What are the next steps and tests to order for a patient suspected of having polycythemia vera, beyond complete blood count (CBC) and comprehensive metabolic panel (CMP)?

Polycythemia Vera: Diagnostic Workup Beyond CBC/CMP

Order serum erythropoietin (EPO) level immediately, followed by JAK2 mutation testing—these two tests will confirm or exclude PV in the vast majority of cases without requiring bone marrow biopsy. 1, 2

Understanding Polycythemia Vera

PV is a myeloproliferative neoplasm affecting approximately 65,000 people in the US, with median survival ranging from 14-27 years. 2 The disease is characterized by:

• Erythrocytosis (hemoglobin >16.5 g/dL in men or >16.0 g/dL in women) 2
• Thrombocytosis (present in 53% of patients) and leukocytosis (49% of patients) 2
• Thrombotic risk: 16% have arterial thrombosis and 7% have venous thrombosis at or before diagnosis, including unusual sites like splanchnic veins 2
• Progression risk: 12.7% develop myelofibrosis and 6.8% develop acute myeloid leukemia 2

Essential Next Tests to Order

First-Line Testing (Order These Now)

1. Serum Erythropoietin (EPO) Level

• **Low EPO (<2 U/L)** strongly favors PV with >90% specificity 3, 1
• Normal EPO (2-12 U/L) does NOT exclude PV—sensitivity of low EPO is only <70% 3, 1
• High EPO (>12 U/L) suggests secondary polycythemia and requires evaluation for hypoxia-driven conditions, pathologic EPO production, or congenital causes 1

2. JAK2 Mutation Testing

• More than 95% of PV patients harbor a JAK2 mutation (JAK2V617F or exon 12 mutations) 2, 4
• Presence of JAK2 mutation highly suggests PV diagnosis 4
• Absence of JAK2 mutation combined with normal or increased EPO essentially excludes PV 4

Second-Line Testing (If Diagnosis Remains Equivocal)

3. Bone Marrow Examination with Cytogenetic Studies

• Order when EPO and JAK2 results are inconclusive 3, 1
• Characteristic findings include:
  • Hypercellularity 1
  • Increased megakaryocytes with cluster formation and giant pleomorphic forms 3, 1
  • Mild reticulin fibrosis (12% of patients) 3
  • Decreased bone marrow iron stores 3, 1
• Cytogenetic abnormalities appear in only 13-18% at diagnosis, limiting diagnostic value 3, 1

Additional Supportive Tests

4. Iron Studies (Serum Ferritin, Iron, Transferrin Saturation)

• Low MCHC (<32%) suggests iron deficiency that can mask true erythrocytosis 5
• Iron deficiency is common in PV due to chronic phlebotomy or occult bleeding 5

5. Vitamin B12 and Leukocyte Alkaline Phosphatase

• These may be elevated but lack sensitivity and specificity for diagnosis 1
• Not recommended as primary diagnostic tools 1

Clinical Features That Support PV Diagnosis

Look specifically for these symptoms during your evaluation:

• Aquagenic pruritus (33% of patients)—itching after warm water exposure 1, 2
• Erythromelalgia (5.3%)—burning pain in extremities with erythema 1, 2
• Transient visual changes (14%) 2
• Splenomegaly (36%) with abdominal discomfort 2
• Unusual thrombosis sites (e.g., Budd-Chiari syndrome, splanchnic veins) 1, 2

Critical Diagnostic Pitfalls to Avoid

• Do NOT rely on red cell mass (RCM) measurement—normal RCM does not rule out PV and adds unnecessary cost 1
• Do NOT dismiss the diagnosis if EPO is normal—up to 30% of PV patients have normal EPO levels 3, 1
• Exclude dehydration before intensifying workup—this is the most common cause of falsely elevated hematocrit 5
• Check for iron deficiency—low MCHC can mask true erythrocytosis while falsely normalizing hematocrit 5
• Do NOT wait for splenomegaly, leukocytosis, or thrombocytosis—these traditional markers lack sufficient sensitivity and specificity 1

Practical Diagnostic Algorithm

1. Confirm erythrocytosis with repeat CBC if not already done (hemoglobin >16.5 g/dL men, >16.0 g/dL women) 2
2. Order EPO level and JAK2 mutation simultaneously 1, 4
3. If JAK2 positive + low/normal EPO → PV diagnosis confirmed 4
4. If JAK2 negative + high EPO → Evaluate for secondary polycythemia (smoking, sleep apnea, chronic lung disease, renal tumors) 1
5. If results equivocal → Proceed to bone marrow biopsy 3, 1
6. Consider specialized testing only in rare equivocal cases (c-mpl immunohistochemistry, PRV-1 expression, endogenous erythroid colony assay)—these should constitute <10% of cases 3

Risk Stratification After Diagnosis

Once PV is confirmed, stratify thrombotic risk:

• High risk: Age >60 years OR prior thrombosis history 6
• Low risk: Age ≤60 years AND no thrombosis history 6
• Additional risk factors: JAK2V617F mutation, cardiovascular risk factors (hypertension, smoking), and leukocytosis increase thrombotic risk 4, 6