Evaluation of plasma retinol-binding protein as a surrogate measure for plasma retinol concentrations

Plasma retinol-binding protein (RBP) concentrations have been suggested as surrogate indicators for plasma retinol concentrations in the assessment of vitamin A status in less technologically developed settings. Plasma RBP was measured by radial immunodiffusion and plasma retinol by high performance liquid chromatography in a cross-sectional study of 900 pregnant women at the Queen Elizabeth Central Hospital, Blantyre, Malawi. The Spearman correlation coefficient between plasma RBP and retinol concentrations was 0.95 (p < 0.0001). By linear regression, 0.70 μmol l^-1 retinol was equivalent to 21.1 mg l^-1 RBP. With these cut-off points for defining vitamin A deficiency, there was high concordance between categorical descriptions of deficiency using retinol and RBP by chi-square analysis (p < 0.001). Measurement of plasma RBP by radial immunodiffusion is simple, inexpensive, and does not require expensive instrumentation: Plasma RBP concentrations measured by radial immunodiffusion are highly correlated with plasma retinol and can be used as a simple surrogate measure for vitamin A concentrations in large field studies.