Decreased Kidney Function in the Elderly: Clinical and Preclinical, Neither Benign

Decreased kidney function is extremely common in elderly persons and is also an important independent predictor of cardiovascular disease. An advance in detecting decreased kidney function and quantifying its prognostic and therapeutic implications is, therefore, important news for clinicians. In this issue, Shlipak and colleagues (1) advance our understanding of the broad consequences of mildly decreased kidney function in elderly persons. They also demonstrate that cystatin C has some advantages over serum creatinine as a marker of decreased glomerular filtration rate (GFR) in predicting future complications.

Direct GFR measurements are expensive and inconvenient. Most large studies and most clinicians use indirect estimates of GFR. The Modification of Diet in Renal Disease (MDRD) Study equation has provided a rigorously developed method for calculating estimated GFR based on serum creatinine level, age, sex, and race. The abbreviated equation has recently been updated to use standardized serum creatinine assays (2). The MDRD equation has several strengths but also has a number of limitations, as detailed in a recent review (3). By using age and sex as markers for lower muscle mass among women and older individuals, the equation accounts for differences in creatinine production, which results in substantial improvements in estimating GFR over serum creatinine alone. However, the MDRD equation accurately accounts for only the average loss of muscle mass with age. Thus, creatinine-based GFR estimates will be less accurate in individuals with unusual body habitus or diet. At the higher GFR range, the MDRD equation is less accurate for many reasons, including a greater influence of poor standardization of serum creatinine measurement, greater biological variability in GFR, greater ratio of nonrenal to renal determinants of serum creatinine, and the effect of having developed the MDRD equation to predict GFR in a low-GFR group. Thus, the MDRD equation has a limited …