Kidney markers together: CREA, SDMA, BUN and phosphorus
Read filtration markers, the effects of food and hydration, and blood phosphorus as related questions rather than interchangeable warning flags.
Prepared by TengRip · Guide updated：2026-10-25
Not veterinarian-reviewed; a report-reading guide, not a diagnosis.
Read these findings together
Start with creatinine and SDMA, then consider BUN, phosphorus and urine tests. Muscle mass, diet, hydration and testing methods affect interpretation. A normal result in one test does not settle the others. This is a reading guide, not a diagnostic or automatic staging tool.
This site does not list fixed normal values: reference ranges differ by analyzer and method, so always use the range printed on your report.
Work through your report
- Record dates, values, units, reference intervals and analyzer differences.
- Bring notes on food, medicines, body condition, drinking and urination.
- Ask which urine findings and repeat tests are needed.
| Test | Clues and limits | Related information |
|---|---|---|
| CREA | A filtration marker; low muscle mass can mask a problem. | SDMA, BUN, phosphorus, urine specific gravity and prior results. |
| SDMA | Another filtration clue, less affected by muscle mass; an abnormal result still needs context. | Creatinine, BUN, urine findings and thyroid information when relevant. |
| BUN | Reflects protein breakdown and excretion; diet, hydration and other factors also matter. | Creatinine, urine findings, proteins, blood cells and liver-related results. |
| PHOS | A blood mineral result, distinct from phosphorus on a food label. | Calcium, BUN, creatinine and the veterinarian's monitoring plan. |
Creatinine (CREA)
One of the most common kidney function tests, reflecting how well the kidneys filter blood. It is used to stage kidney disease in cats, but it can underestimate problems in cats with low muscle mass.
What it measures
Creatinine is a waste product of muscle metabolism that is removed mainly by kidney filtration. When the kidneys filter blood more slowly (lower glomerular filtration rate), creatinine builds up in the blood. That makes it one of the most commonly used kidney tests, and the International Renal Interest Society (IRIS) bases its chronic kidney disease staging on it.
What a high value may mean
A high value means kidney filtration has dropped. The causes fall into three groups: problems in the kidney itself (such as chronic kidney disease), problems before the kidney (such as dehydration reducing blood flow to the kidneys), and problems after the kidney (such as a urinary blockage). So a high creatinine alone does not confirm kidney disease; vets usually add a urine test. A clear rise within a short time (for example, within two days) can point to acute kidney injury and needs a prompt recheck.
What a low value may mean
Because creatinine comes from muscle, animals with little muscle have lower values, such as cats that have been eating poorly and are clearly thin. Importantly, in senior cats that have lost muscle, creatinine can look normal while underestimating a kidney problem. This is one reason vets sometimes add an SDMA test.
Usually read together with
Urea nitrogen (BUN), SDMA, phosphorus, and a urine test (especially urine specific gravity), which helps show whether the kidneys can still concentrate urine.
When to follow up
After chronic kidney disease is diagnosed, IRIS stages it from at least two results taken while the cat is fasting, stable, and well hydrated. The creatinine stage cut-offs for cats are: Stage 1 below 1.6 mg/dL, Stage 2 1.6–2.8, Stage 3 2.9–5.0, and Stage 4 above 5.0 (mg/dL). Staging applies only to confirmed chronic kidney disease and should not be done from a single value on your own. Because creatinine varies a lot between individual cats, comparing with the cat's own past values is most useful.
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SDMA (Symmetric Dimethylarginine)
A newer kidney marker that is not driven by muscle mass like creatinine, so it may detect declining kidney function earlier. It can also be high for unclear reasons, so a recheck is often needed.
What it measures
SDMA is made by the body's cells at a steady rate and removed mainly by the kidneys, so like creatinine it reflects kidney filtration. Unlike creatinine, it does not come from muscle, so it is less likely to be underestimated when a cat loses weight and muscle. A research review describes it as a sensitive early marker of declining kidney function.
