Longitudinal measurements of renal function.

Cross-sectional studies have long suggested that renal function declines with age among adults. However, there are limited longitudinal studies with data on within-individual changes in renal function over time. The Baltimore Longitudinal Study of Aging provided some of the first data on within-person changes in renal function over time. In a subsample of this cohort without renal disease, followed up to 23 years, creatinine clearance declined by a mean of -0.75 mL/min/1.73 m(2) per year. Interestingly, 35% of individuals in this study did not experience a decline in renal function. Similar findings from longitudinal studies of patients with established chronic kidney disease have been reported. Among 1,094 participants with up to 10 years of follow-up evaluation in the African-American Study of Kidney Disease, the mean annual rates of decline in estimated glomerular filtration rate were -2.11 mL/min/1.73 m(2) and -1.50 mL/min/1.73 m(2) during the trial and observational phases, respectively. Of participants with at least 7 years of follow-up evaluation, 33.5% experienced a mean annual decline in estimated glomerular filtration rate of less than -1 mL/min/1.73 m(2). More studies are needed to better define longitudinal changes in renal function. Such data will provide important information for guiding treatment choices for patients, research endeavors, and health policy.