In a male patient with hemoglobin below 17 g/dL, can polycythemia vera still be considered and should a JAK2 exon 12 mutation test be performed?

Can JAK2 Exon 12 Testing Be Considered in Men with Hemoglobin Below 17 g/dL?

Yes, JAK2 exon 12 mutation testing should be performed in men with hemoglobin below 17 g/dL when polycythemia vera is suspected, because the WHO diagnostic criteria do not require meeting the hemoglobin threshold if other major and minor criteria are satisfied, and exon 12 mutations account for 2–3% of PV cases that would otherwise be missed. 1

Understanding the WHO Diagnostic Framework

The WHO diagnostic algorithm for polycythemia vera requires either:

• Both major criteria (elevated Hb/Hct AND JAK2 mutation) plus ≥1 minor criterion, OR
• The first major criterion (elevated Hb/Hct) plus ≥2 minor criteria 2, 1

Critical point: A hemoglobin of 16 g/dL in men does not satisfy the first major criterion threshold of ≥18.5 g/dL (or ≥17 g/dL with a sustained ≥2 g/dL rise from baseline). 2, 1 However, this does not automatically exclude PV—it simply means the diagnosis must rely on the alternative pathway using minor criteria.

When to Order JAK2 Exon 12 Testing

Primary Indication

• JAK2 V617F-negative patients with clinical features suggestive of PV should undergo exon 12 testing, as it detects an additional 2–3% of PV cases 1, 3
• Exon 12 mutations are recognized as fulfilling the second major criterion in WHO criteria 2, 1

Clinical Scenarios Supporting Testing Even Below Threshold Hemoglobin

Patients with JAK2 exon 12 mutations frequently present with:

• Isolated erythrocytosis (elevated RBC count with normal WBC and platelets) in approximately two-thirds of cases 4, 5
• Higher hemoglobin levels than V617F-positive PV but often with lower platelet and leukocyte counts 5
• Increased RBC count with low MCV (56–60 fL), which can mask the true degree of erythrocytosis when hemoglobin is measured 4

Important caveat: Up to half of exon 12-positive patients may have hemoglobin/hematocrit below the diagnostic threshold for PV at presentation, particularly when iron deficiency coexists 4. This creates a diagnostic pitfall where relying solely on hemoglobin cutoffs will miss these cases.

Diagnostic Algorithm for Hemoglobin <17 g/dL in Men

Step 1: Assess for Supporting Features

Order the following tests to evaluate whether PV remains in the differential despite subthreshold hemoglobin 1, 6:

• Complete blood count with indices: Look for elevated RBC count (>5.5 × 10¹²/L) with low-normal MCV, which suggests masked erythrocytosis 4
• Serum erythropoietin level: Low or low-normal EPO supports primary erythrocytosis (minor criterion) 2, 1
• Iron studies (ferritin, transferrin saturation): Iron deficiency can artificially lower hemoglobin while RBC count remains elevated 2, 4
• Peripheral blood smear: Assess red cell morphology 6

Step 2: JAK2 Mutation Testing Strategy

• First-line: JAK2 V617F (exon 14) testing—detects >90–95% of PV cases 1, 3
• If V617F is negative AND clinical suspicion persists (low EPO, elevated RBC count, thrombocytosis, leukocytosis, splenomegaly, aquagenic pruritus): Order JAK2 exon 12 mutation analysis 2, 1, 3

Step 3: Bone Marrow Biopsy Consideration

If JAK2 exon 12 is positive but hemoglobin remains <18.5 g/dL:

• Bone marrow biopsy showing hypercellularity with trilineage growth (panmyelosis) serves as a minor criterion 2, 1
• Combined with positive exon 12 mutation (second major criterion) and low EPO (another minor criterion), this establishes PV diagnosis even without meeting the hemoglobin threshold 1

Note: Approximately 50% of exon 12-positive patients may lack bone marrow hypercellularity at presentation, which can complicate diagnosis 4. In such cases, endogenous erythroid colony formation (third minor criterion) may be required 2, 1.

Common Diagnostic Pitfalls

Iron Deficiency Masking PV

• Iron deficiency causes microcytic erythrocytosis with elevated RBC count but paradoxically lower hemoglobin 2, 4
• The WHO criteria explicitly state that formal PV diagnosis may require demonstrating criteria after iron replacement in research/clinical trial settings 2
• In routine practice, a working diagnosis of PV can be made despite iron deficiency if other criteria are met 2

Relying Solely on Low EPO

• Low EPO is only a minor criterion and cannot diagnose PV alone 1
• A male patient with Hb 16 g/dL and low EPO requires JAK2 testing; if negative, systematic evaluation for secondary erythrocytosis is mandatory 1

Missing Exon 12 Mutations

• Failing to order exon 12 testing after negative V617F results will miss 2–3% of PV cases 1, 3
• These patients often present with isolated erythrocytosis that mimics idiopathic erythrocytosis 2, 5

Evidence Hierarchy and Strength

The strongest guideline evidence comes from:

• 2008 WHO revision 2 and 2007 WHO proposals 2, which established exon 12 mutations as a major diagnostic criterion
• Praxis Medical Insights 1, which synthesizes current WHO criteria and emphasizes that exon 12 testing is mandatory when V617F is negative

Research evidence 4, 5 demonstrates that exon 12-positive patients have distinct clinical features (isolated erythrocytosis, higher hemoglobin, lower platelets) but similar thrombotic risk and outcomes to V617F-positive PV, justifying aggressive case-finding.

Bottom Line

For a male patient with hemoglobin below 17 g/dL:

1. Low EPO alone is insufficient for diagnosis 1
2. Order JAK2 V617F first 1, 3
3. If V617F is negative and clinical suspicion remains (low EPO, elevated RBC count, thrombocytosis), proceed with JAK2 exon 12 testing 2, 1
4. If exon 12 is positive, obtain bone marrow biopsy to document trilineage proliferation (minor criterion) 1, 4
5. If both JAK2 tests are negative, systematically exclude secondary causes (sleep apnea, COPD, smoking, renal tumors, testosterone use) 1, 6

The key is recognizing that PV can be diagnosed without meeting the hemoglobin threshold when the second major criterion (JAK2 mutation) is present along with sufficient minor criteria 2, 1.