Can Polycythemia Vera Present with Low MCV Without Bleeding or Phlebotomy?
Yes, polycythemia vera commonly presents with low MCV (microcytosis) even without chronic bleeding or prior therapeutic phlebotomy, because the clonal erythroid expansion in PV creates massive iron demand that depletes iron stores through increased utilization by the proliferating erythroid compartment alone. 1
Mechanisms of Iron Deficiency and Microcytosis in PV
Iron deficiency in PV arises from two distinct pathways:
Increased iron utilization: The pathologically expanded erythroid compartment in PV consumes iron at rates far exceeding normal erythropoiesis, depleting body iron stores even without any blood loss 1
Therapeutic phlebotomy: This is the second mechanism, not the only one—repeated phlebotomies further exacerbate pre-existing iron deficiency 1
Virtually all PV patients are iron deficient at presentation and/or during disease course, representing a physiological disconnect where polycythemia and iron deficiency coexist 2. This occurs because the JAK2-driven clonal expansion creates relentless iron demand that outstrips absorption capacity.
Clinical Significance of Low MCV in PV
Low MCV serves as a strong diagnostic clue for PV and carries important clinical implications:
Microcytosis is a surrogate marker for active aquagenic pruritus in PV and signals underlying iron deficiency characteristic of the disease 1
Iron deficiency can mask the true red cell mass elevation: When iron deficiency superimposes on PV, the total red cell mass may be reduced to within normal reference range, making hemoglobin and hematocrit appear deceptively normal or only borderline elevated 1, 3
MCV becomes unreliable for screening iron deficiency in erythrocytosis: In the context of elevated RBC count, serum ferritin, transferrin saturation, and iron levels are required for accurate iron status assessment 4
Critical Diagnostic Pitfall
The presence of borderline-high or even normal hematocrit WITH microcytosis should trigger immediate PV workup when accompanied by other PV-related features (thrombocytosis, leukocytosis, splenomegaly, aquagenic pruritus, unusual thrombosis, or erythromelalgia) 1, 5. The possibility of PV must not be excluded solely because hemoglobin is normal or mildly elevated when microcytosis is observed—iron deficiency masks the hemoglobin rise that would otherwise be diagnostic 1.
Recommended Diagnostic Approach When Low MCV Is Detected
When microcytosis is found with erythrocytosis or borderline-high hemoglobin:
Repeat CBC with red cell indices to confirm persistent microcytosis and reinforce suspicion of iron deficiency masking PV 1
Measure serum ferritin, iron, and transferrin saturation to quantify iron deficiency that can suppress hemoglobin and obscure PV diagnosis 1, 4
Perform JAK2 V617F mutation testing (present in >95% of PV cases) and serum erythropoietin level (typically low in PV, with >90% specificity) to provide definitive molecular and biochemical confirmation despite misleading red cell indices 1, 5
Evidence on RBC Count vs. Hematocrit in Iron-Deficient PV
The RBC count may more precisely reflect total red cell mass than hematocrit in PV patients with microcytosis, because hematocrit equals RBC count × MCV, and low MCV artificially suppresses the hematocrit value 3. A good correlation of RBC versus hematocrit was found only when individuals with microcytosis were excluded (R² = 0.87 in PV) 3.
Pathophysiology: Why Hepcidin Regulation Fails in PV
Insufficiently suppressed hepcidin given the degree of iron deficiency in PV patients strongly suggests disordered iron metabolism is an important component of PV pathobiology 2. Both decreased circulating iron and increased erythroferrone levels (from erythroid hyperplasia) should suppress hepcidin and enable recovery from iron deficiency, yet this homeostatic mechanism fails in PV 2. Inflammation accompanying PV likely counteracts hepcidin suppression, but relatively low serum ferritin levels suggest inflammation is not the major contributor 2.