Effects of Storage Temperature, pH and Time on Urinary Albumin Level

The storage of urine samples at 2-8°C and at -20°C for several weeks is a common laboratory practice in research and epidemiological studies to facilitate batch analysis. Despite numerous studies, there is no agreement on how stable albumin is in urine and the best way to store such urine specimen. Random urine samples were obtained from 15 patients with nephropathy and end stage renal failure. Urinary pH and specific gravity of freshly voided urine were determined before storage. A portion of each urine samples were stored at voided pH and another portion stored at neutral pH (7.0) after adjustment with acid or base at 4°C and -20°C for 10 weeks. Pre -analytical treatment of samples involved vortex mixing or centrifuging urine samples before measurement of urinary albumin. Albumin levels were determined at 0, 2, 4 and 10 weeks in both vortex mixed and centrifuged samples using standard spectrophotometric methods. Mean pH and specific gravity of fresh urine samples were 5.82 ± 0.71 and 1.009±0.004 respectively. Significant decreases (p< 0.05) were observed in urinary albumin levels at 4°C at the 4^th week. Significant changes were seen at the 10^th week in samples stored at -20°C in pH unadjusted samples. Vortex mixing or centrifuging of sample of did not restore decreases in albumin level. No significant difference was observed in the pH adjusted group after 10 weeks of storage at -20°C.These results suggest that where long term storage (6-10 weeks) of urine samples is required, samples should be stored at -20°C. However, in medium term storage (2-4 weeks) storage at 4°C may require the adjustment of the pH to neutral (7.0) before storage to obtain reliable results.