Retinol binding protein as a surrogate measure for serum retinol: Studies in vitamin A-deficient children from the Republic of the Marshall Islands

Abstract

Background: Serum retinol is transported by retinol binding protein (RBP), which has one high-affinity binding site for retinol; consequently, the molar ratio of retinol to RBP in the circulation is ≈1 to 1. In vitamin A deficiency (VAD), both serum retinol and RBP decline. However, the retinol-RBP relation has not been well studied in populations with a high incidence of severe VAD. Objective: The purpose of this study was to determine whether RBP is a good surrogate for serum retinol at the very low retinol concentrations encountered in VAD. Design: The stoichiometric relation between retinol and RBP was studied in 239 Marshallese children: 65 with severe VAD (≤0.35 μmol retinol/L), 94 with moderate VAD (0.36-0.70 μmol retinol/L), and 80 with vitamin A sufficiency (>0.70 μmol retinol/L). Results: Excellent correlation between retinol and RBP (r = 0.94) was observed across all retinol concentrations. Severe VAD was predicted with 96% sensitivity and 91% specificity on the basis of an RBP cutoff of ≤0.48 μmol/L, whereas moderate VAD was predicted with 87% sensitivity and 98% specificity on the basis of an RBP cutoff of ≤0.70 μmol/L. Conclusions: The use of RBP results in the classification of essentially the same children with VAD as does retinol, and RBP is an excellent surrogate for serum retinol. Considering the relative ease of measuring RBP with immunodiagnostic kits compared with that of serum retinol by HPLC, the use of RBP concentrations to assess VAD may be particularly advantageous in field settings. Consequently, measuring RBP concentrations may be a practical alternative to measuring serum retinol in population surveys assessing the prevalence of VAD.