Further evaluation revealed vitamin Bl 2 level 292 pg/mL, serum folate 0.9 ng/mL, and RBC folate 24 ng/mL. Methylmalonic acid level was normal, while homocysteine was found to be elevated. What is the most likely diagnosis?

A. Decreased levels of serum and RBC folate in conjunction with normal levels of methyl-malonic acid are consistent with the diagnosis of folic acid deficiency. Elevation of both methylmalonicacid and homocysteine can be used to detect cases of early vitamin Bl2 deficiency, as methylmalonic acid levels are normal in folic acid deficiency, while homocysteine is usually elevated. RBC folate levels are considered to be a better reflection of tissue folate stores compared with serum folate, which can fluctuate widely following meals. However, studies have shown that the information provided by both tests is comparable, and thus the more expensive RBC folate test is reserved for cases of high suspicion for folate deficiency despite normal serum folate levels. The m�thylfolate trap hypothesis predicts higher serum folate levels in conjunction with lower RBC folate levels associated with vitamin Bl2 deficiency, as reduced methionine synthase activity causes reduced intracellular methylfolate metabo-lism and slow cellular loss by diffusion into the circulation. However, this hypothesis has never been definitely demonstrated in humans (J Clin Pathol. 2003;56(12):924, Br J Haematol. 2006;1 32(5):623).