Beta-trace protein is not better than cystatin C as an indicator of reduced glomerular filtration rate.

To the Editor: Recent studies suggest that serum cystatin C [reviewed in Refs. (1, 2)] and -trace protein (3, 4) identify reductions in glomerular filtration rate (GFR) better than creatinine does. Woitas et al. (5 ) compared for the first time the diagnostic performance of cystatin C, -trace protein, 2microglobulin, and creatinine in relation to inulin clearance. They noted that we reported a higher diagnostic accuracy of -trace protein in a study with 115 diabetic patients (4 ). As explanation, Woitas et al. (5 ) offered different patient characteristics. In this letter we would like to compare our data with regard to the diagnostic accuracy of cystatin C in comparison with -trace protein in our diabetic cohort. We have expanded our former investigations of -trace protein with the additional determination of cystatin C measured by two methods: N Latex Cystatin C, from Dade Behring, on the BNA analyzer; and Cystatin C PETKit, from Dako, on the Hitachi 717. Our data supplement the findings of Woitas et al. (5 ). The ROC plots and calculated areas for creatinine, -trace protein, and cystatin C, based on 80 mL min 1 1.73 m 2 as the cutoff for GFR, are shown in Fig. 1. The areas for cystatin C (Dade Behring) and -trace protein do not differ significantly (0.89 and 0.85, respectively; P 0.334). The ROC areas for cystatin C measured by the Dako method and for creatinine are smaller than the ROC areas for cystatin C measured by the Dade Behring method (P 0.01). Using samples stored at 80 °C, we repeated the measurements of cystatin C by the Dade Behring method after 12 months and confirmed these results (area under the ROC curve SE, 0.88 0.033). At 95% sensitivity, the specificity of cystatin C was 0.62 (95% confidence interval, 0.51–0.72), whereas the specificity of -trace protein was only 0.27 (95% confidence interval, 0.18–0.37). Thus, the Dade Behring cystatin C discriminates even better than -trace protein and our data confirm, in a quite different patient population, the conclusion of Woitas et al. (5 ) that cystatin C may be the best low-molecular weight protein marker at present to indicate reduced GFR.