Assessing Kidney Function

Glomerular filtration rate (GFR) is used to characterize kidney function. Its underlying determinants, nephron number and single nephron GFR, better characterize kidney function but are not easy to measure in patients. Using GFR to characterize chronic kidney disease (CKD) requires an understanding of its physiological variation, as well as its decline with healthy aging. The measurement of GFR is based on the urinary or plasma clearance of an ideal or near ideal exogenous marker such as inulin, iohexol, or iothalamate. This requires infrastructure, expense, and patient burden that are not feasible in most clinical settings. Endogenous circulating markers cleared primarily by glomerular filtration, particularly creatinine, are more practical and useful to detect level and loss of kidney function. Estimated GFR can improve the interpretation and application of endogenous markers of GFR. However, estimated GFR still reflects the biology of the underlying markers, which may be unrelated to kidney structure and function. There is not one GFR estimating equation that accurately estimates GFR in all clinical settings, optimally predicts CKD outcomes, and performs the same as measured GFR in its association with CKD risk factors and outcomes.