Serum homocysteine in routine evaluation of potential vitamin B12 and folate deficiency

Total serum homocysteine (Hcy) was measured in patients with either low serum folate, low serum vitamin B12 (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 of B12 or folate of clinical consequence. Children with inherited disturbances of B12 metabolism, and whose serum B12 was 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.