INTRAINDIVIDUAL AND INTERINDIVIDUAL VARIATION IN URINE OSMOLALITY ANDURINE SPECIFIC-GRAVITY IN HEALTHY PET DOGS OF VARIOUS AGES

Urine specific gravity (Usg) and urine osmolality (Uosm) are used rout inely to assess renal concentrating ability, but limited data on these variables are available for healthy dogs. Consequently, we studied th e intra- and interindividual variations in Usg and Uosm in healthy dog s as well as the influence of age and gender on these variables. Dogs were selected for health and anestrus in female dogs through the use o f a detailed questionnaire. Eighty-nine owners collected morning and e vening urine samples from their dogs on 2 consecutive days. In 8 dogs in which the Uosm of different samples varied more than 50%, owners co llected urine for 24 hours at 2-hour intervals during the day and at 4 -hour intervals at night. The possible effect of changes in adrenocort ical function with age was assessed by measurements of urinary cortico id/creatinine (C/C) ratios. Among all samples, Uosm ranged from 161 to 2,830 mOsm/kg and Usg from 1.006 to > 1.050. In the morning, Uosm (1, 541 +/- 527 mOsm/kg, range 273-2,620 mOsm/kg) and Usg (1.035 +/- 0.010 , range 1.009- > 1.050) were higher than in the evening (Uosm 1,400 +/ - 586 mOsm/kg; range 161-2,830 mOsm/kg; Usg 1.031 +/- 0.012, range 1.0 06-> 1.050). The interindividual coefficient of variation in Uosm was 34.2% for morning urine samples and 41.9% for evening samples. In 8 do gs with large differences in urine concentration, there were 2- to 3-f old increases or decreases in Uosm during the day, and the intraindivi dual coefficient of variation was 33.0%. There was no relation between gender and urine concentration. Urine concentration in both the morni ng and evening samples decreased with age. Urinary corticoid/creatinin e ratios did not change with age. It can be concluded that Uosm and Us g vary widely among healthy dogs. Urine concentration is generally low er in the evening than in the morning and is not related to gender. Ur ine concentration decreases with age, and this cannot be ascribed to a n associated increase in endogenous corticoids. In some dogs, Uosm var ies widely during the day, with an intraindividual coefficient of vari ation approaching the interindividual coefficient of variation. This m ay be regarded as a biologic variation but also could represent an ear ly undiagnosed clinical abnormality. Copyright (C) 1997 by the America n College of Veterinary Internal Medicine.