Background Vitamin B12 deficiency can cause severe intramedullary hemolysis and cytopenias. Myeloproliferative neoplasms (MPNs), particularly polycythemia vera (PV), are typically characterized by hyperproliferation but may remain undetected when masked by concomitant deficiencies. Case Presentation A 48‐year‐old woman presented with fatigue, weight loss, and pancytopenia. Laboratory tests showed severe macrocytic anemia, hemolysis, and markedly reduced vitamin B12 levels. Gastroscopy and antibody testing confirmed autoimmune atrophic gastritis with pernicious anemia. Bone marrow biopsy revealed hypercellularity, panmyelosis, and myelofibrosis (MF‐2), initially interpreted as reactive. After vitamin B12 supplementation, cytopenias resolved; however, follow‐up demonstrated polycythemia, leukocytosis, and thrombocytosis. Molecular analysis identified a JAK2 V617F mutation (variant allele fraction 40%), confirming PV with progression toward myelofibrosis. The patient was treated with phlebotomy, low‐dose aspirin, and hydroxyurea, alongside continued vitamin B12 replacement. In addition, mild alpha‐thalassemia was diagnosed in the course of an increasingly microcytic hypochromic blood count. Conclusion Severe vitamin B12 deficiency may mask an underlying MPN, as well as other hematologic disorders like alpha‐thalassemia. Close follow‐up after hematologic recovery is essential to avoid delayed diagnosis of coexisting hematologic malignancies and disorders. This case presents a combination of complex and extremely rare hematological scenarios, where various hematological disorders exert conflicting influences on red blood cell indices, making diagnosis challenging.
Rohner et al. (Thu,) studied this question.