Analytical validation and reference intervals for freezing point depression osmometer measurements of urine osmolality in dogs.

Urine osmolality (UOsm) is considered the most accurate measure of urine concentration and is used to assess body fluid homeostasis and renal function. We performed analytical validation of freezing point depression measurement of canine UOsm, to establish reference intervals (RIs) and to determine the effect of age, sex, and reproductive status on UOsm in dogs. Clinically healthy dogs ( n = 1,991) were retrospectively selected and stratified in groups by age (young [0-12 mo], adults [13-84 mo], and seniors [>84 mo]), sex (females and males), and reproductive status (intact and neutered). RIs were calculated for each age group. Intra- and inter-assay coefficients of variation were <1% in all cases. Good linearity ( r2 = 1, p < 0.001) and recovery (89-98%) were observed. The limit of detection and limit of quantification were zero. Urine specific gravity and UOsm had a highly significant positive correlation ( r = 0.96, p < 0.001) but had inconsistent agreement. The 95% RI for canine UOsm was 369-2,416 mOsm/kg in young and adult dogs, and 366-2,178 mOsm/kg in seniors. Senior dogs had a significantly lower UOsm than young and adult dogs ( p < 0.000). Neutered females had a significantly lower UOsm than intact female dogs ( p < 0.002). These results indicate that the method evaluated is adequate for UOsm measurement and that RIs based on age and reproductive status should be used in dogs.