Mechanism of Action Causing Hypersegmented Neutrophils in Vitamin B12 Deficiency

Pathophysiological Mechanism

Vitamin B12 is essential for DNA synthesis through its role as a cofactor in one-carbon metabolism, and its deficiency impairs DNA synthesis, leading to disordered DNA replication, maturation arrest, and ineffective early hematopoiesis in a population with vitamin B12 deficiency 1, 2
The functional interdependence between vitamin B12 and folate is critical, as B12 deficiency leads to a functional folate deficiency, known as the "folate trap", affecting individuals with vitamin B12 deficiency 1

Vitamin B12 functions as a cofactor for methionine synthase, which converts homocysteine to methionine, and its deficiency decreases methionine synthase activity, causing accumulation of 5-methyltetrahydrofolate (5-MTHF) and impaired conversion of homocysteine to methionine in individuals with vitamin B12 deficiency 1, 3
This disruption affects rapidly dividing cells, particularly in bone marrow, leading to abnormal nuclear maturation in neutrophils, characterized by hypersegmentation, in a population with vitamin B12 deficiency 1, 2

Measuring methylmalonic acid (MMA) or active B12 (holotranscobalamin) provides more sensitive detection of B12 deficiency than standard total B12 tests, according to the guideline society, in individuals suspected of having vitamin B12 deficiency 4, 5

The National Comprehensive Cancer Network recommends checking serum B12 level as part of initial workup for any unexplained cytopenia 6
A low reticulocyte count despite anemia indicates decreased RBC production, and this finding is associated with vitamin B12 deficiency 7

Vitamin B12 deficiency can cause macrocytosis (MCV >100 fL) with megaloblastic changes, according to the National Comprehensive Cancer Network 7

The American College of Physicians and other organizations imply that parenteral vitamin B12 supplementation results in rapid normalization of blood counts and bone marrow morphology, confirming the diagnosis retrospectively, although the article does not directly cite the American College of Physicians, the standard treatment is 1000 mcg intramuscularly for patients with severe hematologic manifestations 6