Despite elevated serum vitamin B12 levels, functional (i.e., metabolically manifest) vitamin B12 deficiency has been reported in a significant proportion of MPN patients. This study aimed to assess the prevalence of functional vitamin B12 deficiency and elevated serum vitamin B12 levels among Indian MPN patients.

This cross-sectional study evaluated serum vitamin B12, homocysteine, methyl malonic acid (MMA) and holotranscobalamin levels among 176 MPN patients. Serum vitamin B12 levels between 150 and 950 pg/mL were considered normal. Functional vitamin B12 deficiency was diagnosed based on combination of elevated serum MMA (> 0.4 nmol/mL) and homocysteine (> 15 µmol/L) levels and low serum holotranscobalamin levels (< 25 pmol/L), irrespective of serum vitamin B12 levels and clinical manifestations of deficiency.

Among patients enrolled (n = 176), 108 (61%) had chronic phase CML, 37 (21%) PV, 23 (13%) PMF and 8 (4%) ET. Nearly one-third were newly diagnosed to have MPNs and treatment naïve. Elevated serum vitamin B12 levels (> 950pg/mL) were seen in about a quarter (n = 45) of MPN patients. Half (n = 89) were detected to have functional vitamin B12 deficiency. No association was found between functional vitamin B12 deficiency and serum vitamin B12 levels and functional deficiency prevalence did not differ significantly between different types of MPNs and between newly diagnosed (treatment-naïve) and already treated MPNs.

About a quarter of MPN patients had elevated serum B12 levels and half had functional vitamin B12 deficiency. Functional deficiency was diagnosed independent of serum vitamin B12 levels. Hence, evaluation for functional deficiency should be carried out irrespective of serum vitamin B12 levels. Future studies should evaluate potential benefit of vitamin B12 replacement in MPN patients with functional vitamin B12 deficiency.