Total serum homocysteine (Hcy) was measured in patients with either low serum folate, low serum vitaminB12(B12), or potential metabolic defects, in order to evaluate Hcy as an indicator of the tissue status of the two vitamins. An increased Hcy supported the diagnosis of frank tissue deficiency in those patients in whom tissue deficiency was evident by other means. A Hcy within the reference interval enabled the clinician to identify those patients whose low serum vitamin level and symptoms did not reflect a tissue deficiency ofB12or folate of clinical consequence. Children with inherited disturbances ofB12metabolism, and whose serumB12was within the reference interval, were correctly identified by an increased Hcy. A declining Hcy was evidence of correction of a deficiency even when other laboratory or clinical measurements of a response were obscured.
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