Decreases in albumin/creatinine and n-acetylglucosaminidase/creatinine ratios in urine samples stored at -20°C

Abstract

The effects of storage for 6 months or 2 years at -20°C were studied in urine samples from Type II diabetic patients by assaying albumin by immunoturbidity, N-acetylglucosaminidase (EC 3.2.1.30) by methoxynitrovinylphenol release, and creatinine by the Jaffé method. There were significant decreases (P <0.001) in albumin/ creatinine ratios from 1.14 (0.63-2.98) to 0.83 (0.32-2.12) g/mol (median + interquartile ranges) after 6 months (n = 97), and from 1.64 (0.74-5.72) to 1.00 (0.37-4.54) g/mol after 2 years (n = 89). The percentage of samples with results below the detection limit of the albumin assay (2 mg/L) increased from 5% to 21% after 6 months and from 0% to 34% after 2 years. N-Acetylglucosaminidase/creatinine ratios decreased (P <0.001) from 520 (358-832) to 380 (263-695) U/mol after 6 months and from 520 (330-865) to 258 (82-462) U/mol after 2 years. The effect of storage was greater in samples with concentrations in the normal range (<2.5 g/mol for albumin/creatinine, <500 U/mol for N-acetylglucosaminidase/creatinine). Samples with albumin concentrations more than twice the normal range were still detected as abnormal after storage at -20°C; e.g., 18% were >5 g/mol (albumin/ creatinine) initially, with 17% >5 g/mol after 6 months of storage. We therefore recommend storage of urine samples at 4°C for no longer than 7 days before assay.