the body will start to accumulate potassium and that will, lead to a corresponding increase in the serum potassium concentrations. However, you should also be aware that we can have increased plasma potassium concentrations in animals with acute kidney injury, so it's not specific for post renal azotemia.
And also just be aware that we can have some other causes of increases in serum, potassium concentrations, particularly if there is EDTA contamination of our serum sample, or if we have delayed, separation of the serum. So if you take a, a sample and don't separate the serum from the clot, within the first hour, Then we can start to get leaching your potassium out of the platelets within the clot which can increase the serum potassium concentrations by about 0.3 to 0.5 millimolars or sometimes even more if the circulating platelet count is higher.
So this table just summarises the clinical features that we can use to distinguish between pre-renal, renal and post renal causes of azotemia. So we can do this using a combination of urine specific gravity, same potassium concentration, the urine volume, the history, and clinical examination findings, and the response of the aotemia to IV fluid therapy. So, pre-renal azotemia is usually diagnosed by a combination of azotemia with a urine specific gravity that's considered concentrated, which, The values for dogs and cats are listed there.
And these animals will normally have, serum potassium concentration that's normal. They would have reduced urine volumes because the kidneys are trying to conserve as much water as possible in those circumstances, and their history and clinical exam findings would be consistently cause of pre-relasia, such as dehydration, shock, blood loss, or perhaps decreased cardiac output. And if the azotemia is secondary to dehydration or hypovolemia, then we usually see complete response of the azotemia, complete resolution of the azotemia, in response to IV fluid therapy.
Animals with renal lasotemia, if they have renal laoenia secondary to acute kidney injury, the urine specific gravity can be quite variable. However, animals with renal lasotemia secondary to chronic kidney disease will have isosuric urine or suboptimally concentrated urine. Serum potassium concentrations will also vary according to the aetiology of the renal lasotemia.
So animals with acute kidney injury can have normal or increased serum potassium concentrations, whereas, animals with chronic kidney disease would have normal or sometimes reduced serum potassium concentrations. Urine volume will be reduced, can be reduced in animals with acute kidney injury, and it's usually increased in animals with chronic kidney disease because they're polyuric because they're unable to concentrate their urine effectively. History and clinical exam findings in animals with acute kidney injury would be consistent with an acute and sudden onset, whereas in animals with chronic kidney diseases, they tend to be polyuric, polydipsic, and on the physical examination, you may document anaemia and pallor of membranes.
animals with renal lasotemia may show some initial improvement in the degree of their azotemia, in response to IV fluid therapy if they have some concurrent pre-renal component. But once that's resolved, then the renal aotemia doesn't tend to, improve much with fluid therapy alone. And animals with post renal causes of azotemia, the urine specific criity is variable and not helpful.
These animals usually have an elevated serum potassium concentration. They usually have reduced urine volume or urine production, and, the historical and clinical examination findings would be consistent with, the cause of post renal aotemia. So they may have a history of dysuria or enlarged and ruptured bladder, and they may have your abdomen.
And again, the response of the asotemia to IV fluid therapy in post renal asotemia is usually minimal if the cause of the obstruction or rupture is not resolved. So hopefully now you know how to interpret azotemia in a clinical patient and to different and how to differentiate between pre-renal, renal and post renal assotemia. So to do that, we need to consider a combination of historical and clinical presentation, the neurine specific gravity, the serum potassium concentration, and some of the other factors like PCD and plasma protein.
And also you should now know how to interpret urine specific gravity and some of the factors that influence urine specific gravity. So essentially urine specific gravity is the most helpful criterion for distinguishing animals with pre-renal and renal aoenia, and animals with hyposphenuric urine are unlikely to have renal disease. So the next learning objective is to identify the circumstances in which measurements of serum STMA concentrations may be warranted.
So a lot of you may have heard about SDMA, through, IDEX, particularly who've been marketing this test. But for those of you who don't know much about this, so this SDMA is symmetrical dimethyarginine, and it's a product of proteolysis. And it's a molecule that's produced in the nucleus of all cells at a constant rate.
