What is being tested?
Vitamin A is a fat-soluble vitamin that is naturally present in many of the foods we eat. Retinol is the primary form of vitamin A in humans. This test measures the level of retinol in the blood.
Vitamin A is an essential nutrient required for healthy vision and skin, bone formation, immune system function, and reproduction. It is required to produce photoreceptors in the eyes and to maintain the lining of the surface of the eyes and other mucous membranes. A lack of vitamin A can affect night vision, cause eye damage, and in severe cases, lead to blindness. Too much vitamin A can be toxic, causing a range of symptoms, and sometimes leading 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 level 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.
Deficiencies in vitamin A are rare in the United States, but they are a major health problem in as many as half of all countries, especially in resource-poor nations where high numbers of people have limited diets, according to the World Health Organization (WHO). One of the first signs of vitamin A deficiency is night blindness. Young children and pregnant women are most commonly affected, say the WHO.
In the U.S., vitamin A deficiencies are primarily seen in those with malnutrition, chronic diarrhea, malabsorption disorders such as celiac disease, cystic fibrosis, or chronic pancreatitis, in the elderly, and in those with alcoholism, liver disease and blockage of the bile ducts. Surgery either on the intestine or pancreas can have the similar consequences.
Vitamin A deficiency is a risk factor for severe measles; treatment with vitamin A can shorten the duration of the disorder and may reduce the severity of symptoms and risk of death.
Vitamin A toxicity occurs primarily from overuse of vitamin supplements. However, it can sometimes occur when the diet includes a high proportion of foods that are high in vitamin A, such as liver.
How is the test used?
Vitamin A testing is used to help diagnose a vitamin A deficiency in people with signs and symptoms, such as night blindness, or in people 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 consuming large amounts of vitamin A.
When is it ordered?
A vitamin A test may be ordered when you have signs and symptoms that suggest a vitamin A deficiency or you have general malnutrition. These signs and symptoms include:
- Night blindness
- Dry eyes, skin, and hair
- Ulcers and damage to the cornea, the outermost layer of the front of the eye that covers the iris and pupil
- Skin thickening and lesions
- Grayish spots on eye lining (Bitot spots)
- Repeated infections
Testing may be done periodically when you have a disease that is associated with malabsorption of nutrients, such as:
- Celiac disease
- Irritable bowel syndrome
- Crohns disease, a type of inflammatory bowel disease
Testing may be performed when your signs and symptoms and medical history suggest vitamin A toxicity. If you have vitamin A toxicity, you may experience:
- Nausea and vomiting
- Double or blurred vision
- Muscle pain
- Bone and joint pain
- Weight loss
- Hair loss
- Mucous membrane dryness
- Liver dysfunction
- Cracks (fissures) at the corners of the mouth
- Inflammation of the tongue (glossitis)
What does the test result mean?
A normal vitamin A blood level means you currently have enough vitamin A, but it does not indicate how much you have stored in reserve. The body will maintain vitamin A in the blood at a relatively stable level until stores are low.
A low vitamin A blood test result means you lack enough vitamin A (deficiency). This can lead to night blindness, among other complications.
A high vitamin A blood level typically means too much vitamin A is circulating in the blood and may be deposited in other parts of the body, leading to toxicity. Getting too much vitamin A, usually from supplements or certain medications, can cause dizziness, headaches, and nausea. In serious cases, it can lead to coma and be life-threatening. In pregnant women, too much vitamin A can cause birth defects in their babies.
Is there anything else I should know?
People with a minimal amount of vitamin A stored may have marginal but adequate vitamin A until they have an illness or other condition, such as pregnancy, that puts increased demands on their body. This is one of the reasons that night blindness is prevalent in many parts of the world during pregnancy.
Eating excessive amounts of foods with beta carotene, such as carrots, can cause a person’s skin to turn yellow-orange but it does not cause vitamin A toxicity. The body does not convert beta carotene to vitamin A when there is sufficient vitamin A present.
What are the dietary sources of vitamin A?
Sources of preformed vitamin A include meat (especially liver), eggs, dairy products, and fortified foods. Sources of carotene (mainly beta carotene) include brightly colored fruits and vegetables such as carrots, spinach, apricots, sweet potatoes, and cantaloupe.
What are the recommended dietary allowances (RDA) for vitamin A?
For teens and adults age 14 and older, the RDA is 900 micrograms (mcg) (3,000 IU) for males and 700 mcg (2,310 IU) for females. The recommendations for others, such as infants, children and pregnant women, vary. For these, see the lists provided by the National Institutes of Health Office of Dietary Supplements.
How are vitamin A deficiencies treated?
Mild to moderate deficiencies are treated with supplements and/or with alterations in diet. More severe deficiencies may require monitored therapeutic doses of vitamin A.
Can vitamin A testing be done in my healthcare practitioner's office?
No, this test requires specialized equipment and is not offered by every laboratory. In many cases, your blood will be sent to a reference laboratory.