Renal function in community-dwelling frail elderly. Comparison between measured and predicted glomerular filtration rate in the elderly and proposal for a new cystatin C-based prediction equation

Background and aims: There is a great need to evaluate renal function regularly in elderly people. This study aimed at analyzing renal function in stable, community-dwelling elderly people of 75 years and over, to compare measured and predicted glomerular filtration rates (GFR) and to develop an accurate prediction equation for this age group. Methods: Forty-five ambulatory elderly people in stable health in ordinary living were randomly selected into four age-classes, aged 75–95. Demographic data, personal activities of daily living, continuous drug prescriptions, body composition, blood pressure and blood chemistry were analysed. GFR was measured as Iohexol clearance based on three time-points 3, 4 and 7 hours after lohexol injection. Results: Mean GFR was well preserved in all four age-classes. The GFR range was 18–83 mL/min and declined with age. The Cockcroft-Gault prediction equation systematically underestimated measured GFR. A new ‘GFRA’ prediction equation is presented, based on the inverse of serum cystatin C and independent of gender, body surface area, body weight, lean body mass or serum creatinine. The proposed equation underestimated measured GFR with a mean of only 0.1 mL/min, had better precision compared with the Cockcroft-Gault equation, and was evaluated by the method of cross-validation. Conclusions: GFR exhibits extensive heterogeneity in frail, community-dwelling elderly people. The proposed GFRA was clearly more precise than the Cockcroft-Gault prediction equation in the study group. However, it needs to be validated in a larger population of elderly subjects, including more individuals in stable health with substantially reduced renal function in whom GFR is measured by a reference method with adequate sampling time.