Biochemical indicators of vitamin A deficiency: serum retinol and serum retinol binding protein.

Two biochemical indicators are currently recommended for determining whether vitamin A deficiency (VAD) is a public health problem: serum retinol and serum retinol-binding protein (RBP). After consideration of 40 data sets and the original rationale for previously proposed cut-offs, a cut-off for serum retinol concentration was proposed at <0.70 micro mol/L (20 micro g/dL) in > or =15% of the sampled population. This cut-off should be applied to a representative group of preschool age children (6-71 mo). Because measurement of low serum retinol concentrations requires high precision, analysis should be done by HPLC. For serum RBP, a cut-off cannot be reliably specified, because available data are too few and too variable. However, because serum RBP concentration correlates well with serum retinol concentration, it can be used to determine whether VAD is a public health problem in those populations for which the relationship between serum concentrations of retinol and RBP have been established. More efforts to establish a reliable cut-off for RBP is warranted, because analysis, in particular radial immunodiffusion (RID), is relatively simple and inexpensive. Whereas HPLC and RID analyses must be done in a laboratory, methods are being developed for assessing serum retinol and RBP under more remote conditions.