Alkalinization of urine samples preserves albumin concentrations during prolonged frozen storage in patients with diabetes mellitus

Abstract

Background In epidemiological studies in patients with diabetes, urine samples are often stored frozen prior to assessment of urinary albumin concentration (UAC). However, prolonged frozen storage may result in a falsely low UAC. In the current study, we investigated whether adjustment of urinary pH to alkaline values prior to frozen storage can prevent this problem. Methods Urine samples were collected in 90 patients from our diabetes outpatient clinic and divided into two portions. One portion was first adjusted to pH > 8.0 with 0.1 m sodium hydroxide, the other was left unprocessed. Both portions were divided into aliquots. UAC was assessed in fresh samples and after 7 days, 1, 6 and 12 months of storage at −20 and −80°C. Results Until 1 month of storage there were no significant changes in UAC. After longer storage, UAC fell significantly in pH unadjusted samples stored at −20°C, with a −7.6% (27.8) and −13.6% (31.7) change after 6 and 12 months storage, respectively. No significant change in UAC occurred in pH adjusted samples stored at −20°C or when samples were stored at −80°C, both with and without pH adjustment. Variation in UAC assessed after 12 months of storage was larger for samples stored at −20°C without adjustment of pH than for the samples stored with pH adjustment or stored at −80°C. Conclusions Urine alkalinization to pH > 8.0 prevents the decline in UAC associated with 12 months of frozen storage at −20°C and results in lower variation between samples after storage.