Background: Cystatin C is a new interesting marker of glomerular filtration rate (GFR). However, data regarding its biological variance are scarce and conflicting. The ability of cystatin C to longitudinally follow renal function in patients therefore remains questionable. Methods: 12 healthy subjects (6 men and 6 women) were included in the final statistical analysis. Serum creatinine, plasma cystatin C and GFR were measured twice after a 1-week interval on the same day, at the same time, and under the same preanalytical and analytical conditions. GFR was measured with an iohexol method. Serum creatinine was measured with a compensated Jaffé and an enzymatic method. Plasma cystatin C was measured by a particle-enhanced immunonephelometric method. Analytical (CVA) and within-subject (CVI) variances were classically calculated. Results: CVA for creatinine (Jaffé and enzymatic methods) and cystatin C was 2.5, 0.97 and 1.29%, respectively. CVI was 5.8, 5 and 4.5% for the Jaffé creatinine, enzymatic creatinine and cystatin C determinations, respectively. Conclusion: Our study confirms that intraindividual variation of cystatin C and creatinine are similar. Therefore, from a biological point of view, cystatin C seems as accurate as creatinine for the longitudinal follow-up of renal function in daily clinical practice.

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