Urine Osmolal Gap in Healthy Horses

The urine osmolality is a direct measure of the number of particles and represents the measurement of the density of the urine concentration as expression of the function of the tubular component of the nephron. In some pathological conditions in urine appear large solutes such as proteins (nephrotic syndrome) and glucose (diabetes mellitus) and urine specific gravity may differ significantly from osmolality, leading the clinician to wrong diagnostic conclusions. Such direct measurement of the urinary density is mainly affected by the presence of urinary salts and from urea and glucose, the most represented metabolites. Dosage of these constituents can estimate the urinary osmolality measured by special osmometers. Under physiological conditions the measured and calculated osmolality are quite similar and are only slightly different (urine osmolal gap, UOG). In pathological conditions UOG can rise because of the presence of unmeasured anions such as ketoacids (acetoacetate, beta-hydroxybutyrate), bicarbonates and organic acids of intestinal derivation (D-Lactate). The aim of this study was to establish the reference ranges of urine osmolal gap in horses.