How is it used?
Vitamin A testing is used to help diagnose vitamin A deficiency in people with symptoms, such as night blindness, or in people with diseases that impair intestinal absorption of nutrients and who are at risk of vitamin A deficiency. Testing for this purpose is not common because vitamin A deficiencies are rare in the United States. Sometimes testing may be used to detect toxic levels caused by ingestion of large amounts of vitamin A.
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.
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). 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.