Renal hyperfiltration revisited—Role of the individual body surface area on mortality

Background: Higher than normal estimated glomerular filtration rate (eGFR), i.e. renal hyperfiltration (RHF), has been associated with mortality. Methods: A population-based screening program in Finland identified 1747 apparently healthy middle-aged cardiovascular risk subjects in 2005–2007. GFR was estimated with the creatinine-based Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI) equation indexed for 1.73 m² and for the actual body surface area (BSA) of the subjects. This individually corrected eGFR was calculated as eGFR (ml/min/BSA m²) = eGFR (ml/min/1.73 m²) x (BSA/1.73). BSA was calculated by the Mosteller formula. RHF was defined as eGFR of more than 1.96 SD above the mean eGFR of healthy individuals. All-cause mortality was obtained from the national registry. Results: The higher the eGFR, the greater was the discrepancy between the two GFR estimating equations. During the 14 years of follow-up, 230 subjects died. There were no differences in mortality rates between the categories of individually corrected eGFR (p = 0.86) when adjusted for age, sex, body mass index, systolic BP, total cholesterol, new diabetes, current smoking, and alcohol use. The highest eGFR category was associated with increased standardized mortality rate (SMR) when CKD-EPI formula indexed for 1.73 m² was used, but SMR was at the population level when individually corrected eGFR was applied. Conclusions: Higher than normal eGFR calculated by the creatinine-based CKD-EPI equation is associated with all-cause mortality when indexed to 1.73 m², but not when indexed to actual BSA of a person. This challenges the current perception of the harmfulness of RHF in apparently healthy individuals.