[Cr]s is the most widely used parameter for GFR. However, [Cr]s is influenced by muscle mass and diet as well as kidney function. [Cystatin C]s, a 13-KDa protein of the Cystatin super-family of cysteine proteinase inhibitors, has been proposed as a better marker for GFR than [Cr]s since it appears to correlate with GFR but is not influenced by muscle mass or diet. We compared [Cr]s with [Cystatin C]s for estimation of GFR using deep frozen samples of serum from children who had simultaneous inulin (Cin) and creatinine clearances (Ccr)-the latter obtained using a cimetidine protocol. Ccr closely approximates Cin using this protocol (Hellerstein et al, Pediatr Nephrol, in press). We have initiated a prospective study comparing [Cystatin C]s and [Cr]s in patients being evaluated for renal function using the cimetidine protocol. The data from 20 children derived from frozen samples and 8 in the prospective group show that [Cystatin C]s and [Cr]s have a similar relationship with GFR (r = 0.76 and 0.71, respectively). Using a GFR of 70 mL/min1.73m2 as the lower limit of normal, [Cystatin C]s and [Cr]s were as follows: Table One child with a normal GFR had a[Cystatin C]s level of 1.45 mg/L. All with normal GFR had [Cr]s < 1.42 mg/dL. There were 4 normal values of [Cystatin C]s and 4 of [Cr]s among the children with Ccr <70 mL/min/1.73m2. Three of the 4 'normal' values of [Cystatin C]s and of [Cr]s were in the same patients. These data show that[Cr]s and [Cystatin C]s are equally valid markers for screening for GFR. However, [Cystatin C]s may prove more useful in general application since it is not influenced by diet or muscle mass (i.e., age or sex).

Table 1 No caption available.
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