And that's the major advantage of this, molecule because it means that the serum SDMA concentrations are less affected by the lean muscle mass than creatinine is. And like creatinine, SCMA is also a small molecule that's freely filtered at the glomerulus. It's not reabsorbed or secreted in the tubules and it's excreted in the urine.
And again, it's inversely correlated with the glomerular filtration rate. So CerMS GMA and concentrations, as I said, are inversely correlated with the era filtration rate. And actually, the correlation between creatinine and GFR is similar to the correlation between SCMA and GFR.
Serum SDMA concentrations are also affected by hydration status, however, so similar to urea and creatinine. And so it's best to assess serum SDMA concentrations in well-hydrated animals, and you should also, interpret the SGMA in conjunction with the urine-specific gravity as for creatinine in order to avoid, false positive results. So the main advantage I think of STMA is this, is the fact that it's produced by all cells rather than just muscle cells, and this is particularly important because we have this decrease in the muscle mass as animals get older.
And this was shown in this study, in cats in which they demonstrated that the serum creatinine concentration decreased with age in cats, so the decrease was around 20 micromoles per litre, and that was despite a concurrent decrease in the GFR of around 20%. So given that the creatinine should be adversely proportional to the GFR, we should expect the creatinine to increase over time as that GFR decreases. Remember what they saw in this study was that actually the creatinine decreased with age as and that was probably due to this, decrease in the lean muscle mass.
However, in the same study, they demonstrated that the SCMA concentration increased with age and therefore more accurately reflecting the decrease in the glomerular filtration rate that was occurring in those cats over time. And then that was being followed up by a series of longitudinal studies in which the the serum SDMA and the creatinine were followed over time. And the serum SDMA concentration was shown to increase above the reference.
Prior to the development of azotemia or hypercreatinemia in three longitudinal studies in dogs and cats, which suggested that an elevated STMA tends to occur earlier in the time course of the development of renal disease than, the The elevation in the serum gasinine concentration. And serum SDMA concentrations were also more sensitive than creatinine for detection of concurrent but masked chronic kidney disease and untreated hyperthyroid cats in one study. Although in a, a very recent study, it's been suggested that very mild elevations in STMA may occur.
Hyperthyroid caps that might normalise after treatment. So, if we have a hyperthyroid cap with a mildly elevated STMA, it's probably worth following up on what happens to the STMA after treatment to see if that STMA, remains elevated, as in some cases, the STMA may normalise. So SDMA, because of these advantages, as a biomarker of renal disease, has now been incorporated into the iris staging system for chronic kidney disease, the International Real Inter Society.
And so what iris have said is that any non-azotemic animals with persistently increased serum SCMA concentrations should be regarded as potentially having iris stage one chronic disease. However, we're making this diagnosis virus stage one chronic kidney disease, it's best to, also assess the results of urine analysis and imaging. And so we'd want to combine an elevation in the, serum creatinine, sorry, serum SDMA concentration with evidence of persistent poor urine concentrating ability and or renal proteinuria or, imaging findings consistent with chronic disease before we would make a definitive diagnosis of iris stage one, CKD.
However, it's also important to note that the iris, board don't, recommend the institution of renal diets in animals in iris stage 1 CKD. So essentially, in these animals, we just need to monitor the renal function, avoid nephrotoxic drugs, and allow free access to water. And also the Irish staging system recommend that if you have animals, with reduced body muscle mass, then, which have chronic kidney disease, then STMA may be better for staging chronic kidney disease in those patients more accurately.
So what are the limitations of SCMA? Well, it's, it's a marker that we're still, just getting to grips with, and, there was a report in last year's ECBIM Congress which suggested that some apparently healthy young cats can have mildly increased SDMA. So in that study, all the cats had an SDMA of less than 20 mcg per deciliter, so they're only very mildly elevated SDMA, concentrations.
And these were young cats without evidence of renal dysfunction at that point, however, they were, planning to follow up on these cats and see whether any of them developed, AT in the near future. So again, I think this reiterates the importance of combining the SGMA concentration with results of your analysis and other suggestive features of chronic kidney disease such as imaging findings or compatible clinical history, before, making a definitive diagnosis of chronic disease based on the STMA alone. The other factor that might influence STMA is also neoplasia, so there have been reports of some animals with neoplasia having elevated STMA.
