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Stability of serum retinol-binding protein at −20⁰C

Introduction Retinol is a common indicator of vitamin A status. Retinol forms a complex with retinol binding protein (RBP) at a ratio of 1:1 and is thus transported in the serum. RBP is easily measured in serum to provide information on retinol and vitamin A concentrations. Retrospective or small-scale clinical research studies may store serum samples for long durations prior to RBP analysis. The aim of this study was to evaluate the stability of RBP concentrations during prolonged storage at -20⁰C. Methods RBP was measured using the quantitative sandwich enzyme immunoassay technique (R&D Systems). A total of 29 serum samples, stored at -20⁰C for 6, 9, 12, 18, 30 and 36 months were analysed and the results compared with the original results to evaluate sample stability and reproducibility. Results The immunoassay had a mean minimum detectable concentration of 0.000224 mg/L, with <10% inter- and intra-assay coefficient of variance. The concentrations of RBP in the samples analysed ranged between 15.80 and 48.11 mg/L. The manufacturer quoted reference range is 18 - 50 mg/L. All 29 serum sample results were comparable and showed no significant difference when compared using linear regression analysis and T-test assuming equal variances: 6 months' storage (r=0.97, p=0.18), 9 months' storage (r=0.94, p=0.45), 12 months' storage (r=0.99, p=0.19), 18 months' storage (r=0.80, p=0.27), 30 months' storage (r=0.96, p=0.27), 36 months' storage (r=0.98, p=0.35). Conclusions Our study showed that retinol binding protein concentrations are stable for up-to 36 months when stored at -20⁰C.