Biological variation of cystatin C and creatinine

Objective: To evaluate the day-to-day biological variation of cystatin C in comparison with creatinine in healthy subjects and in patients with impaired renal function. Material and methods: Eight weekly morning blood samples were taken from 20 healthy subjects (13 females and 7 males, median age 44 years, range 25-61) and 19 patients with impaired renal function (8 females and 11 males, median age 61 years, range 35-70). Serum cystatin C was measured using Dade Behring N Latex Cystatin C assay and serum creatinine by an enzymatic method (Roche). Results: In the healthy subjects mean serum cystatin C was 0.70 mg/L (range 0.44-1.09) and mean serum creatinine 77 mu mol/L (range 54-100). The analytical variance was 2.0% for cystatin C and 1.6% for creatinine. The intra-individual variance was greater for cystatin C than for creatinine (8.6% vs. 4.7%). The inter-individual variance was similar for both analytes (cystatin C 15.1% vs. creatinine 14.4%). In the patients with impaired renal function mean serum cystatin C was 1.86 mg/L (range 0.45-3.31) and mean serum creatinine 224 mu mol/L (range 103-430). The analytical variance was 1.8% for cystatin C and 1.4% for creatinine. The intra-individual variance was greater for cystatin C than for creatinine (16.0% vs. 8.9%). Conclusion: In the present study, the intra-individual variance was greater for cystatin C than for creatinine in both healthy subjects and in patients with impaired renal function. Accordingly, serum creatinine is the preferred marker for serial monitoring of renal function in individuals with stable muscle mass.