So there was one study in which animals with lymphoma had, increased SDMA and actually the SDMA decreased after those animals received chemotherapy, which would suggest that the cause of that elevated SDMA was the increased tumour burden. And then there was another report in which animals with a variety of neoplas were reported to have elevated STMAs. Although in that study, some of those patients did have histopathological evidence of renal disease, which might have counted for the elevation in STMA in those cases.
So when is it worth measuring STMA then in our patients? Well, I would say if we have any non-azotenic animals, so animals with normal urea and creatinines, but which have reduced body muscle mass and that are also considered at risk of chronic kidney disease, then it's certainly worth measuring SCMA in those. Animals, because I think it's likely that STMA would be a more sensitive marker of, renal dysfunction in those patients.
However, it's still important to combine the results of the STMA with urinalysis. So we want to be documenting, current poor urine concentrating ability in those patients in order to get a, a, a more, definitive diagnosis. Also, I think if we have any azotemic animals that have reduced body muscle mass, then measuring STMA is also worthwhile, as it will probably enable more accurate iris staging, which may have consequences for how you would manage that case in terms of targets for phosphate restriction.
And there also could be an argument that it may be worth measuring STMA in uncontrolled hyperthyroid cats because it's a more sensitive marker of concurrent renal disease. However, the presence of massed azotema in hyperthyroid cats is not a negative prognostic indicator. So hyperthyroid cats that develop azotemia, only after they're treated, so which have this mass mass chronic disease, will not have a worse prognosis.
Than cats that remain on Atenia after treatment. So actually, finding out whether that cat has mass chronic kidney disease may not be that clinically relevant, although further studies really are needed to look at whether the STMA in the uncontrolled hyperthyroid cats is, ultimately a prognostic indicator. And also just bear in mind that some hyperthyroid cats could have mildly elevated SGMA's, which might, Which might normalise after treatment.
So the specificity of a mildly elevated STMA in a hyperthyroid cap might not be as, as high as the specificity of SCMA in a, in a non hyperthyroid cap for renal disease. So, that's covered the, identification of the circumstances in which measurements of serum SDNA concentrations warranted. So I think it's warranted in non-naotenic animals with reduced muscle mass that are considered at risk of chronic kidney disease, and it will be warranted in animals with chronic kidney disease that have reduced muscle mass in order to enable more accurate iris staging.
So the final part of the talk is to understand the pathogenesis of proteinuria and its interpretation. So, first of all, just a little bit about how proteins handled by the kidneys. So the urine of normal animals will contain only a small amount of albumin and other proteins, and that's because we have a combination of selected permeability of the glomerul basement membrane, which will minimise the amount of protein that's filtered into the ultrafiltrate.
And also most of the proteins that are filtered and go into the ultra urine ultrafiltrate are reabsorbed in the proximal convoluted tubules by hemocytosis. So the amount of urine in the final, The amount of protein in the final urine that's excreted is usually very low. When we're wanting to quantify urine protein excretion, we tend to use the urine protein to creatinine ratio or the what's called the UPC or sometimes UPCR, and that's to avoid problems with the interpretation of the urine protein concentration in the context of urine concentration.
So UPC helps us to correct the amount of protein in the urine, for the changes in urine volume that occur over time. And so this is demonstrated by this diagram here. So when we're measuring, UPC we're essentially, and proteinuria, we're essentially interested in wanting to know the amount of protein that's excreted in the urine over a 24 hour period.
And obviously, obtaining the full 24 hours of urine from our patients is not very practical. So we want to be able to do this on a spot urine sample. But the reason why we don't just look at the protein concentration is as follows.
So if we imagine that we have these two urine samples from these two from the same patient taken on 2 different days. And the protein excretion, 24 hour protein excretion over those two days was the same. So if we take the first sample, which was only 500 mLs and imagine that during the time that that urine was produced, only 4 grammes of proteins, each of these red dots represents 1 gramme of protein, were, excreted, then our urine protein concentration in this sample would be 4 grammes divided by 500 mLs or 8 grammes per litre.
