How and how long to store urine samples before albumin radioimmunoassay: a practical response

We used three study protocols to check the dependence of albumin stability, measured by an RIA, on different temperatures, durations, and materials (i.e., assay tubes) of urine storage. Albumin values obtained for samples stored in three types of assay tubes (glass, polystyrene, and polypropylene) throughout the 2 months of the first (prospective) protocol were superimposable. The 24-h storage of six urine samples at room temperature or at 4 degrees C, as well as 72-h storage at 4 degrees C, did not affect the albumin measurement by RIA. After 2 months of storage of these same six urine samples at -20 degrees C, there was still no albumin decrease. A significant albumin decrease occurred (a mean of approximately 5% per year, throughout the range of albumin concentrations tested) when samples stored at -20 degrees C were reassayed by RIA after > or = 2 years (second protocol, retrospective). Finally, 3 of 21 (14.3%) urine pools stored at -20 degrees C for various periods (4-21 months) showed a significant albumin loss after storage; the time of storage as well the decrement rates of these 3 pools differed from each other (third protocol, retrospective). Short- and medium-term (2-6 months) freezing of urine samples at -20 degrees C does not significantly affect the stability of immunoreactive albumin. For longer preservation periods, storage of urine samples at -70 degrees C may be preferable.