TABLE 6.

Summary of biomarkers for vitamin A status from the BOND initiative¹

Biomarkers	Type	Use	Utility
Serum RBP	Status	Population	Not released from the liver when retinol is limiting. Used as a proxy for serum retinol in identification of vitamin A deficiency.
Serum/plasma retinol	Status	Population	Most commonly used biomarker. Correlates with the prevalence and severity of xerophthalmia and may change in response to interventions.
Dried blood spot retinol	Status	Population	Surrogate measure of serum retinol. Correlates with serum retinol measured by HPLC.
Relative dose response	Status	Population, individual	Based on hepatic accumulation of RBP during vitamin A depletion. Requires blood sample before and after an oral retinyl ester dose.
Modified relative dose response	Status	Population, individual	More responsive than serum retinol. Qualitatively identifies low or adequate liver vitamin A reserves.
Retinol isotope dilution	Status, marker of excess	Population	Although technically challenging, it is the most sensitive test to measure vitamin A status and intervention impact on vitamin A reserves. Minimally invasive and accurate.
Breast-milk retinol	Status, exposure	Population	Good indicator of vitamin A status in areas where breastfeeding is common until ≥6 mo of age. Milk retinol varies with milk fat; measurement of milk fat is recommended.
Retinyl esters	Status, marker of excess	Population, individual	Validated qualitative measure of hypervitaminosis A. May be confounded by liver disease at the individual level.
Dark adaptation	Function	Population (small scale), individual	Dark-adapted final threshold is inversely and sensitively correlated with serum vitamin A concentrations in the low-to-deficient ranges.
Electroretinography	Function	Population, individual	Measures the bioelectrical response of the retina to a flash of light. Invasive and not suitable for children.
Pupillary threshold testing	Function	Population, groups of individuals	Inversely correlates with serum vitamin A values in the low-deficient range and the concentration of vitamin A in the retina. Noninvasive and can be used in field conditions
Dietary assessment	Exposure	Population, individual with repeated testing	Qualitative measure of exposure. Provides useful information to support biochemical biomarkers. Seasonality of fruit and vegetables must be included.

¹Data are from reference 185. BOND, Biomarkers of Nutrition for Development; RBP, retinol-binding protein.