Evaluating the performance of equations for estimating glomerular filtration rate

Glomerular filtration rate (GFR) is an important indicator of kidney function, critical for detection, evaluation and management of chronic kidney disease (CKD). GFR cannot be practically measured in most clinical or research settings; therefore, estimating equations are used as a primary measure of kidney function. A considerable body of literature now evaluates the performance of GFR estimating equations. The results of these studies are often not comparable, because of variation in GFR measurement methods, endogenous filtration marker assays and tools by which the equations were evaluated. In this article, methods for the evaluation of GFR estimating equations are discussed. Topics addressed include statistical methods used in development and validation of equations; explanation of measures of performance used for evaluation, with focus on distinction between bias, precision and accuracy, and with reference to examples of published evaluations of creatinine-and cystatin C-based equations; explanation of errors in GFR estimates; and challenges and questions in reporting performance of GFR estimating equations.