Cystatin C blood test: how to read your result

Cystatin C is a small protein produced at a steady rate by most cells in the body. Because the kidneys filter it out, the level in blood reflects how well filtration is working — and unlike creatinine, it is not strongly affected by muscle mass. It is read alongside creatinine, eGFR, BUN, electrolytes and urine albumin.

Cystatin C is increasingly used as a complementary view of kidney filtration alongside creatinine. Because creatinine is generated from muscle, it can read lower in people with less muscle (older adults, some chronic illness) and higher in people with more muscle (athletes, certain body compositions). Cystatin C is largely independent of muscle, so a cystatin-C-based eGFR can help refine the picture — particularly when the creatinine-based eGFR does not fit the clinical context.

A high cystatin C can suggest lower kidney filtration and is most informative when read together with creatinine, the creatinine-based and cystatin-C-based eGFR, BUN and urine albumin. Because cystatin C is less influenced by muscle mass than creatinine, the two views sometimes disagree — that disagreement itself is often the most useful clinical signal. One value does not diagnose a condition; clinicians look at the pattern and the trend.

A low cystatin C is uncommon on routine reports and generally not a clinical concern by itself. Reference ranges vary by lab; a low value within or just below the range is interpreted alongside creatinine, eGFR and the rest of the kidney panel rather than on its own.

Compare cystatin C with creatinine and both eGFR views (creatinine-based and cystatin-C-based) on the same panel. Cystatin C is especially useful when muscle mass is low, very high, or changing — that is where creatinine alone is least informative. Read alongside urine albumin or albumin/creatinine ratio when kidney filtration is being followed over time