What is the typical red blood cell count in untreated masked polycythemia vera for men and women?

Red Blood Cell Count in Untreated Masked Polycythemia Vera

In untreated masked polycythemia vera, hemoglobin levels remain below the WHO diagnostic thresholds—specifically <18.5 g/dL in men and <16.5 g/dL in women—despite an elevated red cell mass, because concurrent plasma volume expansion dilutes the blood and masks the true erythrocytosis. 1, 2

Defining Masked Polycythemia Vera

Masked PV (also termed "inapparent polycythemia") occurs when increased red cell mass coexists with expanded plasma volume, resulting in normal or borderline hemoglobin/hematocrit values that fail to meet standard diagnostic criteria. 3, 2

Specific Hemoglobin Ranges in Masked PV

• Men with masked PV typically present with hemoglobin between 16.0–18.4 g/dL (below the WHO threshold of 18.5 g/dL but above the lower normal limit). 4
• Women with masked PV typically present with hemoglobin between 15.0–16.4 g/dL (below the WHO threshold of 16.5 g/dL but above the lower normal limit). 4
• These patients have objectively elevated red cell mass when measured directly, but the concurrent plasma volume increase of approximately +36% (compared to only +9.5% in overt PV) dilutes the blood sufficiently to normalize hemoglobin and hematocrit. 2

Prevalence and Clinical Context

Masked PV comprises 11–45% of all PV cases, depending on whether BCSH (hematocrit-based) or WHO (hemoglobin-based) criteria are applied. 5, 4

• Using WHO hemoglobin thresholds, approximately 35–45% of JAK2-mutated PV patients present with masked disease. 5, 4
• Using BCSH hematocrit thresholds (>52% in men, >48% in women), only 11–15% are classified as masked PV, because hematocrit is less affected by plasma volume expansion than absolute hemoglobin concentration. 5, 4

Distinguishing Laboratory Features

Patients with masked PV demonstrate several characteristic laboratory patterns that differ from overt PV:

• Higher platelet counts at diagnosis are more common in masked PV compared to overt disease. 5, 4
• Lower JAK2V617F allele burden is observed in masked PV patients at presentation. 5
• Increased bone marrow reticulin fibrosis occurs more frequently in masked PV. 4
• Male predominance is more pronounced in masked PV cohorts. 4

Critical Diagnostic Pitfall

The most dangerous error is dismissing the possibility of PV solely because hemoglobin and hematocrit are normal or only mildly elevated. 2, 6

• In the context of unexplained thrombosis (especially portal vein thrombosis), isolated thrombocytosis, leukocytosis, or splenomegaly, JAK2 mutation testing and red cell mass measurement must be performed even when hemoglobin is normal. 2, 6
• Iron deficiency can further mask PV by suppressing hemoglobin production; microcytosis in the setting of thrombocytosis or splenomegaly should trigger immediate PV workup regardless of hemoglobin level. 7
• A case report documented JAK2-positive PV presenting with hemoglobin of 14.1 g/dL—well within the normal range—discovered only after investigation of portal vein thrombosis. 6

Prognostic Implications

Masked PV carries a worse overall survival compared to overt PV when diagnosed by WHO criteria (P = 0.011) and by BCSH criteria (P = 0.0019). 4

• The survival disadvantage is driven primarily by patients with age >65 years and white blood cell count >15 × 10⁹/L. 4
• Masked PV patients without these risk factors have survival equivalent to overt PV, suggesting that some cases with hemoglobin below WHO thresholds should still be managed as overt disease. 4
• However, one study found no difference in thrombosis rate, bleeding, or transformation risk between masked and overt PV, with masked patients actually achieving higher rates of complete hematological response (79% vs. 58%, P = 0.001). 5

Practical Diagnostic Algorithm

When masked PV is suspected (thrombosis, splenomegaly, thrombocytosis, or leukocytosis with borderline-high or normal hemoglobin):

1. Order JAK2V617F mutation testing immediately—present in >95% of all PV cases, including masked variants. 1, 7
2. Measure serum erythropoietin—suppressed levels (<2.9 mU/mL) have >90% specificity for PV even when hemoglobin is normal. 7
3. Perform bone marrow biopsy to document hypercellularity with trilineage growth and pleomorphic megakaryocytes, which are diagnostic hallmarks. 1, 7
4. Consider red cell mass measurement if JAK2 is positive but hemoglobin remains equivocal, to objectively demonstrate elevated RCM despite normal hemoglobin/hematocrit. 2, 8

Red cell mass is best expressed as percentage of mean expected value (±2 SD), not mL/kg body weight, because the latter lacks precision in obese patients due to hypovascular adipose tissue. 8