What laboratory workup is indicated for evaluating polycythemia?

Laboratory Workup for Polycythemia

The laboratory workup for polycythemia should begin with complete blood count (CBC) and JAK2 mutation testing, followed by serum erythropoietin level and bone marrow examination to confirm polycythemia vera (PV) according to WHO 2016 diagnostic criteria. 1

Initial Laboratory Tests

Complete Blood Count with Differential

• Hemoglobin/Hematocrit thresholds: Hemoglobin >16.5 g/dL (men) or >16 g/dL (women); hematocrit >49% (men) or >48% (women) 1, 2
• Red blood cell count: RBC >6.8 × 10⁶/μL for men or >5.9 × 10⁶/μL for women suggests PV over secondary causes 3
• Platelet count: Thrombocytosis (≥450 × 10⁹/L) is present in 53% of PV cases and rarely occurs in secondary erythrocytosis 1, 2
• White blood cell count: Leukocytosis (≥11 × 10⁹/L) occurs in 49% of PV cases 1, 2
• Red cell indices: Low MCV or low ferritin favor PV diagnosis 3

JAK2 Mutation Testing

• JAK2V617F mutation is present in >95% of PV patients and is a major diagnostic criterion 1, 2
• JAK2 exon 12 mutations should be tested if JAK2V617F is negative but clinical suspicion remains high 1
• This test distinguishes PV from secondary polycythemia and is essential for diagnosis 1

Secondary Laboratory Tests

Serum Erythropoietin Level

• Subnormal serum erythropoietin is a minor diagnostic criterion for PV 1
• Low erythropoietin levels strongly suggest PV over secondary causes 3, 4
• This test helps differentiate primary from secondary erythrocytosis 4

Bone Marrow Examination

• Bone marrow biopsy showing age-adjusted hypercellularity with trilineage myeloproliferation and pleomorphic, mature megakaryocytes is a major criterion 1
• Bone marrow morphology is generally more cellular in PV versus secondary erythrocytosis and assesses disease progression 3
• While morphologic confirmation is advised, it is not mandated if other criteria are met 5

Additional Molecular Testing

• CALR and MPL mutations should be tested if JAK2 is negative, though these are more common in essential thrombocythemia 1
• Karyotype analysis: Abnormal karyotype is seen in 15-20% of PV patients 5
• Additional mutations (TET2, ASXL1, SRSF2, IDH2, RUNX1, U2AF1) may have prognostic significance 5

Inflammatory Markers (Emerging Evidence)

• Neutrophil-to-lymphocyte ratio (NLR), platelet-to-lymphocyte ratio (PLR), and systemic immune-inflammation index (SII) are significantly elevated in PV compared to secondary polycythemia 6
• These markers show >85% sensitivity and specificity for distinguishing PV from secondary causes, with SII being particularly useful 6

Tests to Exclude Secondary Causes

• Arterial oxygen saturation to exclude hypoxic secondary polycythemia 4
• Chest radiograph to identify pulmonary disease 7
• Smoking history: Current smoking strongly favors secondary erythrocytosis 3

Red Cell Mass Measurement (Historical)

• Red cell mass measurement is no longer required for diagnosis under current WHO criteria if hemoglobin/hematocrit thresholds are met with JAK2 mutation 1, 5
• This test may still be useful in borderline cases without clear diagnostic features 4, 7

Common Pitfalls to Avoid

• Do not order blood volume studies in patients with obvious pulmonary disease or active smoking—address these first 8
• Male patients with hematocrit >60% and female patients with hematocrit >55% always have absolute polycythemia and do not require red cell mass measurement 8
• Patients with splenomegaly, elevated white blood cell count, or thrombocytosis in addition to elevated hematocrit likely have PV and may not need extensive secondary workup 8
• JAK2 mutation testing is expensive and not universally available; inflammatory markers may serve as preliminary screening tools 6