Vitamin B6 (Tryptophan Challenge) - Deficiency Test
Measurement of urine xanthurenic acid after a challenge dose of tryptophan confirms deficiency of vitamin B6 long before symptoms appear.
Although vitamin B6 isn't directly involved in energy metabolism, it's essential for reactions that occur in protein metabolism and for amino acid synthesis. Vitamin B6 deficiency can cause hypochromic microcytic anemia without iron deficiency and central nervous system disturbances. When normal magnesium levels accompany a vitamin B6 deficiency, urinary citrate and oxalate solubility may decrease, causing formation of urinary calculi.
Procedure and posttest care
Normal excretion of xanthurenic acid after a tryptophan challenge dose is less than 50 mg/24 hours.
Urine levels of xanthurenic acid exceeding 100 mg/24 hours indicate vitamin B6 deficiency. This rare disorder may result from malnutrition, malignancy, pregnancy, familial xanthurenic aciduria, or use of oral contraceptives, hydralazine, D-penicillamine, or isoniazid.
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