Stability of osmolality in previously frozen canine serum and urine samples

Background: The measurement of osmolality is of interest in several clinical conditions and has been investigated in multiple veterinary studies. However, the stability of osmolality over time in frozen canine serum and urine has not been established. Objectives: The purpose of this study was to determine the stability of osmolality in canine serum and urine stored frozen at −20°C and −80°C up to 90 days, and to assess the effect of storage temperature on osmolality measurement. Methods: Serum and urine samples collected from 5 healthy Greyhound dogs were aliquoted and stored at −20°C and −80°C until measurement. Osmolality, assessed by freezing-point depression, was measured at 0, 7, 14, 30, and 90 days of storage. Results: For both serum and urine, osmolality at day 7 was not statistically different from day 0, but osmolality at days 14, 30, and 90 was significantly lower than at day 0. There was no significant effect of storage temperature on serum osmolality. However, the osmolality of urine samples stored at −20°C was slightly, but significantly lower than the osmolality of those stored at −80°C. Conclusions: Osmolality measurement in the serum and urine of healthy dogs appears to be stable for at least 7 days in frozen samples. The small changes observed after day 7 are unlikely to be relevant for individual patients, but could have implications in research study protocols.