Cystatin C-based glomerular filtration rate estimating equations in early chronic kidney disease.

OBJECTIVE To compare performance of combined creatinine and cystatin C-based equation with equations based on either cystatin C or creatinine alone, in early chronic kidney disease. DESIGN Diagnostic accuracy study. SETTING Tertiary-care hospital. PATIENTS One hundred children with chronic kidney disease who underwent 99mTc diethylenetriamine pentaacetic acid (DTPA) glomerular filtration rate measurement. METHODS Estimating equations for glomerular filtration rate (GFR) based on serum cystatin C alone and in combination with serum creatinine were generated using regression analyses. These equations and the creatinine-based equation [0.42 x height/creatinine] were validated in 42 children with glomerular filtration rate between 60 and 90 mL/min/1.73 m2. Bias, precision and accuracy of estimating equations using DTPA glomerular filtration rate as gold standard. RESULTS Cystatin C-based equation (GFR=96.9 - 30.4 x cystatin) overestimated while the combined cystatin C-and creatinine-based equation [GFR=11.45 x (height/creatinine) 0.356 x (1/cystatin) 0.188] underestimated the measured GFR. Cystatin C-based equation had less bias (1.9 vs. 12.4 ml/min/1.73 m2), and higher precision (13.1 vs. 25.6 mL/min/1.73 m2) and accuracy (92.1% vs. 75.7%) than creatinine-based equation. The combined cystatin C and creatinine equation had bias (-1.4 mL/min/1.73 m2) precision (15.2 mL/min/1.73 m2) and accuracy (91.2%) similar to cystatin C-based equation. CONCLUSIONS Cystatin C-based equation has a better performance in estimating glomerular filtration rate than creatinine-based equation in children with early chronic kidney disease. Addition of creatinine equation does not improve the performance of the cystatin C-based equation.