Creatinine (CREA)

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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