The Vitamin A test measures the level of vitamin A in the blood. Vitamin A is a fat-soluble vitamin that promotes normal vision, contributes to growth of bone, teeth, and soft tissues and helps maintain skin and mucous membranes. 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. It may also be used to detect toxic levels caused by ingestion of large amounts of vitamin A.