VetVaultCat kidney values explained: creatinine, BUN, SDMA and IRIS stages
High creatinine in a cat means the kidneys were filtering less blood than expected when the sample was taken. Causes range from dehydration (pre-renal) to kidney disease (renal) to a urinary blockage (post-renal). Vets read creatinine with BUN, SDMA and urine concentration, and repeat it, before diagnosing kidney disease or assigning a stage.
Cat kidney values at a glance
Each kidney test answers a different question, so one high number rarely tells the whole story. Ranges below are typical, approximate examples, not cut-offs for your cat.
| Value | What it measures | Typical adult cat range (approximate) | Common non-kidney influences |
|---|---|---|---|
| Creatinine | Filtration; a waste product of muscle | Upper limit often about 1.8–2.4 mg/dL (160–210 µmol/L) | Dehydration raises it; low muscle mass lowers it |
| BUN (or urea) | Filtration; a waste product of protein | Roughly 15–35 mg/dL | Dehydration, protein meals, gut bleeding raise it |
| SDMA | Filtration; released by protein turnover in all cells | Upper limit usually 14 µg/dL | Little muscle effect; occasionally raised for other reasons |
| USG | How well the kidneys concentrate urine | Usually above about 1.035–1.040 | Fluids, wet diet, diabetes, hyperthyroidism lower it |
| UPC | Protein leaking into urine | Below 0.2 | Urinary infection, blood or inflammation raise it |
| Phosphorus | A mineral the kidneys excrete | Varies by laboratory; higher in kittens | Growth and a damaged (hemolyzed) sample raise it |
Your report's range comes first. Reference ranges differ between laboratories and analyzers, so always compare your cat's result with the range printed on its own report.
When to call the vet urgently
Contact your vet or an emergency clinic straight away if your cat:
- has not eaten for more than 24 hours;
- is vomiting repeatedly;
- is not passing urine or is straining in the litter box with little or nothing coming out (a blockage is an emergency, especially in male cats);
- is suddenly weak, wobbly or hiding;
- has very high kidney values on a new test, especially alongside any of the signs above.
Creatinine and BUN: what they measure and why they rise
Creatinine
Creatinine is a waste product made at a fairly steady rate by muscle. The kidneys filter it out, so it rises when the GFR (how much blood the kidneys filter each minute) falls. It is the main staging value but a late marker: the Cornell Feline Health Center notes it typically rises only after roughly three quarters of kidney function is lost. Because it comes from muscle, a thin senior cat can show a reassuring creatinine despite reduced function. Healthy Birman cats also tend to run higher creatinine and SDMA.
BUN (blood urea nitrogen) or urea
Urea is made by the liver from broken-down protein and filtered by the kidneys. More of it is reabsorbed when a cat is dehydrated, so BUN often climbs faster than creatinine with dehydration. It also rises after a high-protein meal or bleeding into the gut, and falls with liver disease, making it useful support but less specific than creatinine.
Pre-renal, renal or post-renal?
Raised creatinine and BUN is called azotemia, sorted by where the problem sits:
| Type | Where | Common examples | Clues |
|---|---|---|---|
| Pre-renal | Before the kidneys: too little blood reaching them | Dehydration, vomiting, diarrhea, heart disease | Urine usually well concentrated (above about 1.040); improves with fluids |
| Renal | The kidney tissue itself | Chronic kidney disease, kidney infection, toxins such as lilies or antifreeze | Urine less concentrated than expected (below about 1.035–1.040) |
| Post-renal | After the kidneys: urine cannot leave | Urethral blockage, stones, a ruptured bladder | Straining, little or no urine; an emergency |
These can overlap: a cat with kidney disease that stops drinking adds a pre-renal component on top, which is one reason vets recheck once a cat is rehydrated and stable.
SDMA: an earlier marker, less affected by muscle
SDMA is released as cells break down proteins and is removed almost entirely by the kidneys. It is not tied to muscle mass, which makes it especially helpful in older or thin cats, and Cornell describes it as rising earlier than creatinine, at around 40% loss of kidney function.
SDMA methods are not standardized between laboratories (IRIS bands are based on one laboratory's method), and SDMA occasionally rises for other reasons, such as lymphoma. A single mildly high SDMA is a reason to recheck, not a diagnosis. IRIS treats a persistently increased SDMA above 14 µg/dL with normal creatinine as evidence of early (stage 1) kidney disease.
Urine specific gravity and UPC
Urine specific gravity (USG)
USG compares the density of urine with water (1.000). Concentrating urine is one of the first jobs failing kidneys lose, so USG is an essential partner to the blood tests. Healthy cats usually produce urine above about 1.035–1.040, often well above; around 1.008–1.012 means the kidneys are hardly concentrating at all. Fluid therapy, a mostly wet diet, diuretics, diabetes and hyperthyroidism can lower USG, and some cats with kidney disease still reach 1.035 or more. Ideally urine is collected before any fluids are given.
