A comparison between cystatin C, plasma creatinine and the Cockcroft and Gault formula for the estimation of glomerular filtration rate

Background. In clinical practice, the glomerular filtration rate (GFR) is often estimated from plasma creatinine. Several studies have shown cystatin C (cys C) to be a better parameter for the diagnosis of impaired renal function. No data are available, however, on the performance of cys C in follow-up of patients, compared with creatinine. Also, comparisons of cys C with the Cockcroft and Gault (C&G) formula for estimation of GFR are few. Methods. Plasma samples were obtained from 93 consecutive patients seen for GFR determination and from 30 patients with diabetes mellitus type 2, of whom 23 were investigated a second time after 2 years. GFR was determined with [I-125]iothalamate. Plasma creatinine was determined enzymatically and the creatinine clearance calculated according to C&G. Cys C was measured with a particle-enhanced immunonephelometric method. Results. GFR correlated with 1/cys C (r=0.873) as well as with C&G (r=0.876). The area under the curve (AUC) of the receiver operating curves (ROCs), a measure of diagnostic accuracy, for cys C (0.93 1) and C&G (0.938) were equal (P=0.815) and both better than the creatinine AUC (0.848; P=0.006). Bland and Altman analysis showed that the simple formula GFR=-4.32+80.35 x 1/cys C, derived from our data, gave more accurate (P < 0.0001) and more precise (P=0.024) GFR estimates than obtained with the C&G formula. The day-to-day variation (biological + analytical) for cys C was small (3.1 %, SID 2.51 %) in diabetic patients. In the follow-up study in diabetic patients, cys C was the parameter which had the best correlation (r = 0. 66) with changes in GFR. Conclusions. Cys C shows a high correlation with GFR. With a very simple formula, cys C gives a good estimate of GFR, more accurate and precise than C&G. Because biological variation is low, cys C gives also a good assessment of GFR changes during follow-up. Cys C is the preferred endogenous parameter for GFR.