Falsely Low Urinary Albumin Concentrations After Prolonged Frozen Storage of Urine Samples (Technical Briefs‪)‬

Microalbuminuria, defined as a urinary albumin concentration (UAC) of 20-200 mg/L, is an early predictor of diabetic nephropathy (1-6). In addition, microalbuminuria is a marker of cardiovascular morbidity and mortality, both in patients with diabetes mellitus and in the general population (7-17). Consequently, there is great interest in screening for microalbuminuria in these groups. In cohort studies, urine samples are often kept frozen at -20[degrees]C before analysis. Although some study results have indicated no effect of freezing on UAC (18-23), other studies have found erroneously low values when samples were frozen at -20 [degrees]C (24-27). Only a few studies have investigated the effect of longer storage periods, but these studies were small, and samples were in the macroalbuminuric range (200 mg/L). We investigated the effects of storage at -20 [degrees]C for up to 24 months, mixing methods, and baseline UAC on samples in the normo- and microalbuminuric ranges. Urine samples were collected during the prospective PREVEND study in the general population initiated to investigate urinary albumin excretion as a predictor of renal and cardiovascular disease (28). The participants were asked to collect urine for two 24-h periods and to store it in two 3-L plastic containers at 7 [degrees]C and deliver it within 2 days to the clinic. Immediately after delivery, the urine sample volume was determined, and a portion of each of the 24-h samples was stored in 2-mL polypropylene aliquots at -20 [degrees]C for albumin assessment after freezing. For measurement of UAC in fresh samples, portions were kept at 7[degrees]C in 10-mL polystyrene tubes until the UAC was measured.