The products of vitamin B metabolism are excreted in the urine, the major product of which is 4-pyridoxic acid.

Urinary 4-pyridoxic acid is also an indicator of vitamin B deficiency; levels of less than 3.0 mmol/day is suggestive of vitamin B deficiency.

Several studies have shown that 4-pyridoxic acid is undetectable in the urine of vitamin B deficient subjects, making it a useful clinical marker to assess the vitamin B status of an individual.