This test measures the level of retinol in the blood; retinol is the primary form of vitamin A in animals. Vitamin A is an essential nutrient required for healthy vision, skin growth and integrity, bone formation, immune function, and embryonic development. It is required to produce photoreceptors in the eyes and to maintain the lining of the surface of the eyes and other mucous membranes. Deficiencies in vitamin A can impair night vision, cause eye damage, and in severe cases lead to blindness. Acute or chronic excesses of vitamin A can be toxic, cause a range of symptoms, and sometimes lead to birth defects.
The body cannot make vitamin A and must rely on dietary sources of vitamin A. Meat sources provide vitamin A (as retinol), while vegetable and fruit sources provide carotene (a substance that can be converted into vitamin A by the liver). Vitamin A is stored in the liver and fat tissues (it is fat-soluble), and healthy adults may have as much as a year's worth stored. The body maintains a relatively stable concentration in the blood through a feedback system that releases vitamin A from storage as needed and increases or decreases the efficiency of dietary vitamin A absorption.