Creatinine in cats and dogs

What it shows

Muscle makes creatinine all the time, the kidneys filter it out and almost none is taken back. So the more creatinine stays in the blood, the less the kidneys are filtering. The link is not a straight line: in early disease a big drop in filtration moves creatinine only a little, and the value becomes sensitive only after about a third of filtering capacity is gone.

The number also depends on how much muscle the animal has. A large, muscular dog has more creatinine; a thin old cat has less — so a value that looks “normal” in a wasted cat can hide reduced kidney function.

Higher than reference — what it may be a sign of

• Not enough blood reaching the kidneys (prerenal): dehydration after vomiting or diarrhoea, heart failure, shock. Often comes back down once fluids are restored.
• Kidney disease itself: chronic kidney disease (CKD), common in older cats; acute kidney injury from toxins (lilies for cats, antifreeze, grapes and raisins for dogs), infection or some drugs.
• Urine cannot leave the body (postrenal): urethral blockage in male cats, ureteral stones, a ruptured bladder.
• Reasons that are not the kidneys: large muscle mass (greyhounds and other sighthounds), a recent meat meal (small rise), and some lab methods that react to ketones or cephalosporin antibiotics.

Lower than reference

Small or young animals, loss of muscle, pregnancy, a liver shunt. In theory, marked jaundice can lower the measured value (not clearly shown in animals). A low creatinine is rarely a problem on its own — a vet will read it next to weight and muscle condition.

Look at together with

• Urea (BUN) — the second kidney marker. Urea high with creatinine normal points to diet, gut bleeding or dehydration; both high — kidneys first.
• SDMA — rises earlier than creatinine and depends much less on muscle mass.
• Urine specific gravity — dilute urine together with high creatinine makes a kidney origin more likely.
• Urine protein : creatinine (UPC), phosphorus, potassium, hematocrit — the usual companions in CKD staging and follow-up.

Units and conversion

European labs report µmol/L, North American labs mg/dL. One mg/dL equals 88.4 µmol/L, so 2.0 mg/dL is about 177 µmol/L. In the app each row keeps the unit printed on the form; the chart converts to one unit so forms from different labs line up.

Reference range and IRIS stages

The reference range is on your lab’s form. It differs between labs, methods, breeds and ages — a value is read against the interval printed next to it, not against a number from the internet. A rise of about 0.3 mg/dL (≈26 µmol/L) between two measurements is already meaningful to a vet if both were measured by the same method, even when both are inside the range, which is why the trend matters more than one result.

After CKD is diagnosed, vets stage it with thresholds set by the International Renal Interest Society (IRIS) — in a stable, hydrated animal, from at least two measurements. These are staging thresholds, not reference ranges:

Source: iris-kidney.com

Cats and dogs

Sources

1. eClinPath (Cornell University College of Veterinary Medicine). Creatinine — chemistry / kidney.
2. Van Vertloo L. Renal Dysfunction in Dogs and Cats. Merck Veterinary Manual, professional version, revised March 2025.
3. International Renal Interest Society. IRIS Staging of CKD, iris-kidney.com.
4. Cornell Feline Health Center. Chronic Kidney Disease.