Retinol-Binding Protein Stability in Dried Blood Spots

Abstract Background: Retinol-binding protein (RBP) is accepted as a surrogate biochemical marker for retinol to determine vitamin A (VA) status. A recently developed enzyme immunoassay for RBP uses serum or whole blood stored as dried blood spots (DBS). However, the stability of RBP in DBS has not been examined. Methods: RBP stability was studied in a laboratory and in field conditions in northern Kenya. For the laboratory study, 63 DBS collected by finger prick and stored sealed in a plastic bag with desiccant were exposed to 1 of 5 time/storage-temperature treatments: (a) baseline, (b) 30 °C/7 days, (c) 30 °C/14 days, (d) 30 °C/28 days, and (e) 4 °C/38 days. Baseline RBP concentrations were compared to those obtained after the storage treatments. For the field study, 50 paired DBS and serum specimens were prepared from venous blood obtained in northern Kenya. DBS were stored in a sealed plastic bag with desiccant at ambient temperature (12 °C–28 °C) for 13–42 days, and sera were stored at −20 °C to −70 °C. Recovered RBP concentrations were compared with serum retinol for stability, correlation, sensitivity, and specificity. Results: RBP in DBS stored in the laboratory at 30 °C remained stable for 2–4 weeks, but specimens stored at 4 °C for 38 days produced values below baseline (P = 0.001). DBS stored under field conditions remained stable for 2–6 weeks, as demonstrated by good correlation with serum retinol, a result that suggests that RBP in DBS will have good sensitivity and specificity for predicting VA deficiency. Conclusion: RBP in DBS can withstand storage at a relatively high ambient temperature and thus facilitate accurate VA assessments in populations in locations where serum collection and storage are unfeasible.