Definition
- Vitamin A is a subclass of a family of lipid-soluble compounds referred to as retinoic acids. There are essentially three forms of vitamin A: retinols, beta-carotenes, and carotenoids. Retinol, also known as preformed vitamin A, is the most active form and is mostly found in animal sources of food. Beta-carotene, also known as provitamin A, is the plant source of retinol from which mammals make two thirds of their vitamin A. Carotenoids, the largest group of the three, contain multiple conjugated double bonds and exist in a free alcohol or in a fatty acyl ester form. Vitamin A promotes normal vision and prevents night blindness; contributes to growth of bone, teeth, and soft tissues; supports thyroxine formation; maintains epithelial cell membranes, skin, and mucous membranes; and acts as an anti-infection agent.
- Normal range: see Table 16.83.
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TABLE 16 " “83Normal Ranges for Vitamin A by AgeView LargeTABLE 16 " “83Normal Ranges for Vitamin A by Age Age Reference Interval (mg/L) 0 " “1 mo 0.18 " “0.50 2 mo " “12 y 0.20 " “0.50 13 " “17 y 0.26 " “0.70 ≥18 y 0.30 " “1.20
Use
- Assist in the diagnosis of night blindness
- Evaluate skin disorders
- Investigate suspected vitamin A deficiency
Interpretation
Increased In
- Chronic kidney disease
- Idiopathic hypercalcemia in infants
- Vitamin A toxicity
Decreased In
- Abetalipoproteinemia
- Carcinoid syndrome
- Chronic infections
- CF
- Disseminated TB
- Hypothyroidism
- Infantile blindness
- Liver, GI, or pancreatic disease
- Night blindness
- Protein malnutrition
- Sterility and teratogenesis
- Zinc deficiency
Limitations
- Alcohol (moderate intake), oral contraceptives, and probucol increase vitamin A levels.
- Alcohol (chronic intake, alcoholism), allopurinol, cholestyramine, colestipol, mineral oil, and neomycin decrease vitamin A levels.
- Serum retinol is typically maintained until hepatic stores are almost depleted. Values >0.30 mg/L represent adequate liver stores, whereas values <0.10 mg/L indicate deficiency.
- Samples that come in contact with plastic tubing or have been exposed to excessive light may show low results.