Summary Aims Microalbuminuria and, to a lesser extent, renal tubular proteins are widely used in the early detection of incipient nephropathy in diabetes mellitus. Recent reports have indicated detrimental effects of storage at −20°C on urine proteins. This study investigated the effects of storage on the measurement of urine proteins and discusses implications for the interpretation of data. Methods Two‐hundred and sixty‐eight specimens, collected from children with Type 1 diabetes, split into duplicate aliquots and stored at −20°C and −70°C, respectively, for 6–8 months, were analysed for albumin, retinol binding protein, N‐acetyl glucosaminidase and creatinine, in the same assays to eliminate inter‐assay variability. Two independent non‐diabetic cohorts of children provided urine specimens, which were stored at −20°C for one cohort and −70°C for the other, to determine normal ranges for urine proteins. Results Storage at −20°C led to a variable underestimation of all three urine proteins in 20% of specimens. Creatinine was unaffected. This underestimation was greater in more concentrated urine (r2 = 0.38, P < 0.001, n = 262). Consequently storage at −20°C increased the varian
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