Vitamin A stress test

Vitamin A (axerophthol) is a fat-soluble vitamin with a molecular weight of 286.44, existing in two natural forms - retinol (A1) and 3-dehydroretinol (A2). The precursor of vitamin A is β-carotene.

In the stress test, vitamin A is administered in the form of a fat-soluble ester, which is hydrolyzed by pancreatic enzymes and then re-esterifies in the enterocyte most often with palmitic acid; in plasma, it binds to a specific lipoprotein, retinol binding protein (RBP).

Execution of stress test
The patient comes on an empty stomach, a blood sample is taken and then a load of 5000 IU vitamin A / kg body weight and fluid (tea) is given. A blood sample is taken 3 and 5 hours after vitamin A administration for analysis. Laboratory determination of vitamin A in serum can be performed by extraction method with spectrophotometric measurement. The basic methodology according to Carr-Price is based on the reaction with antimony trichloride, correction for β-carotene is necessary. Modern methods of vitamin A determination use HPLC separation options.

Serum vitamin A reference values are in the range of 1.8–2.3 µmol / l, and clinically simple determination of serum levels is rarely used. The vitamin A stress tolerance test is evaluated according to the increase in serum levels at 3 and 5 hours after the test load. Normal values in 3 hours are in the range 3.6–12.6 µmol / l, in 5 hours 7.2–24.6 µmol / l. The pathological result of the test is at values <3.6 µmol / l in 3 hours, resp. <7.2 µmol / l in 5 hours.

Clinical significance
The vitamin A tolerance test is of clinical importance in the differential diagnosis of malabsorption syndrome. The positivity of the test correlates with the excretion of fats in the stool.

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