Expected MCV Levels After 7 Years of Iron-Deficient Polycythemia Vera
After seven years of polycythemia vera masked by chronic iron deficiency, expect a mean corpuscular volume well below 70 fL—typically in the range of 60-67 fL—representing severe microcytosis from prolonged iron depletion. 1, 2, 3
Understanding the Pathophysiology
The hematocrit in polycythemia vera reflects both red blood cell count (RBC) and mean corpuscular volume (MCV = HCT ÷ RBC). When chronic iron deficiency develops in PV patients, the elevated RBC count persists while MCV progressively falls, creating "masked" or "inapparent" polycythemia where the hematocrit appears deceptively normal despite true erythrocytosis. 1, 4
Degree of Microcytosis Expected
Severe microcytosis (MCV 60-67 fL) is characteristic of polycythemia vera with prolonged iron deficiency, as documented in patients with microcytic polycythemia where MCV values below 70 fL are common. 2, 3
The MCV rarely falls below 60 fL even with severe chronic iron deficiency, though values of 61 fL have been reported in extreme cases of anemia of chronic disease with microcytosis. 2
Patients with PV demonstrate significantly lower MCV ratios compared to other myeloproliferative neoplasm suspects when iron deficiency is present, with the correlation between RBC and hematocrit breaking down when microcytosis develops (R² = 0.87 only when microcytic individuals are excluded). 4
Associated Laboratory Findings
Beyond the low MCV, several other parameters help confirm this diagnosis:
Serum ferritin will be markedly reduced (typically <15-30 μg/L), confirming absent or severely depleted iron stores after years of consumption by the expanded red cell mass. 1, 5
Red cell distribution width (RDW) is typically elevated (>14-16%), distinguishing iron deficiency from thalassemia trait, which would show normal or minimally elevated RDW despite microcytosis. 1, 5, 6
Transferrin saturation falls below 16-20%, reflecting inadequate circulating iron for ongoing erythropoiesis despite the myeloproliferative drive. 5, 6
The RBC count remains elevated despite the microcytosis, which is the key distinguishing feature from simple iron deficiency anemia where RBC count would be reduced. 4, 3
Clinical Correlation with Symptoms
- Pruritus severity correlates inversely with MCV in polycythemia vera patients, meaning that worsening microcytosis over seven years would be associated with more severe itching symptoms. 7
Diagnostic Pitfalls to Avoid
Do not assume microcytosis excludes polycythemia vera—the combination of elevated RBC count with MCV <70 fL should prompt consideration of PV with iron deficiency rather than thalassemia minor alone. 3
Do not rely on hematocrit alone for PV diagnosis when microcytosis is present, as the HCT may appear falsely normal or only mildly elevated despite true erythrocytosis (the RBC count more accurately reflects the hypercoagulable state). 4
Do not overlook combined deficiencies—iron deficiency can coexist with folate or B12 deficiency in PV patients, which may partially mask the degree of microcytosis while producing an elevated RDW. 1, 6
Ferritin may be falsely elevated above 12-15 μg/L if concurrent inflammation, malignancy, or hepatic disease is present, though values >100 μg/L make iron deficiency unlikely. 1, 6
Expected Response to Iron Repletion
Upon iron supplementation, the MCV will rise toward normal (80-100 fL) over 3-6 months as iron stores are replenished, while the RBC count and hematocrit will increase further, unmasking the true severity of the polycythemia. 5, 3
Hemoglobin should increase ≥10 g/L within 2 weeks of starting oral iron therapy if iron deficiency is contributing to the clinical picture. 5, 6