By The Pawline veterinary teamVeterinary-reviewed by Alastair Greenway, BVM&S MRCVS · 25 July 2026
Emerging marker
This marker has no established reference range in dogs or cats yet, so it is read as a trend rather than measured against a threshold.
Cystatin C is a small protein filtered freely by the kidneys, so it corroborates early kidney decline alongside SDMA and creatinine — and, unlike creatinine, it is not swayed by muscle mass. Canine and feline reference intervals remain lab-dependent, so it is read as a corroborating trend rather than against a fixed threshold.
Cystatin C is a small protein produced at a steady rate by virtually every cell in the body and then filtered freely by the kidneys. That combination — constant production, filtration-dependent clearance — makes it a natural marker of glomerular filtration, the same property that underlies creatinine and SDMA. Its particular appeal is what it isn't affected by: unlike creatinine, cystatin C is not meaningfully influenced by muscle mass, which is creatinine's best-known blind spot in thin, elderly or muscle-wasted patients.
Cystatin C is filtered freely by the kidneys and, unlike creatinine, is not swayed by muscle mass — so it corroborates the early kidney picture.
Why this is an emerging marker
Cystatin C sits in the frontier layer at the "emerging" end — a step more established than GDF-15 or klotho, but still short of a validated band. It is a real, measurable assay used in veterinary practice, and it has a solid physiological rationale. However, canine and feline reference intervals remain laboratory- and method-dependent, without a single agreed clinical cut-off. So here it is:
read as a corroborating trend on a single laboratory's assay;
excluded from the in/out-of-range tally and any health score;
never used as a stand-alone diagnosis of kidney disease.
What a rising trend might mean
Read as a direction, a rising cystatin C is consistent with declining filtration — the same message SDMA and creatinine carry. Its value is in corroboration and confidence: when SDMA is drifting upward and cystatin C is moving the same way, the renal picture becomes more convincing than either marker alone. It can be particularly interesting in the patient where creatinine is unreliable — the thin, ageing cat or the muscle-wasted dog whose "normal" creatinine may be flattering the kidneys. Even so, in current veterinary practice it supplements rather than replaces the validated markers.
What a stable trend might mean
A stable cystatin C alongside stable SDMA and creatinine adds reassurance to the renal picture. As with any frontier marker, it should not be read as independent proof of kidney health — that judgement rests on the validated renal panel, including urine concentration (USG) and protein loss (UPC).
Cat vs dog
Cystatin C has been studied in both species, with the muscle-mass advantage relevant to each — arguably most valuable in older cats, in whom age-related muscle loss and chronic kidney disease so often coincide and where creatinine can lag behind true function. But assay performance and intervals differ by method and species, and no universal band exists for either, so the trend-only treatment applies to both.
How it's measured
Cystatin C is measured from a blood sample, reported in mg/L, using assays that vary between laboratories. Because intervals are method-dependent, only results from the same laboratory and assay should be compared over time. Interpret any result with your veterinary surgeon alongside the validated renal markers.
What to do next
Cystatin C is best used as a corroborating measure within a kidney work-up rather than a trigger on its own — especially useful where creatinine's muscle-mass dependence makes it untrustworthy. Decisions about staging and treating kidney disease continue to rest on SDMA, creatinine, urine concentration, UPC and phosphorus; cystatin C adds confidence to the direction those markers indicate.
This page is educational and is not a diagnosis. Lab results are interpreted in the full clinical context by your veterinary surgeon; a single value rarely tells the whole story.
