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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.

Purpose

• To detect vitamin B6 deficiency

Patient preparation

• Explain to the patient that this test determines the body's stores of vitamin B6
• Tell him he'll receive an oral dose of medication.
• Explain that this test requires urine collection over a 24-hour period.

Procedure and posttest care

• Administer L-tryptophan by mouth (usually, 50 mg/kg for children and up to 2 g/kg for adults).
• Have the patient void and discard the urine. Immediately begin collection of a 24-hour urine specimen.
• Inform the patient with vitamin B6 deficiency that yeast, wheat, corn, liver, and kidneys are good sources of pyridoxine.

Precautions

• Make sure the specimen bottle contains a crystal of thymol, a preservative.
• Tell the patient not to contaminate the urine specimen with toilet tissue or stool.
• Refrigerate the specimen, or place it on ice during the collection period.

Reference values

Normal excretion of xanthurenic acid after a tryptophan challenge dose is less than 50 mg/24 hours.

Abnormal findings

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.

Interfering factors

• Contamination of the specimen with toilet tissue or stool
• Failure to properly handle the specimen
• Oral contraceptives, hydralazine, Dpenicillamine, and isoniazid (decrease)