Detection of functional vitamin B12 and folate deficiencies, while serum levels are normal

Dear Sir,

We read with great interest an article by Scarpa et al.¹ on a patient with functional vitamin B12 deficiency despite serum levels of the vitamin being in the reference interval. We congratulate the authors for diagnosing the patient with their precise biochemical and clinical assessments and for their valuable comments. We would like to add a short note to help readers to interpret the results of the study.

The patient was diagnosed as having megaloblastic anaemia despite serum vitamin B12 and folic acid levels being within normative ranges, at 910 pg/mL and 10.3 ng/mL, respectively. One of the possible explanations for this condition is low tissue levels or functional deficiencies of both vitamin B12 and folic acid. Laboratory errors are much less frequent explanations in well-managed laboratories. The analysis of plasma total homocysteine (tHcy) has been recommended in the diagnosis of folate and cobalamin deficiencies, because these vitamins are both cofactors of the methionine synthase enzyme which has central role in one carbon metabolism². Elevated tHcy concentrations could demonstrate a functional deficiency of folic acid, directly. Additionally, vitamin B12 metabolism could be evaluated through the concentration of methylmalonic acid (MMA) in either the serum or urine, which is accepted as a better approach for the assessment of functional or tissue levels of vitamin B12 deficiency³. Indeed, as in the case described by Scarpa et al., normal serum levels of vitamin B12 do not exclude low tissue levels or a functional deficiency of the vitamin. MMA measurement could, therefore, be a complementary way to evaluate intracellular vitamin B12 metabolism, although there are some problems relative to the specificity, complexity and cost of this test. In conclusion, patients considered to have megaloblastic anemia could be further evaluated by measuring their tHcy and MMA concentrations which would help clinicians to make a precise diagnosis and exclude possible laboratory errors. The case report presented highlights the importance of careful interpretation of clinical laboratory results.