What values for Red Blood Cell (RBC) count, Hemoglobin (Hgb), and Hematocrit (Hct) indicate Polycythemia Vera?

Diagnostic Hemoglobin, Hematocrit, and RBC Values for Polycythemia Vera

For polycythemia vera diagnosis, hemoglobin ≥18.5 g/dL in men or ≥16.5 g/dL in women, or hematocrit >52% in men or >48% in women, should trigger diagnostic evaluation including JAK2 mutation testing. 1

Primary Diagnostic Thresholds

Hemoglobin Criteria

• Men: Hemoglobin ≥18.5 g/dL 2, 1
• Women: Hemoglobin ≥16.5 g/dL 2, 1
• Alternative: Hemoglobin >99th percentile of method-specific reference range adjusted for age, sex, and altitude 2

Hematocrit Criteria

• Men: Hematocrit >52% 1, 3
• Women: Hematocrit >48% 3
• When hematocrit ≥60%, absolute erythrocytosis is virtually always present 4

Red Cell Mass

• Elevated red cell mass >25% above mean normal predicted value 2, 1

Important Diagnostic Nuances

Lower Threshold Considerations

A sustained increase in hemoglobin of ≥2 g/dL from an individual's baseline (not attributable to iron deficiency correction) may indicate PV even without meeting absolute thresholds. 2, 1 This captures early or "masked" polycythemia vera cases that would otherwise be missed 2.

Masked Polycythemia Vera

• Approximately 35% of PV patients present with hemoglobin below the traditional WHO thresholds (16.0-18.4 g/dL in men, 15.0-16.4 g/dL in women) but still have true disease 3
• These patients more frequently present with thrombocytosis, increased bone marrow reticulin fibrosis, and worse overall survival 3
• When hematocrit is 48-52% in men or 48% in women with JAK2 mutation and other PV features, proceed with full diagnostic workup 3, 5

Critical Diagnostic Algorithm

Step 1: Initial Screening

Suspect PV when:

• Hemoglobin/hematocrit exceeds 95th percentile for sex and race 2
• Documented sustained increase in hemoglobin above individual baseline 2
• Borderline-high hematocrit with PV-related features (thrombocytosis, leukocytosis, microcytosis from iron deficiency, splenomegaly, aquagenic pruritus, unusual thrombosis, erythromelalgia) 2

Step 2: Confirmatory Testing

• JAK2 mutation testing (present in >95% of PV cases) 2
• Serum erythropoietin level (typically low in PV, specificity >90%) 2
• Bone marrow biopsy showing hypercellularity with trilineage growth (panmyelosis) 2, 1

Step 3: Red Cell Mass Measurement

Red cell mass measurement should be performed when: 5

• Hematocrit is 48-52% in men or 48% in women with clinical suspicion 5
• Portal vein thrombosis, isolated leukocytosis, thrombocytosis, or splenomegaly present with borderline hematocrit 6

Red cell mass measurement is NOT needed when: 2

• Hematocrit >60% without obvious hemoconcentration (RCM almost always elevated) 2
• Hemoglobin clearly exceeds diagnostic thresholds 4

Common Pitfalls and Caveats

Iron Deficiency Masking

Iron deficiency can lower hemoglobin/hematocrit and mask true polycythemia. 2, 1 In routine clinical practice, you can make a working diagnosis of PV despite not meeting WHO hemoglobin criteria if iron deficiency is present, but for research purposes, demonstrate criteria fulfillment after iron replacement 2.

Hemoglobin vs. Hematocrit Discordance

• Hemoglobin >18.5 g/dL in men has high specificity (predicts absolute erythrocytosis in only 35% of male PV patients due to low sensitivity) 4
• Hematocrit shows better diagnostic accuracy than hemoglobin 5
• Hematocrit ≥60% always indicates absolute erythrocytosis in both sexes 4

Plasma Volume Expansion

Increased plasma volume (average +36% in masked PV vs. +9.5% in overt PV) can mask elevated red cell mass, keeping hemoglobin and hematocrit in normal range despite true polycythemia 6. This defines "inapparent polycythemia vera" affecting approximately 17% of PV patients 6.