Serum cystatin C as an endogenous parameter of the renal function in patients with normal to moderately impaired kidney function

Background: Cystatin C is a proteinase inhibitor with a low molecular weigh t. The serum levels of cystatin C are mainly dependent on glomerular filtra tion rate (GFR) making cystatin C an endogenous parameter of GFR. The aim o f the study was to elucidate the applicability of serum cystatin C as a par ameter of GFR in patients with normal to moderately impaired kidney functio n and to estimate a reference interval for serum cystatin C. Patients and m ethods: Forty-six patients (25 males and 21 females) aged 22 to 83 years wi th various kidney diseases and 250 blood donors (164 males and 86 females) aged 19 to 64 years were included. Cystatin C was measured by an automated particle-enhanced nephelometric immunoassay, serum creatinine by an enzymat ic and by Jaffe method, urine creatinine by an enzymatic method, and GFR by Tc-99m-DTPA clearance. Results: Serum levels of cystatin C and creatinine showed increments with decreasing values of Tc-99mm-DTPA clearance and a li near relationship was found between 99mTc-DTPA clearance and 1/serum cystat in C, 1/serum creatinine (enzymatic method), and creatinine clearance. Comp arison of the non-parametric receiver-operating characteristic (ROC) plots for serum cystatin C (area under the curve (AUC) = 0.996; SE = 0.005), seru m creatinine (enzymatic method) (AUC = 0.899; SE = 0.044), serum creatinine (Jaffe method) (AUC = 0.870; SE = 0.051), measured creatinine clearance (A UC = 0.959; SE = 0.025), and estimated creatinine clearance (0.950; SE = 0. 029) revealed significant differences for serum cystatin C and serum creati nine (enzymatic and Jaffe method) (p values: 0.03 and 0.01). No significant differences were demonstrated between serum cystatin C and measured and es timated creatinine clearance (p value: 0.14 and 0.12). The non-parametric r eference interval for se rum cystatin C was calculated to be 0.51- 1.02 mg/ l (median: 0.79 mg/l; range: 0.33- 1.07 mg/l). Conclusion: Serum cystatin C seems to be a better parameter of GFR than serum creatinine in adults with various types of kidney disease with normal to moderately impaired kidney function.