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

Background: Serum retinol is transported by retinol binding protein (RBP), which has one high-affinity binding site for retinol; consequently, the mol ar ratio of retinol to REP in the circulation is approximate to 1 to 1. In vitamin A deficiency (VAD), both serum retinol and RBP decline. However, th e 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 REP is a good surrogate for serum retinol at the very low retinol concentrations encount ered in VAD. Design: The stoichiometric relation between retinol and REP was studied in 239 Marshallese children: 65 with severe VAD (less than or equal to0.35 mu mol retinol/L), 94 with moderate VAD (0.36-0.70 mu mol retinol/L), and 80 w ith vitamin A sufficiency (> 0.70 mu mol retinol/L). Results: Excellent correlation between retinol and REP (r = 0.94) was obser ved across all retinal concentrations. Severe VAD was predicted with 96% se nsitivity and 91% specificity on the basis of an REP cutoff of less than or equal to0.48 mu mol/L, whereas moderate VAD was predicted with 87% sensiti vity and 98% specificity on the basis of an REP cutoff of less than or equa l to0.70 mu mol/L. Conclusions: The use of REP results in the classification of essentially th e same children with VAD as does retinol, and REP is an excellent surrogate for serum retinol. Considering the relative ease of measuring REP with imm unodiagnostic kits compared with that of serum retinol by HPLC, the use of REP concentrations to assess VAD may be particularly advantageous in field settings. Consequently, measuring REP concentrations may be a practical alt ernative to measuring serum retinol in population surveys assessing the pre valence of VAD.