MCV Levels in Polycythemia Vera
Low MCV (microcytosis) is characteristic of polycythemia vera, not an elevated MCV. This occurs due to iron deficiency from chronic phlebotomy or increased iron utilization by the expanded red cell mass.
The Relationship Between MCV and PV
A low mean corpuscular volume strongly suggests active pruritus in PV and reflects underlying iron deficiency. 1 This finding is clinically significant because:
Iron deficiency develops in PV patients through two mechanisms: 1
- Increased iron consumption by the proliferating erythroid mass
- Repeated therapeutic phlebotomies that deplete iron stores
The pathologic baseline red cell mass in PV can be lowered to within the normal reference range by superimposed iron deficiency or bleeding. 1 This creates a diagnostic pitfall where the hemoglobin/hematocrit may appear deceptively normal despite underlying PV.
Clinical Significance of Low MCV in PV
Iron deficiency with low MCV can mask the true severity of polycythemia vera. 1 When evaluating a patient with:
- Borderline-high or even normal hematocrit
- Low MCV (microcytosis)
- Other PV-related features (thrombocytosis, leukocytosis, splenomegaly, aquagenic pruritus)
This constellation warrants immediate workup for PV regardless of the measured red cell mass value. 1
Diagnostic Algorithm When MCV is Low
Repeat complete blood count with red cell indices to confirm sustained microcytosis and evaluate mean corpuscular volume, as low MCV suggests iron deficiency that can mask true PV. 2
Order serum ferritin, iron, and transferrin saturation studies because iron deficiency can suppress hemoglobin and create fluctuating values that obscure the diagnosis of PV. 2
Proceed with JAK2 V617F mutation testing (present in >95% of PV cases) and serum erythropoietin level (characteristically low in PV, <2.9 mU/mL). 2, 3
Critical Pitfall to Avoid
Do not dismiss the possibility of PV in a patient with normal or borderline hemoglobin if microcytosis is present. 1 The iron deficiency may be suppressing the hemoglobin elevation that would otherwise be diagnostic. Look for associated features such as thrombocytosis, leukocytosis, or splenomegaly that trigger immediate workup. 2