Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1294 page)

   In addition to PLP, the following methods can be used to assess for vitamin B
₆
deficiency:

   Erythrocyte transaminase activity with and without PLP added has been used as a functional test of pyridoxine status.

   Urinary 4-pyridoxic acid excretion >3.0 mmol/day can be used as an indicator of adequate short-term vitamin B
₆
status.

   Urinary excretion of xanthurenic acid is normally <65 mmol/day following a 2-g tryptophan load.

   Drugs that may decrease vitamin B
₆
levels include amiodarone, anticonvulsants, cycloserine, disulfiram, ethanol, hydralazine, isoniazid, levodopa, oral contraceptives, penicillamine, pyrazinoic acid, and theophylline.

   B
₆
may be decreased with pregnancy, lactation, alcoholism, DM, and in an uncommon B
₆
dependency state, vitamin B
₆
–responsive neonatal convulsions. There is evidence of significant neurotoxicity associated with pyridoxine megavitaminosis; tingling, numbness, clumsiness, gait disturbances, and pseudoathetosis, with doses >2 g/day.

VITAMIN C (ASCORBIC ACID)

   Definition

   Ascorbic acid is essential for the enzymatic amidation of neuropeptides, production of adrenal cortical steroid hormones, promotion of the conversion of tropocollagen to collagen, and metabolism of tyrosine and folate. It also plays a role in lipid and vitamin metabolism and is a powerful reducing agent or antioxidant. Specific actions include activation of detoxifying enzymes in the liver, antioxidation, interception and destruction of free radicals, preservation and restoration of the antioxidant potential of vitamin E, and blockage of the formation of carcinogenic nitrosamines. Vitamin C promotes collagen synthesis, maintains capillary strength, facilitates release of iron from ferritin to form hemoglobin, and functions in the stress response. In addition, vitamin C appears to function in a variety of other metabolic processes in which its role has not been well characterized.

   
Normal range:
0.4–2.0 mg/dL.

   Use

   Investigate suspected metabolic or malabsorptive disorders

   Investigate suspected scurvy