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Vitamin A and Carotene - Deficiency Test

The vitamin A and carotene test measures serum levels of vitamin A (retinol) and its precursor, carotene. A fat-soluble vitamin normally supplied by diet, vitamin A is important for reproduction, vision (especially night vision), and epithelial tissue and bone growth. Vitamin A is found mostly in fruits, vegetables, eggs, poultry, meat, and fish. Carotene is present in leafy green vegetables and in yellow fruits and vegetables.

In this serum test, the color reactions produced by vitamin A and related compounds with various reagents provide both quantitative and qualitative information.

Purpose

• To investigate suspected vitamin A deficiency or toxicity
• To aid diagnosis of visual disturbances, especially night blindness and xerophthalmia
• To aid diagnosis of skin diseases, such as keratosis follicularis or ichthyosis
• To screen for malabsorption

Patient preparation

• Explain to the patient that this test measures the level of vitamin A in the blood.
• Instruct him to fast overnight, but that he needn't restrict water intake.
• Tell him that this test requires a blood sample. Explain who will perform the venipuncture and when.
• Reassure him that although he may feel some discomfort from the needle puncture and the pressure of the tourniquet, collecting the sample takes only a few minutes.

Procedure and posttest care

• Perform a venipuncture, and collect the sample in a 7-ml red-top or royal-blue-top tube.
• If a hematoma develops at the venipuncture site, apply warm soaks.
• Remove diet restrictions.

Precautions

• Protect the sample from light, because vitamin A characteristically absorbs light.
• Handle the sample gently, and send it to the laboratory immediately.

Reference values

Normal serum levels for carotene, the precursor to vitamin A, range from 48 to200 ug/dl; for vitamin A (retinol), 360 to1,200 ug/L.

Abnormal findings

Low serum levels of vitamin A (hypovitaminosis A) may indicate impaired fat absorption, as in celiac disease, infectious hepatitis, cystic fibrosis of the pancreas, or obstructive jaundice. Low levels are also associated with protein-calorie malnutrition (marasmic kwashiorkor). Similar decreases in vitamin A levels may also result from chronic nephritis.

Elevated vitamin A levels (hypervitaminosis A) usually indicate chronically excessive intake of vitamin A supplements or of foods high in vitamin A. Increased levels are also associated with hyperlipemia and hypercholesterolemia of uncontrolled diabetes mellitus.

Decreased serum carotene levels may indicate impaired fat absorption or, rarely, insufficient dietary intake of carotene. Carotene levels may also be suppressed during pregnancy. Elevated carotene levels indicate grossly excessive dietary intake.

Interfering factors

• Failure to observe pretest restrictions
• Hemolysis due to rough handling of the sample
• Mineral oil, neomycin, and cholestyramine (possible decrease)
• Glucocorticoids and oral contraceptives (possible increase)