AIM: Although cystatin C has been developed as an alternative marker for estimating glomerular filtration rate (GFR), its clinical use is as yet limited. The significance of cystatin C for differentiating chronic kidney disease (CKD) stages and established cystatin C-based equations estimating GFR were evaluated.

METHODS: The fresh frozen serum samples from CKD (n = 119) and healthy volunteers (n = 22) were evaluated. Serum creatinine (sCr) was measured by the kinetic Jaffé method, and recalibrated to the isotope dilution mass spectrometry (IDMS). Cystatin C was measured using a particle-enhanced nephelometric assay.

RESULTS: CKD stages were more sensitively differentiated by cystatin C compared to sCr, especially in moderate and severe kidney dysfunction. Sex and body mass index did not affect cystatin C level. Pearson's correlation coefficients of reciprocal of cystatin C, measured and recalibrated sCr compared to systemic inulin clearance (Cl(in) ) were 0.757, 0.734 and 0.709, respectively. We derived novel pertinent equations based on cystatin C (model 1: 1.404 × cystatin C(-0.895)  × age(0.006) × weight(1.074) × height(-1.562)  × (0.865; if female); model 2: 43.287 × cystatin C(-0.906) × age(0.101) × (0.762; if female)]. Models 1 and 2 showed superior performance in representing systemic Cl(in) than the IDMS Modification of Diet in Renal Disease (MDRD) study equations did (adjusted r(2) = 0.76 and 0.72 for models 1 and 2, and 0.64 and 0.65 for 4 and 6 variable IDMS MDRD equations, respectively).

CONCLUSION:   Cystatin C reflects kidney dysfunction sensitively, and thus cystatin C-based estimation of GFR could provide a reliable support for clinical practice.