What is the lowest mean corpuscular volume (MCV) that can be seen in iron‑deficient (masked) polycythemia vera?

MCV in Iron-Deficient (Masked) Polycythemia Vera

In masked polycythemia vera with iron deficiency, the MCV can drop below 70 fL, with documented cases showing microcytic polycythemia at this threshold. 1

Understanding Masked PV and Iron Deficiency

Iron deficiency is nearly universal in PV patients, occurring at presentation and/or during disease course in virtually all cases. 2 This creates a physiological paradox where polycythemia coexists with iron deficiency, fundamentally altering red cell indices. 2

The key mechanism: Since hematocrit equals RBC count × MCV, iron deficiency-induced microcytosis can mask the elevated red cell mass by normalizing or reducing the hematocrit despite a truly elevated RBC count. 3

Specific MCV Thresholds in Masked PV

• MCV <70 fL is well-documented in polycythemia vera patients with concurrent iron deficiency 1
• Patients with masked PV demonstrate significantly lower MCV ratios compared to other myeloproliferative neoplasm suspects 3
• The microcytosis in iron-deficient PV can be severe enough to mimic thalassemia minor, requiring careful differentiation 1

Critical Diagnostic Pitfalls

Do not assume microcytic polycythemia is always thalassemia. Among 35 patients with MCV <70 fL and elevated RBC counts, 26% had polycythemia (either vera or secondary) with iron deficiency rather than thalassemia. 1

Distinguishing Features:

• RBC size distribution curves reliably distinguish between thalassemia minor and polycythemia with iron deficiency 1
• Red cell distribution width (RDW) is elevated in iron deficiency (>14%) but normal in thalassemia 4, 5
• Serum ferritin <30 μg/L confirms iron deficiency in the absence of inflammation 4, 5
• When iron is repleted in polycythemia patients, the RBC count remains elevated while MCV normalizes to reveal the underlying polycythemia 1

Clinical Implications of Iron Deficiency in PV

Iron deficiency in PV is not merely a laboratory finding—it has significant therapeutic and prognostic implications:

• Therapeutic phlebotomy exacerbates iron deficiency, contributing to non-hematological symptoms beyond the hematologic abnormalities 2
• Cytoreductive therapy works more effectively in iron-deficient versus iron-replete PV patients for reversing symptoms 2
• Masked PV patients (45% of PV cases by WHO criteria) present with higher platelet counts and lower JAK2V617F allele burden compared to overt PV 6

Practical Diagnostic Algorithm

When encountering microcytic polycythemia (elevated RBC with low MCV):

1. Measure serum ferritin immediately to assess iron status 4
2. Check RDW: High RDW (>14%) suggests iron deficiency; normal RDW suggests thalassemia 4, 5
3. If ferritin is low (<30 μg/L), consider masked PV in the differential alongside iron deficiency anemia 4
4. Evaluate RBC count specifically: In true PV, the RBC count remains elevated despite microcytosis 3, 1
5. Consider JAK2 mutation testing if clinical suspicion for PV exists despite normal/low hematocrit 6

The RBC count may more precisely reflect total red cell mass than hematocrit in iron-deficient states, making it a superior marker for the hypercoagulable state in PV patients. 3