The effect of the freeze-defrost cycle on the long-term stability of the main clinical and laboratory parameters of urine

In sports medicine and research, biological samples, including urine, are often frozen for long-term storage and may undergo multiple freeze-thaw cycles to perform various analyses. To ensure comparability and reliability of research results, standardization of protocols for the storage of biological urine samples is relevant. The aim of this study was to evaluate the effect of a single freeze-thaw cycle of urine samples at –20 °C and subsequent thawing after 30 days on the stability of key clinical and laboratory parameters. Materials and methods. Morning urine samples were collected from 24 healthy volunteers the day after cardiorespiratory exercise testing—a treadmill run to failure. After aliquoting, a portion of the volunteers’ urine samples were subjected to biochemical analysis to determine key clinical and laboratory parameters; the remaining samples were analyzed after a freeze-thaw cycle after 30 days. Results. Analysis of urine samples subjected to a freeze-thaw cycle revealed a statistically significant (p<0.05) decrease in total protein concentration by 56.2%, microalbumin by 71.9%, uric acid by 67.3%, creatinine by 13.2%, and urea by 21.6% compared to the values obtained in samples analyzed ex tempore from the same volunteers. However, the levels of glucose, protein, urobilinogen, and ketone bodies in the samples remained unchanged after low-temperature exposure. The results of the study demonstrate the unacceptability of using the –20 °C freezing mode for analyzing urine bioassay parameters, especially when studying post-load proteinuria, as this can lead to false-negative results. Conclusions. Freezing urine samples at –20 °C followed by thawing leads to a statistically significant and clinically unacceptable decrease in the concentration of key clinical laboratory parameters.