OSMOTIC INDICES AND KIDNEY CONCENTRATING ACTIVITY: POPULATION-BASED DATA ABOUT CORRELATES AND PROGNOSTIC POWER

Background. Research data are limited about indices of osmotic equilibrium and of kidney concentrating activity. This study investigated correlates and prognostic power of these indices in a sample of the general population. Methods. Urine osmolality (U-osm), plasma osmolality (P-osm), plasma creatinine, and other variables were measured by Gubbio Study for the 1988-92 exam (baseline). Plasma creatinine and other variables were re-measured in 2001-07 (follow-up). Kidney concentrating activity was assessed as U-osm/P-osm ratio and kidney function as estimated glomerular filtration rate (eGFR). Results. Baseline data were complete in 4220 adults of whom 852 dead before follow-up and 2795 participated in follow-up. At baseline, there were the following independent cross-sectional associations: female sex and higher urine flow with lower values of U-osm, P-osm, and U-osm/P-osm ratio (P<0.01); obesity with higher values of U-osm, P-osm, and U-osm/P-osm ratio (P<0.01); older age and lower eGFR with lower U-osm, lower U-osm/P-osm ratio, and higher P-osm (P<0.05); hypertension and smoking with lower U-osm and lower U-osm/P-osm ratio (P<0.05) but not with P-osm. From baseline to follow-up, annualized rate was 1.26% for mortality and -0.74±0.76 mL/min x 1.73 m2 for eGFR change. Mortality was independently predicted by baseline U-osm and baseline U-osm/P-osm ratio (hazard ratio for one higher SD ≤0.91, 95% confidence interval ≤0.97, P< 0.01), but not by baseline P-osm. eGFR change was not independently predicted by baseline values of U-osm, P-osm, and U-osm/P-osm ratio (P≥0.4). Conclusions. Sex, age, obesity, eGFR, urine flow, hypertension, and smoking independently correlated with U-osm and kidney concentrating activity. U-osm and kidney concentrating activity independently predicted mortality, but not kidney function change over time.