Estimated Glomerular Filtration Rate from Serum Cystatin C: Significant Differences among Several Equations Results

Objectives: Several equations for the estimation of glomerular filtration rate (GFR) from serum cystatin C have been reported. We compared the results obtained using these equations to test the homogeneity of their results as well as their usefulness in clinical practice. Design and methods: Seven hundred and twenty-seven outpatients were studied. Of these, 439 were male and 288 were female, and their mean age was 60.8 +/- 24.1 years. GFR was estimated from serum creatinine using the abbreviated Modification of Diet in Renal Disease (MDRD-4) equation. GFR was estimated from serum cystatin C levels using five different equations. Results: The simplest (100/cystatin C) formula rendered the highest estimated GFR and the Hoek's equation rendered the lowest GFR, even significantly lower than the MDRD-4 equation (p < 0.001, Student's t-test). From the simplest formula to the Hoek equation the mean difference calculated was 25.1 +/- 8.7 mL/min (p < 0.001, Student's t-test). No differences by gender were found among the results of different equations. All cystatin C-derived equations reduced the number of patients diagnosed of chronic renal failure when compared with MDRD-4 formula. No patient with normal renal function was shifted to the renal disease group. Conclusions: A higher value could be expected when GFR is estimated from cystatin C. Nevertheless, vast differences were found in the results when tested using several equations. Physicians should be aware of this problem to avoid a wrong clinical diagnosis of renal function.