Urinary β₂-microglobulin, orosomucoid, albumin, transferrin and IgG were measured by enzyme-linked immunosorbent assays (ELISA). In urine samples stored at -20°C three of these proteins decreased during the period of freezing. After 1 week at -20°C urinary transferrin decreased by 81%, IgG by 39% and albumin by 26% of the pre-freezing values; however, addition of Tween-20 restored these values. In previously stored urine samples with bovine albumin, the decreased value after freezing at -20°C could be increased by changing the thawing procedure and including addition of Tween-20.

Urine samples thawed at room temperature just before analysis decreased by 80% for transferrin, 57% for IgG, 30% for albumin and 26% for pVmicroglobulin compared with samples thawed at 37°C, had Tween-20 added and then were kept for a few days at room temperature before analysis. Furthermore, previously frozen urine samples that were thawed at 37°C, had Tween-20 added and then were stored at room temperature did not show significant changes in any of the protein results measured the day after thawing and 35 days later. Orosomucoid seemed to be less variable as regards the effect of freezing and thawing procedure.