UPC (urine protein:creatinine ratio)
The UPC measures protein leaking into urine, corrected for urine concentration. Persistent kidney protein loss is linked with faster progression, so IRIS uses it for substaging. It is only meaningful when the urine shows no infection, bleeding or inflammation, ideally based on at least two samples collected over at least two weeks.
Phosphorus and the rest of the panel
Phosphorus is a mineral the kidneys excrete. As filtration falls it builds up in the blood, and high phosphorus is associated with faster progression of kidney disease. That is why kidney diets restrict phosphorus and IRIS treatment recommendations set targets by stage; your vet will tell you your cat's target. It is naturally higher in growing kittens, and a hemolyzed sample can raise it falsely.
Vets also check potassium (which can run low in kidney disease), red blood cell values (anemia becomes more common in later stages) and muscle condition. For the rest of the panel, see cat bloodwork results explained.
Hyperthyroidism can hide kidney disease
Hyperthyroidism (an overactive thyroid) mostly affects older cats, the same group that develops kidney disease. Excess thyroid hormone raises blood flow and filtration through the kidneys and wastes muscle. Both push creatinine down, so kidney values can look better than the kidneys really are, and creatinine may rise once thyroid hormone is controlled. The AAFP hyperthyroidism guidelines caution that an artificially lowered creatinine can give false reassurance. That is why vets often check total T4 in older cats and monitor kidney values during thyroid treatment. Read more in cat thyroid (T4) results explained.
IRIS CKD stages for cats
The International Renal Interest Society stages CKD to guide treatment and monitoring. Staging comes after CKD is diagnosed and is only valid in a stable, well-hydrated cat. It is based on fasting blood creatinine, ideally on at least two occasions, with SDMA alongside. It does not apply to a dehydrated or blocked cat, or to sudden (acute) kidney injury, which IRIS grades separately. The cat cut-offs below are from the IRIS 2023 guidelines and are unchanged in the 2026 revision.
| Stage | Creatinine (mg/dL) | Creatinine (µmol/L) | SDMA (µg/dL) | What it usually looks like |
|---|---|---|---|---|
| 1 | < 1.6 | < 140 | < 18 | No azotemia, but another kidney abnormality (dilute urine, protein loss, rising values) |
| 2 | 1.6–2.8 | 140–250 | 18–25 | Mild azotemia; signs usually mild or absent |
| 3 | 2.9–5.0 | 251–440 | 26–38 | Moderate azotemia; wider signs may appear |
| 4 | > 5.0 | > 440 | > 38 | Rising risk of whole-body illness and uremic crises |
- SDMA and stage 1: a persistently increased SDMA (above 14 µg/dL) with normal creatinine can indicate stage 1.
- When the two disagree: if SDMA stays higher than the creatinine stage suggests (for example above 25 µg/dL with stage 2 creatinine), IRIS suggests staging and treating one stage higher.
- Stage 2 can look normal: the low end of stage 2 sits inside many laboratories' creatinine ranges, so an unflagged value can still be stage 2 in a cat with confirmed CKD.
Substaging by proteinuria (UPC)
| UPC (cats) | Substage |
|---|---|
| < 0.2 | Non-proteinuric |
| 0.2–0.4 | Borderline proteinuric |
| > 0.4 | Proteinuric |
Substaging by systolic blood pressure
| Systolic pressure (mmHg) | Substage | Risk of future organ damage |
|---|---|---|
| < 140 | Normotensive | Minimal |
| 140–159 | Prehypertensive | Low |
| 160–179 | Hypertensive | Moderate |
| ≥ 180 | Severely hypertensive | High |
High blood pressure can damage the eyes, brain, heart and kidneys. Stressed cats can read falsely high, so IRIS uses several measurements after the cat settles. A full result might read “stage 2, non-proteinuric, normotensive”; the trend across tests says more than any single value.
Worked example: a borderline senior-cat result
Suppose an older cat's panel shows:
| Test | Result | Flag | On its own |
|---|---|---|---|
| Creatinine | 2.4 mg/dL (≈ 212 µmol/L) | H | Reduced filtration when sampled |
| BUN | 38 mg/dL (urea ≈ 13.6 mmol/L) | H | Supports reduced filtration; dehydration can also raise it |
| SDMA | 17 µg/dL | H | Above the usual limit of 14 |
| USG | 1.025 | – | Less concentrated than expected with raised creatinine |
Together, raised creatinine, BUN and SDMA with poorly concentrated urine is a pattern compatible with reduced kidney function rather than dehydration alone. But it is one snapshot: it does not confirm CKD or set a stage. If CKD were later confirmed, a stable creatinine of 2.4 mg/dL would fall in the IRIS stage 2 band. A vet would typically want to:
- Repeat creatinine and SDMA once the cat is eating, drinking and well hydrated, to see whether the values persist.
