What does the Cystatin C test measure?

Why is Cystatin C measured?

Cystatin C is the preferred filtration marker when creatinine-based estimates are unreliable or when early or mild renal dysfunction must be detected with greater sensitivity. It is incorporated into the CKD-EPI cystatin C and combined creatinine-cystatin C equations, which improve GFR estimation across diverse populations. Practitioners use cystatin C to confirm borderline creatinine-based eGFR, to assess kidney function in patients with cirrhosis (where creatinine underestimates impairment), and to refine risk stratification for cardiovascular and progression outcomes.

Reference ranges

Standard reference

Approximately 0.50–1.10 mg/L in adults; intervals are age-dependent and assay-specific.

Functional medicine optimal

There is no single 'optimal' value; cystatin C is interpreted through eGFR equations. Lower concentrations correspond to better filtration, but interpretation is equation- and context-dependent rather than threshold-based.

Units

mg/L, µmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.