Laboratory Testing for Polycythemia Vera
The essential lab tests for diagnosing polycythemia vera are: (1) complete blood count with differential to confirm elevated hemoglobin/hematocrit, (2) JAK2 mutation testing (both V617F and exon 12), and (3) serum erythropoietin level to distinguish primary from secondary causes. 1, 2
Initial Laboratory Panel
Order these tests immediately when PV is suspected:
Complete blood count (CBC) with red cell indices and manual differential to document elevated hemoglobin (>18.5 g/dL in men, >16.5 g/dL in women) or hematocrit (>55% in men, >49.5% in women), and assess for accompanying thrombocytosis or leukocytosis 1, 2
Reticulocyte count to evaluate bone marrow response 1
Serum ferritin and transferrin saturation to identify coexisting iron deficiency, which commonly masks the full extent of erythrocytosis in PV 1, 2
Peripheral blood smear review by a qualified hematologist to identify abnormal morphology and assess for left shift 2
Diagnostic Molecular and Biochemical Testing
JAK2 mutation testing is the cornerstone diagnostic test:
JAK2 V617F mutation (exon 14) is present in up to 97% of PV cases and should be ordered as the first-line molecular test 3, 1, 2, 4
JAK2 exon 12 mutations should be tested if JAK2 V617F is negative, as these account for most remaining PV cases 1
Serum erythropoietin (EPO) level is critical for distinguishing PV from secondary polycythemia: low or inappropriately normal EPO (below reference range) is highly specific (>90%) for PV, though sensitivity is only 64-70% 3, 5
WHO Diagnostic Criteria Application
The World Health Organization criteria require either:
- Both major criteria PLUS one minor criterion, OR
- First major criterion PLUS two minor criteria 3, 1, 2
Major criteria:
- Elevated hemoglobin (>16.5 g/dL women, >18.5 g/dL men) OR hematocrit (>49% women, >52% men) OR documented sustained increase >2 g/dL from baseline 3, 1
- Presence of JAK2 mutation (V617F or exon 12) 3, 1
Minor criteria:
- Bone marrow biopsy showing hypercellularity with trilineage growth (panmyelosis with prominent erythroid and megakaryocytic proliferation) 3, 2
- Subnormal serum erythropoietin level 3, 2
- Endogenous erythroid colony formation in vitro 3, 2
Bone Marrow Examination
Bone marrow biopsy is required if JAK2 mutation is positive to confirm PV diagnosis and assess for trilineage myeloproliferation, and is essential for establishing baseline histomorphology 2, 6
Bone marrow biopsy should also be performed if diagnosis remains unclear after initial workup to exclude other myeloid neoplasms 2
Additional Laboratory Tests to Exclude Secondary Causes
When EPO is elevated or normal, systematically evaluate for secondary polycythemia:
Arterial blood gas or pulse oximetry to assess for hypoxemia from chronic lung disease, right-to-left shunts, or sleep apnea 5
Carboxyhemoglobin level if smoking history is present, as smoker's polycythemia from chronic carbon monoxide exposure is the most common secondary cause 5, 2
Renal function tests (creatinine, BUN) to screen for kidney disease causing inappropriate EPO production 5
Liver function tests (AST, ALT, bilirubin) to screen for hepatocellular carcinoma 5
Abdominal imaging (ultrasound or CT) to screen for EPO-producing tumors: renal cell carcinoma, hepatocellular carcinoma, uterine leiomyomas, pheochromocytoma 5, 2
Critical Diagnostic Pitfalls to Avoid
Do not assume normal EPO excludes PV – EPO sensitivity for PV is only 64-70%; normal EPO with elevated hemoglobin still requires JAK2 testing 5
Do not overlook iron deficiency coexisting with erythrocytosis – iron-deficient red blood cells have reduced oxygen-carrying capacity and deformability, and high RDW with normal or low MCV suggests this combination 1, 2
Do not miss masked PV – a subset of patients have normal hemoglobin/hematocrit due to blood dilution or coincidental blood loss but still have underlying PV with JAK2 mutations; these patients often present with thrombosis as their first manifestation 7
Do not rely on a single measurement – repeat hemoglobin and hematocrit measurements to confirm persistent elevation, as single measurements are unreliable 1, 2
When to Refer to Hematology
Immediate hematology referral is indicated for: