Effects of Freeze-Thaw Cycle on Urine Values from Bottlenose Dolphins (Tursiops truncatus)

Urinalysis is a valuable tool for assessing the renal health of marine mammals. While retrospective and population renal health studies often use frozen urine samples, it has not been determined if the freeze-thaw process alters urine values from bottlenose dolphins (Tursiops truncatus). The primary objective of our study was to compare the values of 38 fresh and frozen paired urine samples collected from 20 bottlenose dolphins at the U.S. Navy Marine Mammal Program. Paired t-tests and chi-squared tests were conducted to assess the effects of storage at -80 degrees C for 2 to 301 d, and a subsequent thaw on urine specific gravity; pH; creatinine; protein: creatinine ratio; quantitative protein; uric acid; uric acid: creatinine ratio; and categorical characterizations of color, clarity, glucose, ketones, occult blood, protein levels, and crystals. The freeze-thaw cycle decreased urinary pH and increased urinary uric acid (p = 0.04 and 0.02, respectively). There were no other significant changes in urine variable values, including urinary uric acid concentration by grams of creatinine, when comparing fresh and frozen-thawed urine samples. Urinary uric acid concentration by grams of creatinine is the most accurate uric acid measurement when frozen-thawed samples are used. Urinary pH should be measured in fresh samples to avoid falsely decreased pH.