What is the workup and differential diagnosis for a complete blood count (CBC) showing erythrocytosis, specifically for Polycythemia Vera (PV)?

Workup and Differential Diagnosis for Erythrocytosis

When evaluating a CBC showing erythrocytosis, immediately measure serum erythropoietin (EPO) level and test for JAK2 mutations to distinguish polycythemia vera from secondary causes, followed by bone marrow examination if PV is suspected. 1, 2

Initial Diagnostic Thresholds

Consider polycythemia vera when:

• Hemoglobin >16.5 g/dL (hematocrit >49%) in men or >16.0 g/dL (hematocrit >48%) in women 1, 3, 4
• Sustained hemoglobin increase ≥2 g/dL above patient's baseline, even within normal reference ranges 1, 5

Key clinical features suggesting PV:

• Aquagenic pruritus (33% of patients) 1, 3
• Erythromelalgia (5.3%) 1, 3
• Unusual thrombosis sites (e.g., Budd-Chiari syndrome, splanchnic veins) 1, 3
• Splenomegaly (36%) 3
• Transient visual changes (14%) 3

Diagnostic Algorithm

Step 1: Serum Erythropoietin Level

• Low EPO: >90% specific for PV but only ~70% sensitive 1, 5, 2
• Normal EPO: Does NOT exclude PV 1, 5
• High EPO: Suggests secondary polycythemia 1, 2

Step 2: JAK2 Mutation Testing

• JAK2V617F or exon 12 mutations present in >95% of PV patients 3, 4, 2
• Positive JAK2 mutation distinguishes PV from secondary causes 2
• Negative JAK2 does not completely exclude PV 6

Step 3: Bone Marrow Examination with Cytogenetics

Perform when PV is suspected based on low/normal EPO and/or positive JAK2 1

PV-supportive findings:

• Hypercellularity with panmyelosis 6, 1
• Increased megakaryocytes with cluster formation and pleomorphic giant forms 1
• Decreased iron stores 5
• Abnormal karyotype in 15-20% (most common: +9, -Y, +8, 20q-) 4

Step 4: Additional Testing in Equivocal Cases

• Peripheral blood PRV-1 expression assay 1
• Endogenous erythroid colony formation 6, 1
• Decreased megakaryocyte c-mpl (TPO receptor) expression 1

Revised WHO Diagnostic Criteria for PV

Diagnosis requires BOTH major criteria plus ≥1 minor criterion, OR first major criterion plus ≥2 minor criteria 6

Major Criteria:

1. Elevated hemoglobin (>18.5 g/dL men, >16.5 g/dL women) or hematocrit, or increased red cell mass 6
2. Presence of JAK2V617F or functionally similar JAK2 mutation 6

Minor Criteria:

1. Bone marrow biopsy showing panmyelosis with erythroid and megakaryocytic proliferation 6
2. Serum EPO level below normal reference range 6
3. Endogenous erythroid colony formation 6

Differential Diagnosis: Secondary Polycythemia

Hypoxia-driven causes:

• Chronic lung disease, high altitude, tobacco smoking, sleep apnea 1, 2

Pathologic EPO production:

• Renal cell carcinoma, hepatocellular carcinoma 1

Congenital causes (life-long erythrocytosis):

• High oxygen-affinity hemoglobinopathy 1, 2
• EPO receptor mutations 2
• von Hippel-Lindau gene mutations 2

Drug-associated:

• EPO doping, androgen preparations 1

Critical Pitfalls to Avoid

Do NOT rely on these traditional markers alone—they lack sensitivity and specificity:

• Splenomegaly, leukocytosis, thrombocytosis 1, 5
• Elevated leukocyte alkaline phosphatase 1, 5
• Increased vitamin B12 levels 1, 5

Red cell mass (RCM) measurement is NOT recommended:

• Normal RCM does not exclude PV 1, 5
• Adds unnecessary cost without diagnostic benefit 5

Watch for masked erythrocytosis from iron deficiency:

• Low MCHC (<32%) suggests iron deficiency masking true polycythemia 1, 7, 5
• Check serum ferritin and transferrin saturation 7
• Do NOT give iron supplementation before confirming diagnosis—this worsens hyperviscosity and thrombotic risk 7

Exclude dehydration:

• Most common cause of falsely elevated hematocrit 5
• Ensure adequate hydration and repeat CBC 7

Special Consideration: PV with Iron Deficiency

The combination of elevated RBC count with low MCV, MCH, and MCHC is highly suggestive of PV with iron deficiency 7

• Microcytosis from iron deficiency can mask true erythrocytosis 7
• Ferritin <30 μg/L indicates iron deficiency (absent inflammation) 7
• Initiate phlebotomy FIRST to maintain hematocrit <45% before considering iron replacement 7