The Test Sample
What is being tested?
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.
Deficiencies in vitamin A are rare in the United States, but they are a major health problem in many developing nations where high numbers of people have limited diets. One of the first signs of vitamin A deficiency is night blindness. In a 1995-2005 review of the global prevalence of vitamin A deficiency in populations at risk, the World Health Organization estimated that night blindness affected as many as 5 million preschool age children and nearly 10 million pregnant women. In addition to this, they estimated that another 190 million preschool age children and 19 million pregnant women were at risk of vitamin A deficiency, with low retinol levels that reflected an inadequate supply of vitamin A.
In the U.S., deficiencies are primarily seen in those with malnutrition, with malabsorption disorders such as celiac disease, cystic fibrosis, or chronic pancreatitis, in the elderly, and in those with alcoholism and liver disease.
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 sample collected for testing?
A blood sample is obtained by inserting a needle into a vein in the arm.
NOTE: If undergoing medical tests makes you or someone you care for anxious, embarrassed, or even difficult to manage, you might consider reading one or more of the following articles: Coping with Test Pain, Discomfort, and Anxiety, Tips on Blood Testing, Tips to Help Children through Their Medical Tests, and Tips to Help the Elderly through Their Medical Tests.
Another article, Follow That Sample, provides a glimpse at the collection and processing of a blood sample and throat culture.
Is any test preparation needed to ensure the quality of the sample?
Fasting is required and no alcohol consumption for 24 hours before sample collection.