What a high value may mean
A persistently high SDMA can mean kidney filtration has dropped. IRIS states that SDMA persistently above 14 µg/dL may be used to diagnose early chronic kidney disease. However, newer studies show that SDMA can also be falsely high: for example, some cats with hyperthyroidism had high SDMA before treatment that returned to normal afterward. A single high result is not necessarily kidney disease, and a recheck together with creatinine and a urine test is usually needed.
What a low value may mean
Studies found that many hyperthyroid cats have low SDMA, because hyperthyroidism speeds up kidney filtration. In these cats the true kidney function may only show after thyroid treatment, so vets usually recheck kidney values after treatment.
Usually read together with
Creatinine, urea nitrogen, and a urine test; in senior cats, thyroxine (T4) is often checked too.
When to follow up
The IRIS SDMA stage cut-offs for cats are: Stage 1 below 18 µg/dL, Stage 2 18–25, Stage 3 26–38, and Stage 4 above 38, again only for cats with confirmed, stable chronic kidney disease. When SDMA and creatinine point to different stages, IRIS has separate guidance, so leave this to your vet. After a single high result, simply arrange a recheck as your vet advises; there is no need to panic.
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Blood Urea Nitrogen (BUN)
A waste product of protein breakdown that the kidneys remove. Diet, dehydration, and gut bleeding also affect it, so it is usually read together with creatinine.
What it measures
Breaking down protein produces ammonia, which the liver turns into urea for the kidneys to remove. So urea nitrogen depends on the amount of protein, liver production, and kidney excretion. Like creatinine it reflects kidney filtration, but more outside factors affect it.
What a high value may mean
The most important cause is reduced kidney filtration (including dehydration, kidney disease, and urinary blockage). A high-protein meal, bleeding in the upper digestive tract (the digested blood acts like extra protein), and conditions that increase protein breakdown, such as fever or steroid use, can also raise urea, often without a matching rise in creatinine.
What a low value may mean
Possible causes include a low-protein diet, very young age, poor liver function (less urea is made), a portosystemic shunt (an abnormal liver blood vessel), and conditions that cause a lot of urination, such as diabetes.
Usually read together with
Creatinine, a urine test, total protein, albumin, hematocrit, and liver values.
When to follow up
If urea and creatinine rise together, kidney filtration is usually affected, and your vet will look for the cause. If only urea is high and creatinine is normal, think about whether the cat ate shortly before the blood draw or has shown signs of gut bleeding such as black stools, and tell your vet.
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Phosphorus (PHOS)
Often rises when kidney function drops and is a key part of kidney disease care. It is naturally higher in young animals and can also rise in hyperthyroid cats.
What it measures
Phosphorus is a mineral needed for bones and energy metabolism, and it is removed mainly by the kidneys, so it tends to build up when kidney function declines. In cats with chronic kidney disease, controlling blood phosphorus is a key part of care.
What a high value may mean
The most common cause is reduced kidney filtration. Other possibilities: young, growing animals naturally have higher levels; hyperthyroidism in cats; eating rat poison that contains vitamin D; and phosphate enemas (cats need special caution). Hemolysis during the blood draw can also cause a false high.
What a low value may mean
Possible causes include losses from vomiting or diarrhea, feline hepatic lipidosis (fatty liver), cats that have just started tube feeding, and insulin or glucose treatment, which moves phosphorus into cells. A severely low level needs veterinary treatment.
Usually read together with
Calcium, urea nitrogen, and creatinine.
When to follow up
For cats with confirmed chronic kidney disease, IRIS recommends stage-based limits for blood phosphorus, usually reached with a prescription kidney diet and, if needed, a phosphate binder. Recheck regularly against the target your vet sets. To see which food ingredients relate to phosphorus, see the phosphate additive and high-phosphorus ingredient entries in this site's ingredient encyclopedia.
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Questions for the visit
- Could hydration, diet or muscle mass affect this result?
- What do urine tests add?
- Are we investigating a cause or monitoring an established diagnosis?
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This site is for general reference only and is not a substitute for veterinary diagnosis. If your cat shows unusual signs, please see a vet promptly.