- Run a full urinalysis, possibly with a culture, then a UPC if the urine is clean.
- Measure blood pressure, ideally more than once.
- Check total T4, phosphorus, potassium and red blood cell values.
- Possibly image the kidneys to look for stones, blockage or changes in size and shape.
Useful questions: Was my cat dehydrated when sampled? When should we recheck?
Unit conversions: mg/dL, µmol/L and mmol/L
US laboratories usually report creatinine and BUN in mg/dL. Many European laboratories report creatinine in µmol/L, while Turkish laboratories mostly use mg/dL. Both often report urea instead of BUN, in mmol/L or in mg/dL of urea. Urea and BUN are different numbers in mg/dL, so check the label.
| To convert | Formula | Example |
|---|---|---|
| Creatinine mg/dL to µmol/L | × 88.4 | 2.4 mg/dL ≈ 212 µmol/L |
| Creatinine µmol/L to mg/dL | ÷ 88.4 | 250 µmol/L ≈ 2.8 mg/dL |
| BUN mg/dL to urea mmol/L | × 0.357 | 38 mg/dL ≈ 13.6 mmol/L |
| BUN mg/dL to urea mg/dL | × 2.14 (approximately) | 38 mg/dL BUN ≈ 81 mg/dL urea |
| Urea mmol/L to urea mg/dL | × 6.0 | 10 mmol/L ≈ 60 mg/dL |
| Phosphorus mg/dL to mmol/L | × 0.323 | 5.0 mg/dL ≈ 1.6 mmol/L |
SDMA is reported in µg/dL; USG and UPC have no units.
Frequently asked questions
What does high creatinine mean in a cat?
It means the kidneys were filtering less blood than expected when the sample was taken. The cause can be dehydration (pre-renal), kidney disease (renal) or a urinary blockage (post-renal), so vets interpret creatinine together with urine specific gravity, SDMA and the cat's condition, and usually repeat the test.
Is SDMA better than creatinine for cats?
SDMA tends to rise earlier than creatinine and is less affected by muscle loss, which helps in thin senior cats. It can occasionally rise for non-kidney reasons and its cut-offs are not standardized between laboratories, so vets use both values together.
What urine specific gravity is normal for a cat?
Healthy cats usually concentrate their urine above about 1.035 to 1.040. A lower value in a cat with high creatinine suggests reduced concentrating ability, although fluid therapy, a wet diet, diabetes and hyperthyroidism can also lower it.
What are the IRIS stages of kidney disease in cats?
Based on fasting creatinine: stage 1 is below 1.6 mg/dL, stage 2 is 1.6 to 2.8, stage 3 is 2.9 to 5.0 and stage 4 is above 5.0 mg/dL, with matching SDMA bands. Staging only applies to a stable, hydrated cat already diagnosed with chronic kidney disease, ideally using results from at least two occasions.
Can a cat have high BUN with healthy kidneys?
Yes. BUN rises with dehydration, a high-protein meal or bleeding into the gut, so it is less specific than creatinine. Vets read it alongside creatinine, SDMA and urine specific gravity.
Why can creatinine rise after a cat's hyperthyroidism is treated?
An overactive thyroid increases blood flow through the kidneys and reduces muscle mass, and both lower creatinine, which can hide kidney disease. Once thyroid hormone is controlled, the kidneys' true filtration shows, which is why vets monitor kidney values during thyroid treatment.
How do I convert cat creatinine from mg/dL to µmol/L?
Multiply the mg/dL value by 88.4. For example, 2.0 mg/dL is about 177 µmol/L. To convert back, divide µmol/L by 88.4.
Sources
- International Renal Interest Society. IRIS Staging of CKD (modified 2026) and treatment recommendations. iris-kidney.com/iris-guidelines-1
- Sparkes AH, et al. ISFM Consensus Guidelines on the Diagnosis and Management of Feline Chronic Kidney Disease. Journal of Feline Medicine and Surgery. 2016;18(3):219–239. pmc.ncbi.nlm.nih.gov/articles/PMC11148907
- Cornell Feline Health Center. Chronic Kidney Disease. vet.cornell.edu
- eClinPath, Cornell University College of Veterinary Medicine. Azotemia. eclinpath.com/chemistry/kidney/azotemia
- eClinPath, Cornell University College of Veterinary Medicine. SDMA. eclinpath.com/chemistry/kidney/sdma
- Carney HC, et al. 2016 AAFP Guidelines for the Management of Feline Hyperthyroidism. Journal of Feline Medicine and Surgery. 2016;18(5):400–416. pmc.ncbi.nlm.nih.gov/articles/PMC11132203