Supplementary Material for: Validation of Surrogates of Urine Osmolality in Population Studies

<p><b><i>Background:</i></b> 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. <b><i>Methods:</i></b> We analyzed 2 possible surrogates of U_osm in 4 large population-based cohorts (total <i>n</i> = 4,247) and in patients with chronic kidney disease (CKD, <i>n</i> = 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). <b><i>Results:</i></b> eU_osm is an excellent surrogate of mU_osm, with a highly significant linear relationship and values within 5% of mU_osm (<i>r</i> = 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. <b><i>Conclusion:</i></b> 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.</p