Whereas if we imagine that this second sample was also produced over the same timeframe, so our production of protein over that time frame was the same. So we still have 4 grammes of protein produced in that set time. However, if we were to measure the protein concentration in this sample, it would come out lower at 4 grammes per litre.
So even though our protein excretion rate is the same in these two samples, actually, if we just looked at protein concentration, it would appear that the protein excretion rate was higher in this sample than in this sample. So the way we can overcome that is to measure the urine creatinine concentration as well, and that's because urine creatinine excretion rate is also constant over time, provided that the GFR remains constant. So if we imagine that during the time that this urine sample was produced here, the, the animal produced 5 grammes of creatinine.
And in this sample here, which was taken over the same time point, the 5 grammes of creatinine were also excreted. So if we then normalise the amount of protein to the amount of creatinine, then our UPC in this sample would be 0.8 and also in this sample we'll be 0.8.
So essentially, the UPC helps us to normalise the variation in urine volume, which will occur secondary to the amount of water that's being reabsorbed, secondary to changes in the hydration space of the animal. So UPC is not affected by differences in method of collection, so it's not different between free cap samples and cystocentesis samples. It's not influenced by fasted versus Fed states, and it's not also, it's also not influenced by the time of collection.
And the normal UPC of a dog would be less than 0.5, the cat would be less than 0.2.
So what causes proteinuria, well, increased UPC can occur transiently in what's called physiological proteinuria. And we can see this secondary to exercise, pyrexia, and seizures. So it's important if we document proteinuria that we document persistence of that proteinuria in order to exclude this transient physiological proteinuria.
And when we have a persistent proteinuria, this can have numerous causes, which can be grouped into one of four categories. So we can have pre-renal proteinuria, renal glomerular proteinuria, renal tubular proteinuria, and post renal proteinuria. So taking each of those in turn, so what's pre-renal proteinuria?
So essentially this is caused by conditions that are associated with increased serum concentrations of the small proteins which can pass freely through the glomerular barrier. So we get increased filtration of proteins here and this overwhelms the resorptive capacity of the proximal tubule. So we then get increased protein in the urine.
And this occurs with proteins like myoglobin or haemoglobin or sometimes some of the paraproteins associated with conditions like multiple myeloma, whether this is a relatively uncommon cause of proteinuria. The second group of proteinuria is the renal glomerular proteinurias, and this is caused by reduced selectivity of the glomerulla barrier. So again, we're getting increased filtration of proteins, which overwhelms the resorptive capacity of the tubules, leading to increased protein loss in the final urine.
And usually this is associated with the higher magnitude proteinuria with the UPC of greater than 2. The third category is the renal tubular proteinuria, so this is where we have the normal amount of protein being filtered at the glomerulus. However, because of tubular damage, we don't get the normal reabsorption of that small amount of philtre protein and therefore we get a low magnitude proteinuria in the final urine.
So usually animals with renal tubular proteinuria would have a UPC of less than a year. Then the final category is the post renal proteinurias, and this is important because it's actually the most common cause of proteinuria in animals, and it's associated with inflammation or infection within the urinary tract, so we have normal filtration of protein in the glomerulus, normal reabsorption of that philtre protein in the tubules, but then we have conditions which cause increased protein discretion, within the urinary tract usually associated with inflammation. And in experimental studies where dogs were given UTIs, UPCs of up to 40.8 were recorded, so the magnitude of proteinuria really cannot help you to differentiate this type of proteinuria from the other types.
And so this is why it's important to always check a urine dipstick and sediment in any sample that you're planning to send for a UPC in order to rule out concurrent inflammation that might be associated with proteinuria. However, if we just have blood contamination of the sample alone and that blood contamination is microscopic, that usually shouldn't affect the UPC very much and we'd only expect to see any marked change in the UPC if the urine was grossly premauric. So interpretation of the UPC then, so we have an animal with an elevated UPC, then first of all, you want to consider if there's evidence of an active sediment or if the sample is closely can mature it, because if there is, then it's probable that animal has post renal proteinuria.
