Validation of Surrogates of Urine Osmolality in Population Studies

Background: The importance of vasopressin and/or urine concentration in various kidney, cardiovascular, and metabolic diseases has been emphasized recently. Due to technical constraints, urine osmolality (U-osm), a direct reflect of urinary concentrating activity, is rarely measured in epidemiologic studies. Methods: We analyzed 2 possible surrogates of U-osm in 4 large population-based cohorts (total n = 4,247) and in patients with chronic kidney disease (CKD, n = 146). An estimated U-osm (eU(osm)) based on the concentrations of sodium, potassium, and urea, and a urine concentrating index (UCI) based on the ratio of creatinine concentrations in urine and plasma were compared to the measured U-osm (mU(osm)). Results: eU(osm) is an excellent surrogate of mU(osm), with a highly significant linear relationship and values within 5% of mU(osm) (r = 0.99 or 0.98 in each population cohort). Bland-Altman plots show a good agreement between eU(osm) and mU(osm) with mean differences between the 2 variables within +/- 24 mmol/L. This was verified in men and women, in day and night urine samples, and in CKD patients. The relationship of UCI with mU(osm) is also significant but is not linear and exhibits more dispersed values. Moreover, the latter index is no longer representative of mU(osm) in patients with CKD as it declines much more quickly with declining glomerular filtration rate than mU(osm). Conclusion: The eU(osm) is a valid marker of urine concentration in population-based and CKD cohorts. The UCI can provide an estimate of urine concentration when no other measurement is available, but should be used only in subjects with normal renal function. (C) 2017 S. Karger AG, Basel