Results:

Mean measured GFRs were 68 and 70 ml/min/1.73 m² of body-surface area in the development and validation data sets, respectively. In the validation data set, the creatinine–cystatin C equation performed better than equations that used creatinine or cystatin C alone. Bias was similar among the three equations, with a median difference between measured and estimated GFR of 3.9 ml/min/1.73 m² with the combined equation, as compared with 3.7 and 3.4 ml/min/1.73 m² with the creatinine equation and the cystatin C equation (p = 0.07 and p = 0.05), respectively. Precision was improved with the combined equation (interquartile range of the difference, 13.4 vs. 15.4 and 16.4 ml/min/1.73 m², respectively [p = 0.001 and p < 0.001]), and the results were more accurate (percentage of estimates that were >30% of measured GFR, 8.5 vs. 12.8 and 14.1, respectively [p < 0.001 for both comparisons]). In participants whose estimated GFR based on creatinine was 45-74 ml/min/1.73 m², the combined equation improved the classification of measured GFR as either <60 ml/min/1.73 m² or ≥60 ml/min/1.73 m² (net reclassification index, 19.4% [p < 0.001]) and correctly reclassified 16.9% of those with an estimated GFR of 45-59 ml/min/1.73 m² as having a GFR of ≥60 ml/min/1.73 m².