If not, then you want to consider if there's any evidence of intravascular hemolysis, myopathy, or myeloma that might be contributing to a pre-renal proteinuria and consider that if that is present. If not, and the UPC is greater than 2, then it's probable that animal has a renal glomerullar proteinuria. And if not, then it's probable that that animal has a renal tubular proteinuria.
So if we see severe renal proteinuria, UPC bracing 2, this is usually a hallmark of glomerular disease and its associated with glomerular nephritis or amyloidosis, provided that you've ruled out post renal proteinuria. However, a less severe proteinuria, the UPC less than 2, can be a non-specific finding associated with many diseases. So we can see this in renal tubular diseases like feline chronic kidney disease.
But we can also see it in animals with hypertension and hyperthyroidism dependence of the presence of concurrent renal disease. We can also see it in dogs with hyperadrenal corticism or animals or steroid therapy. And also in our anecdotal experience at Cambridge, I think we also see this quite commonly in animals with systemic inflammation of a variety of causes.
So hopefully you now understand the pathogenesis of protein neuria in its interpretation. So remember that persistence of proteinuria should be documented. Samples sent for UPC should have a sediment and dipstick examination performed to rule out, post renal proteinuria, that's the most common cause of an elevated UPC.
UPC greater than 2 will indicate renal glomerullar proteinuria, provided that post renal protonuria is excluded. And low-level proteinuria can be a non-specific finding that's associated with a variety of renal and non-renal disease. So that's the end of my presentation.
So thanks for listening this afternoon and I'm happy to take any questions if we've got some time now. So back to you, Sophie. Thank you very much, Tim.
Excellent webinar, really, really interesting. So we don't have any questions so far, but if you do have any questions you'd like to ask Tim, just hover over the bottom of the screen. You'll see a Q&A box and just send them through.
We've got a couple of minutes left, so we should have time to read any questions you do have. We'll just, we'll give a couple of minutes and wait for to see if any come through. If you're staying tuned on the clinical pathology stream, we have Ian Ramsey next, so just stay online and we'll introduce Ian soon.
Otherwise, if you do close this webinar, please fill in some feedback so that we can give it to Tim at the end and we can tailor any future webinars to meet your needs as well. Otherwise, stay tuned and please do ask any questions you might have. So Tim, from memory, I think IEX offer the SDMA test for 10 pounds if you run a full biochem and electrolytes with them.
Oh, I'm not sure because, we obviously have our own labs but yeah, I think for a while they were running it as part of their routine biochemistry, but I'm not sure they're doing that anymore. But yeah, that, that might be correct. I'm not sure.
Yeah, yeah, I'm sure they do a discounted offer if you do a full biochem. So we have two questions come through. So one from Dan.
Hi, does the phosphorus level, does the phosphorus level have a prognostic value? Thank you. So, We do think that phosphate is important in animals with chronic kidney disease, and it's important to control that.
So, it, so that's why the International Renal Interest Society recommends to keep phosphate within these specific targets. And actually, it's quite interesting to look at what those targets are because they're frequently well within the normal reference intervals. So, Even an animal with iris stage 2 chronic kidney disease, we want the phosphate to be within the lower half of the normal reference interval usually, so less than 1.45.
And, the reason why we think phosphate is important is because, we think it's associated with, secondary renal hyperparathyroidism. And potentially, metastatic mineralization of the kidneys which might progress, the renal disease more quickly. And so that's the basis for controlling phosphate and putting these animals on phosphate restrictive diets.
So yeah, phosphate, we do think has prognostic importance, and controlling it is, is therefore recommended. Excellent. And just a comment from Louise saying so much useful information.
Thank you very much. And would there be a chance of obtaining a copy of any of the slides? Yeah, I'm sure we can send those out.
No problem. Perfect. So that seems to be all for the questions.
Thank you very much for your time, Tim. Really interesting webinar and enjoy the rest of your day.