Vitamin A deficiency states in developed countries are rare, but many developing countries have vitamin A deficiency of public health significance, associated with overt signs of deficiency, or subclinical levels of vitamin A depletion with marginal liver reserves [27]. However, today, marginal vitamin A status is prevalent and difficult to diagnose. Vitamin A biomarkers have been developed to diagnose different degrees of vitamin A status [28]. There are biological and functional indicators, such as ophthalmic signs of vitamin A deficiency (night blindness, xerophtalmia, Bitot spots), and biochemical indicators. As biochemical indicators, serum retinol concentrations are the most common population indicator. Normal plasma levels are 20–50 µg/dL in infants and increase gradually as children become older. However, serum retinol concentration is affected by infection and inflammation because RBP is an acute phase protein, thus these conditions may mimic a lack of vitamin A. For this reason, RBP linked with markers of inflammation may be used to adjust serum retinol concentration even if the ratio of retinol to RBP is influenced by a status of vitamin A deficiency, because of the increased level of circulating unbound plasma RBP. Furthermore, serum retinol concentration is homeostatically controlled over a wide range of liver reserves, and thus does not reflect the vitamin A liver stores. However, in children, values <10 µg/dL indicate a deficiency of vitamin A [29].