Clinical Impact of Reporting Estimated Glomerular Filtration Rates

When clinicians speak of kidney function, they almost always are talking about the glomerular filtration rate (GFR).3 The kidney has many other functions, however. Most excretory functions are related in some way to glomerular filtration, at least as it is modified by the variable reabsorption of water and some solutes and the addition of yet other solutes via secretion. The kidney also has important endocrine functions (such as the production of erythropoietin and active vitamin D) and metabolic functions (such as the metabolism of insulin and other low molecular weight proteins) that are not related to excretory function; however, as the GFR declines because of chronic kidney disease (CKD), all of these functions also tend to decline, roughly in parallel. Thus, taking the overall kidney function as equivalent to the GFR is a generally useful approximation.Clinicians' equating of the GFR with kidney function has an additional practical utility. Estimating GFR relies on simple tests, which are based mainly on serum creatinine, whereas many other kidney functions are more difficult to measure. For example, the extensive machinery for secreting organic solutes has become increasingly well described in molecular detail, but the major natural or xenobiotic solutes that are secreted are still largely unidentified. Hence, estimating the residual secretory function by measuring the serum concentrations of such solutes is not feasible, regardless of how important to health this excretory mode might be.Creatinine was found to be a good marker of the GFR more than 70 years ago. Measurements of the urinary clearance of endogenous creatinine are a reasonable approximation of the GFR, as measured by the infusion of such exogenous markers as inulin or iothalamate. Measuring creatinine clearance, however, requires the timed collection of urine, which is cumbersome and notoriously inaccurate. To address this problem, researchers have developed several …