What is the appropriate workup and management for a patient presenting with unexplained polycythemia (elevated red blood cell count)?

Workup for Unexplained Polycythemia

Begin with serum erythropoietin (EPO) level measurement and JAK2 mutation testing as first-line diagnostic tests, followed by bone marrow examination if the diagnosis remains unclear. 1, 2

Initial Confirmation and Triggers for Workup

Before initiating a full polycythemia workup, confirm true polycythemia and exclude apparent (relative) polycythemia:

• Verify adequate hydration status, as dehydration is the most common cause of falsely elevated hematocrit 3
• Repeat hemoglobin/hematocrit measurements to confirm sustained elevation 4
• Proceed with workup if:
  • Hemoglobin >16.5 g/dL (men) or >16.0 g/dL (women) 5
  • Sustained increase ≥2 g/dL from baseline, even within normal reference ranges 4, 3
  • Borderline-high hematocrit accompanied by PV-related features (thrombocytosis, leukocytosis, splenomegaly, aquagenic pruritus, unusual thrombosis, erythromelalgia) 1, 4

Check MCHC and iron studies early: Low MCHC (<32%) suggests iron deficiency, which can mask true erythrocytosis 3. Iron deficiency is common in polycythemia vera and can normalize hemoglobin despite underlying disease 1.

First-Line Diagnostic Tests

JAK2 Mutation Testing

• Order JAK2V617F (exon 14) mutation testing immediately 3
• If negative, proceed to JAK2 exon 12 mutation testing 3
• JAK2 mutations are present in >95% of polycythemia vera cases 4, 5
• A positive JAK2 mutation strongly supports polycythemia vera diagnosis 2, 6

Serum Erythropoietin Level

• Measure serum EPO simultaneously with JAK2 testing 1, 2
• Low EPO (below reference range): Highly suggestive of polycythemia vera with >90% specificity 1, 4
• Normal EPO: Does not exclude polycythemia vera (sensitivity only 64-70%) 1, 4
• Elevated EPO: Strongly suggests secondary polycythemia; systematically evaluate for underlying causes 1, 2

Critical pitfall: Do not assume normal EPO excludes polycythemia vera—EPO sensitivity is only 64-70%, and normal EPO with elevated hemoglobin still requires JAK2 testing 2. Additionally, in hypoxia-driven secondary polycythemia, EPO may initially be elevated but can normalize once hemoglobin stabilizes at a compensatory higher level 2, 4.

Evaluating for Secondary Polycythemia (When EPO is Elevated or Normal with Negative JAK2)

Hypoxia-Driven Causes

• Obtain detailed smoking history: Smoker's polycythemia from chronic carbon monoxide exposure is the most common secondary cause and resolves with smoking cessation 2
• Arterial blood gas or pulse oximetry to assess for hypoxemia 2
• Chest X-ray to evaluate for chronic lung disease 2
• Sleep study if sleep apnea is suspected 2
• Consider high-altitude habitation, right-to-left cardiopulmonary shunts, or hypoventilation syndromes 1

Hypoxia-Independent Causes

• Abdominal ultrasound or CT to screen for EPO-producing tumors: renal cell carcinoma, hepatocellular carcinoma, uterine leiomyomas, pheochromocytoma 2
• Review medication history for exogenous erythropoietin or testosterone use 1, 2
• Assess for post-renal transplant erythrocytosis in transplant recipients 1, 2
• Consider parathyroid carcinoma (check serum calcium, PTH, phosphorus) 2
• Evaluate for congenital causes: high oxygen-affinity hemoglobinopathy, Chuvash polycythemia 1, 2

Bone Marrow Examination

Perform bone marrow biopsy when:

• JAK2 mutation is positive (to confirm diagnosis before cytoreductive therapy) 3
• Diagnosis remains equivocal after initial testing 1, 4
• EPO is low or normal but JAK2 is negative 1

Bone marrow findings supporting polycythemia vera:

• Hypercellularity for age with trilineage growth (panmyelosis) 4, 3
• Increased megakaryocytes with cluster formation 1, 4
• Giant megakaryocytes with pleomorphic morphology 1, 4
• Decreased bone marrow iron stores 1
• Mild reticulin fibrosis (in 12% of patients) 1

Cytogenetic studies have limited diagnostic value (abnormalities in only 13-18% at diagnosis) 1, 4

Specialized Testing for Equivocal Cases (<10% of patients)

When diagnosis remains unclear after JAK2 testing, EPO measurement, and bone marrow examination:

• Decreased megakaryocyte c-mpl (thrombopoietin receptor) expression supports polycythemia vera 4
• Neutrophil PRV-1 expression assay can distinguish PV from secondary polycythemia (limited availability) 4
• Spontaneous erythroid colony assay can distinguish PV from secondary polycythemia but requires considerable expertise 4

Complete Blood Count Findings That Support Polycythemia Vera

• Thrombocytosis (present in 53% at diagnosis, often >400 × 10⁹/L) 3, 5
• Leukocytosis (present in 49% at diagnosis, typically >10 × 10⁹/L) 3, 5
• Microcytosis from iron deficiency 1, 4

Critical Diagnostic Pitfalls to Avoid

• Do not rely solely on red cell mass (RCM) measurement: Modern JAK2 testing and EPO levels have largely replaced the need for RCM studies except in truly equivocal cases 1, 2. A normal RCM does not rule out polycythemia vera 4.
• Do not miss masked polycythemia vera: A subset of patients with JAK2-positive PV present with normal hemoglobin/hematocrit due to blood dilution or coincidental blood loss anemia 7. Investigate for myeloproliferative neoplasm in patients with unusual site thrombosis (e.g., splanchnic veins) even with normal blood counts 7.
• Beware of elevated EPO in polycythemia vera: While rare, polycythemia vera can present with elevated EPO levels 8. The WHO diagnostic criteria can still be met despite elevated EPO, as decreased EPO is only a minor criterion 4, 8.
• Traditional markers lack specificity: Splenomegaly, elevated leukocyte alkaline phosphatase, and increased vitamin B12 levels lack both sensitivity and specificity for polycythemia vera 4

Algorithmic Summary

1. Confirm true polycythemia (exclude dehydration, check iron studies)
2. Order JAK2 mutation testing + serum EPO simultaneously
3. If JAK2 positive: Proceed to bone marrow biopsy to confirm diagnosis
4. If JAK2 negative with low/normal EPO: Bone marrow biopsy and consider specialized testing
5. If JAK2 negative with elevated EPO: Systematically evaluate for secondary causes (smoking history, imaging for tumors, arterial blood gas, medication review)
6. If diagnosis remains equivocal: Consider specialized testing (c-mpl expression, PRV-1 assay, spontaneous erythroid colonies)