The ratio of urinary cystatin C to urinary creatinine for detecting decreased GFR

Glomerular filtration rate (GFR) and urine and serum concentrations of cystatin C and creatinine were measured in 40 boys and 42 girls. The fractional excretion of cystatin C (FE Cyst C) increased in proportion to the decrease in GFR. Since serum creatinine concentration (S-Creatinine) in the numerator of the fractional excretion equation and serum cystatin C concentration (S-Cystatin C) in the denominator have similar numerical values, they cancel out. The result is an equation in which the FE Cyst C is equal to the ratio of urinary cystatin C to urinary creatinine (u[cystatin-C/Cr]). The ratio of u[cystatin C/Cr] was compared with GFR. Using a receiving operating characteristic (ROC) plot, the data showed that a ratio of u[cystatin C/Cr]*100 that is ≥0.100 has a sensitivity of 90.0% for identification of children with GFR ≤60 ml/min per 1.73 m². The false-positive rate is 16.1%. The u[cystatin C/Cr] ratio is a reliable screening tool for detecting decreased GFR that does not require a serum sample.