What laboratory tests are needed to diagnose polycythemia?

Laboratory Tests for Diagnosing Polycythemia

Begin with a complete blood count (CBC), serum erythropoietin (EPO) level, and JAK2 mutation testing—these three tests will diagnose polycythemia vera in over 95% of cases. 1, 2

Initial Essential Laboratory Tests

Complete Blood Count (CBC)

• Hemoglobin/Hematocrit thresholds: Hemoglobin >16.5 g/dL in men or >16.0 g/dL in women, or hematocrit above the 95th percentile for sex and race 1, 2
• Additional CBC findings suggestive of PV: Leukocytosis (present in 49% of cases), thrombocytosis (present in 53% of cases), and microcytic hypochromic red blood cells indicating iron deficiency 3, 2
• A sustained increase of ≥2 g/dL hemoglobin from baseline warrants investigation even if within normal reference ranges 4, 1

Serum Erythropoietin (EPO) Level

• Low EPO is highly specific: >90% specificity for polycythemia vera, making it the most important discriminating test 4, 1
• Normal EPO does not exclude PV: Sensitivity is only ~70%, so normal levels are still consistent with PV 5, 4
• High EPO suggests secondary polycythemia: Directs evaluation toward hypoxia-driven conditions, pathologic EPO production, or other secondary causes 5, 1

JAK2 Mutation Testing

• Present in >95% of PV patients: JAK2V617F mutation (exon 14) or exon 12 mutations confirm the diagnosis 2, 6
• This test distinguishes PV from secondary causes of erythrocytosis such as smoking or sleep apnea 2

Iron Studies (Critical Complementary Tests)

• Serum ferritin, iron studies, and transferrin saturation: Essential because iron deficiency commonly masks true erythrocytosis in PV 4, 7
• Low MCHC (<32%): Suggests iron deficiency that can falsely normalize hematocrit while true erythrocytosis persists 4, 7
• Decreased bone marrow iron stores: Characteristic finding in PV 5, 1

When to Proceed to Bone Marrow Examination

Bone marrow biopsy with cytogenetic studies is the next step when initial tests are equivocal or when EPO is low/normal but JAK2 is negative 5, 1:

• Characteristic histology: Hypercellularity, increased megakaryocytes with cluster formation, giant megakaryocytes with pleomorphic morphology, and mild reticulin fibrosis 5, 1
• Cytogenetic studies: Show abnormalities in only 13-18% of patients at diagnosis, thus have limited diagnostic value 5, 1

Specialized Tests (Only for Equivocal Cases <10%)

These are rarely needed in routine practice 5:

• Decreased megakaryocyte expression of TPO receptor (c-mpl): Supports PV diagnosis when combined with bone marrow morphology 5, 1
• PRV-1 expression assay: Peripheral blood neutrophil assay distinguishes PV (high expression) from secondary polycythemia 5, 1
• Spontaneous erythroid colony assays: Limited availability and require considerable expertise; negative results do not exclude PV 5, 1

Critical Pitfalls to Avoid

• Do not order red cell mass (RCM) measurement routinely: Normal RCM does not rule out PV and adds unnecessary cost 4, 1
• Exclude dehydration first: The most common cause of falsely elevated hematocrit before intensifying workup 4
• Do not rely on traditional markers: Splenomegaly, elevated leukocyte alkaline phosphatase, and increased vitamin B12 levels lack sensitivity and specificity 4, 1
• Recognize iron deficiency masking: Low MCHC with elevated RBC count creates a distinctive pattern specific to PV with iron deficiency 7, 3

Practical Diagnostic Algorithm

1. CBC showing erythrocytosis → Confirm true polycythemia (exclude dehydration) 1
2. Serum EPO level → Low = probable PV; Normal = possible PV; High = evaluate secondary causes 5, 1
3. JAK2 mutation testing → Positive confirms PV in >95% of cases 2
4. Iron studies → Assess for iron deficiency that may mask erythrocytosis 4, 7
5. Bone marrow biopsy → Only if JAK2 negative or equivocal presentation 5, 1