In a typical adult with masked polycythemia vera, how much decrease in mean corpuscular volume is expected over five years?

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Expected MCV Drop in Masked Polycythemia Vera Over 5 Years

In masked polycythemia vera (PV), mean corpuscular volume (MCV) typically decreases progressively over time due to ongoing iron depletion from the hyperproliferative erythroid compartment, though the exact magnitude of decline over 5 years is not precisely quantified in the literature.

Understanding Masked PV and Microcytosis

Masked PV occurs when iron deficiency-related microcytosis suppresses hemoglobin and hematocrit to below diagnostic thresholds, obscuring the diagnosis despite an underlying clonal myeloproliferative disorder. 1, 2

  • Iron deficiency in PV arises from two mechanisms: (1) increased iron utilization by the proliferating erythroid compartment and (2) repeated therapeutic phlebotomies that deplete iron stores 1
  • When iron deficiency superimposes on the pathologically elevated red cell mass of PV, the total red cell mass can be reduced to within the normal reference range, making hemoglobin and hematocrit appear deceptively normal 1
  • Low MCV serves as a strong surrogate marker for active disease in PV and signals underlying iron deficiency 1

Clinical Characteristics of Masked PV

Patients with masked PV consistently demonstrate JAK2 V617F mutation positivity (100%), frequently elevated RBC count (73.9%), and PV-compatible bone marrow findings (83.3%), despite nonelevated hemoglobin/hematocrit at presentation. 2

  • Masked PV comprised 35% of one cohort when using WHO hemoglobin criteria (HB <18.5 g/dL in males, <16.5 g/dL in females) 3
  • Using BCSH hematocrit-based criteria (HCT ≤52% in men, ≤48% in women), masked PV decreased to 15% of cases 3
  • Masked PV patients more frequently present as males, with higher platelet counts and increased bone marrow reticulin fibrosis compared to overt PV 3

Quantifying MCV Changes

While specific longitudinal data on MCV decline over exactly 5 years in masked PV is not available in the provided evidence, the progressive nature of iron depletion in untreated or inadequately managed PV suggests continuous MCV reduction until iron stores are completely exhausted.

  • Patients with PV had significantly lower MCV ratios and serum ferritin compared to MPN suspects 4
  • The presence of microcytosis (low MCV) with borderline-high or normal hematocrit should prompt immediate diagnostic work-up for PV, particularly when accompanied by thrombocytosis, leukocytosis, splenomegaly, or aquagenic pruritus 1
  • Iron deficiency can mask the hemoglobin rise that would otherwise be diagnostic; clinicians must actively search for accompanying signs to trigger further evaluation 1

Prognostic Implications

Masked PV patients demonstrate worse overall survival compared to overt PV when diagnosed by WHO criteria (P = 0.011) and BCSH criteria (P = 0.0019). 3

  • Risk factors for inferior survival in masked PV include age >65 years and white blood cell count >15 × 10⁹/L 3
  • Without these risk factors, masked PV patients have the same survival as overt PV, suggesting that some patients with hemoglobin lower than WHO diagnostic thresholds should still be considered as having overt PV 3

Diagnostic Approach When Microcytosis Is Present

When low MCV is detected in a patient with suspected myeloproliferative disease, repeat CBC with red cell indices to confirm persistent microcytosis, measure serum ferritin/iron/transferrin saturation to quantify iron deficiency, and perform JAK2 V617F mutation testing (present in >95% of PV cases) plus serum erythropoietin level. 1

  • The possibility of PV must not be excluded solely because hemoglobin is normal or only mildly elevated when microcytosis is observed 1
  • RBC count may more precisely reflect the total red cell mass and the hypercoagulable state of the PV patient than hematocrit in the presence of microcytosis 4
  • A combination of RBC and ESR has been proposed as a novel tool to substitute hemoglobin concentration and hematocrit in the diagnosis of PV 4

References

1
Guideline

Secondary Polycythemia Diagnosis and Management

Praxis Medical Insights: Practical Summaries of Clinical Guidelines, 2026

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This information is intended for healthcare professionals. Any medical decision-making should rely on clinical judgment and independently verified information. The content provided herein does not replace professional discretion and should be considered supplementary to established clinical guidelines. Healthcare providers should verify all information against primary literature and current practice standards before application in patient care. Dr.Oracle assumes no liability for clinical decisions based on this content.

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