Fasting Urinary Osmolality, CKD Progression, and Mortality: A Prospective Observational Study

Rationale & Objective Chronic kidney disease (CKD) characterized by decreased glomerular filtration rate (GFR) is often accompanied by various degrees of impaired tubular function in the cortex and medulla. Assessment of tubular function may therefore be useful in establishing the severity of kidney disease and identifying those at greater risk for CKD progression. We explored reductions in urinary concentrating ability, a well-known feature of CKD, as a risk factor for GFR decline and end-stage renal disease (ESRD). Study Design Prospective longitudinal cohort study. Setting & Participants 2,084 adult patients with CKD stages 1 to 4 from the French NephroTest Cohort Study. Predictor Fasting urinary osmolality measured using delta cryoscopy. Outcomes ESRD, mortality before ESRD, and measured GFR (mGFR) assessed using 51 Cr-EDTA renal clearance. Analytical Approach Cause-specific hazards models were fit to estimate crude and adjusted associations of urinary osmolality with ESRD and death before ESRD. Linear mixed models with random intercepts were fit to evaluate the association of urinary osmolality with slope of decline in mGFR. Results At baseline, mean age was 58.7±15.2 (SD) years with a median mGFR of 40.2 (IQR, 29.1-54.5) mL/min/1.73m 2 and a median fasting urinary osmolality of 502.7±151.7mOsm/kg H 2 O. Baseline fasting urinary osmolality was strongly associated with mGFR ( R =0.54; P P Limitations Fasting was self-reported. Conclusions Fasting urinary osmolality may be a useful tool, in addition to GFR and albuminuria, for assessing nonglomerular damage in patients with CKD who are at higher risk for CKD progression.