Since it is formed at a constant rate and freely filtered by the healthy kidney, cystatin C is a good marker of renal function. Serum concentrations of cystatin C are almost totally dependent on glomerular filtration rate (GFR).1,2 A reduction in GFR causes a rise in the concentration of cystatin C.

Cystatin C has not been shown to be affected by factors such as muscle mass and nutrition, factors which have been demonstrated to affect creatinine values. In addition, a rise in creatinine does not become evident until the GFR has fallen by approximately 50%.

1. Grubb A. Diagnostic value of analysis of cystatin C and protein HC in biological fluids. Clin Nephrol. 1992; 38(Suppl 1):S20-7. PubMed 1284235

2. Coll E, Botey A, Alvarez L, et al. Serum cystatin C as a new marker for noninvasive estimation of glomerular filtration rate and as a marker for early renal impairment. Am J Kidney Dis. 2000; 36:29-34. PubMed 10873868

Overnight fasting is preferred.