[Comparison between serum cystatin C and estimation of GFR by the MDRD study equation for chronic kidney disease]

Glomerular filtration rate(GFR) can be estimated from serum (s-) creatinine using the modification of diet in renal disease (MDRD). However, its calculation is sometimes cumbersome in clinical use. Cystatin C is less influenced by age, gender and muscle mass than serum creatinine, and it has been proposed as an alternative marker for estimating GFR (eGFR). The comparison of s-cystatin C with MDRD-eGFR from 245 Japanese outpatients with chronic kidney disease (CKD)resulted in the equation of eGFR = 82.8/s-cystatin C - 10.7 (r = 0.85, n = 245). Based on this equation, there were 22 patients above + SD, which was the high-group in which s-cystatin C levels were higher than the corresponding eGFR, and there were 21 patients below -SD, which was the low-group in which s-cystatin C levels were lower than the corresponding eGER. Between the two groups there was no significant difference in age, gender, weight, and body mass index. The high-group included 1 case of hyperthyroidism and 7 cases of steroid user. The low-group included 4 cases of hypothyroidism and 1 case of steroid user. In healthy individuals, MDRD-eGFR is unsuitable for estimating GFR. Thyroid dysfunction or glucocorticoid excess are known to influence s-cystatin C levels. An improved eGFR equation was provided from 144 cases excluding 88 with normal renal function (eGFR > 90 mL/min/1.73 m2), 5 with thyroid dysfunction and 8 steroid users. eGFR = 86.1/s-cystatin C - 13.6 (r = 0.94, n = 144). Each GFR estimation provided from males or from females yielded nearly the same results as this equation. The prediction of eGFR using s-cystatin C may be convenient and useful in clinical practice, and the comparison of s-cystatin C with creatinine-based eGFR may reveal some factors that affect s-cystatin C or s-creatinine levels independent of GFR.