Abstract 1965

Cystatin C, a 13.3 KD protein, has a constant rate of production, is freely filtered at the glomerulus and is exclusively removed by the kidney. Studies have shown that glomerular filtration rate is the major determinant of serum cystatin C levels. When kidney function is normal the proximal tubular cells reabsorb and catabolize most of the filtered cystatin C, but with decreased GFR increased quantities appear in the urine. We hypothesized that the fractional excretion of cystatin C might correlate with GFR. If this proves to be correct, it should be possible to estimate GFR using a non-timed urine specimen and a single blood sample.

Twenty-eight urine and blood specimens were obtained from 23 children just prior to starting a two hour timed-urine collection for estimation of GFR using a cimetidine protocol. We have shown that the clearance of creatinine obtained employing this protocol closely approximates GFR measured using inulin clearance (Pediatr Nephrol;1998 12:49-54). The fractional excretion of cystatin C (FE Cys C) with a normal GFR (>70 mL/min/1.73m2) was compared with that with decreased GFR. (Table)

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One had interstitial nephritis of unknown etiology and the other IgA nephropathy with proteinuria. These two data points may reflect outliers or differences in the proximal tubular handling of filtered cystatin C associated with tubular dysfunction. These studies show that the FE Cys C obtained from measurement of creatinine and cystatin C in a random urine and serum from children pretreated with cimetidine increases as GFR decreases. The acquisition of more data, particularly from children with decreased GFR, should enable us to determine if the FE Cys C can be used to estimate GFR.