What is the recommended workup for a patient with elevated hemoglobin and hematocrit indicating erythrocytosis?

Workup of Erythrocytosis

Begin with JAK2 mutation testing (V617F and exon 12) and serum erythropoietin measurement as the cornerstone first-line investigations to distinguish polycythemia vera from secondary erythrocytosis. 1, 2, 3

Initial Diagnostic Confirmation

• Repeat hemoglobin and hematocrit measurements using an automated cell counter to confirm true erythrocytosis, as a single measurement is unreliable. 1
• Use hemoglobin rather than hematocrit for diagnosis and monitoring because hemoglobin remains stable during sample storage (hematocrit can falsely increase 2–4% with prolonged storage) and is unaffected by hyperglycemia. 1
• True erythrocytosis thresholds are hemoglobin >18.5 g/dL (men) or >16.5 g/dL (women), and hematocrit >52% (men) or >48% (women). 1, 2

Mandatory Initial Laboratory Panel

• Complete blood count with red cell indices (MCV, MCH, MCHC, RDW) to assess red cell morphology and identify concurrent cytopenias or thrombocytosis/leukocytosis suggesting myeloproliferative disease. 1
• Serum ferritin and transferrin saturation to detect iron deficiency, which frequently coexists with erythrocytosis and can mask true red cell mass elevation by lowering hemoglobin. 1, 2
• Reticulocyte count to evaluate bone marrow response. 1
• C-reactive protein (CRP) to identify concurrent inflammatory conditions. 1
• Peripheral blood smear review by a qualified hematologist to identify abnormal morphology, Rouleaux formation, or left-shifted myeloid cells. 1

Molecular and Hormonal Testing

• JAK2 V617F mutation testing is mandatory as the first-line molecular assay, detecting >90–95% of polycythemia vera cases and fulfilling the second WHO major diagnostic criterion. 1, 2, 3
• If JAK2 V617F is negative, immediately test for JAK2 exon 12 mutations, which account for an additional 2–3% of polycythemia vera cases, bringing total detection to >97%. 1, 2
• Serum erythropoietin (EPO) level distinguishes primary from secondary erythrocytosis: subnormal EPO suggests polycythemia vera (WHO minor criterion), while normal or elevated EPO indicates secondary causes. 1, 2, 3

WHO 2016 Diagnostic Criteria for Polycythemia Vera

Diagnosis requires EITHER:

• All 3 major criteria, OR
• First 2 major criteria PLUS the minor criterion 1, 2

Major Criteria:

1. Elevated hemoglobin/hematocrit: Men ≥18.5 g/dL or hematocrit >49%; Women ≥16.5 g/dL or hematocrit >48% (or sustained rise ≥2 g/dL reaching ≥17 g/dL in men or ≥15 g/dL in women). 2
2. Bone marrow biopsy showing hypercellularity with trilineage (panmyelosis) growth, pleomorphic mature megakaryocytes, and reduced iron stores. 2
3. Presence of JAK2 mutation (V617F or exon 12). 2

Minor Criterion:

• Subnormal serum erythropoietin below the laboratory reference range. 2

Critical caveat: Up to 30% of confirmed polycythemia vera patients have EPO values within the normal range, so normal EPO does not exclude the diagnosis. 2

Evaluation for Secondary Erythrocytosis (If JAK2-Negative)

Hypoxic Causes:

• Sleep study if nocturnal hypoxemia suspected (obstructive sleep apnea). 1
• Pulmonary function tests and chest imaging for chronic obstructive pulmonary disease or other chronic lung disease. 1
• Arterial oxygen saturation measurement: <92% indicates hypoxia-driven secondary erythrocytosis. 2
• Smoking history and carbon monoxide exposure assessment: chronic smoking causes "smoker's polycythemia" through tissue hypoxia. 1
• Altitude of residence: physiologic adaptation increases hemoglobin by 0.2–4.5 g/dL depending on elevation (1,000–4,500 meters). 1

Non-Hypoxic Causes:

• Renal imaging (ultrasound or CT) to exclude renal cell carcinoma, hydronephrosis, or cystic disease producing erythropoietin. 1
• Medication review: testosterone therapy (prescribed or unprescribed) is a common cause in young adults and requires dose adjustment or discontinuation. 1
• Screen for erythropoietin-producing tumors: hepatocellular carcinoma, pheochromocytoma, uterine leiomyoma, meningioma. 1

Congenital Causes (If Life-Long Erythrocytosis):

• High-oxygen-affinity hemoglobin variants (decreased P50). 3, 4
• Erythropoietin receptor mutations (enhanced EPO effect). 3, 4
• VHL gene mutations (Chuvash polycythemia—altered oxygen sensing). 3, 4
• PIEZO1 mutations (recently identified cause). 4

Bone Marrow Biopsy Indications

• Mandatory when JAK2 mutation is positive to confirm polycythemia vera diagnosis and assess for trilineage myeloproliferation. 1, 2
• Required when JAK2 is negative but diagnosis remains unclear after initial workup to exclude other myeloid neoplasms and satisfy WHO minor criteria. 1, 2
• Consider if unexplained cytopenias coexist with erythrocytosis, warranting immediate hematology referral. 1

Immediate Hematology Referral Criteria

• JAK2 mutation positive. 1
• Hemoglobin >20 g/dL with symptoms of hyperviscosity (headache, blurred vision, confusion, bleeding). 1
• Unexplained splenomegaly. 1
• Unexplained cytopenias in the setting of elevated red blood cell count. 1
• Diagnosis remains unclear after initial workup. 1

Common Diagnostic Pitfalls to Avoid

• Do not diagnose polycythemia vera solely on low EPO: low EPO is only a WHO minor criterion and cannot establish diagnosis without major criteria. 2
• Do not overlook iron deficiency: it can mask true erythrocytosis by lowering hemoglobin while red cell mass remains elevated; formal diagnosis requires demonstration of WHO criteria after iron replacement. 1, 2
• Do not use standard PV thresholds at high altitude without adjustment: physiologic adaptation can increase hemoglobin by 0.2–4.5 g/dL depending on elevation. 1
• Do not rely on MCV to screen for iron deficiency in erythrocytosis: serum ferritin, transferrin saturation, and iron levels are required for accurate diagnosis. 1
• Do not assume JAK2 positivity alone confirms PV: the mutation also occurs in ~50% of essential thrombocythemia and primary myelofibrosis; full WHO criteria must be satisfied. 2

Management Considerations Based on Diagnosis

Confirmed Polycythemia Vera:

• Maintain hematocrit strictly <45% through therapeutic phlebotomy to reduce thrombotic risk (CYTO-PV trial: 2.7% vs 9.8% event rate, P=0.007); target ~42% for women and African Americans. 1
• Initiate low-dose aspirin (81–100 mg daily) as second cornerstone of therapy for thrombosis prevention. 1

Secondary Erythrocytosis:

• Treat the underlying condition: smoking cessation for smoker's polycythemia, CPAP for obstructive sleep apnea, management of COPD, dose adjustment or discontinuation of testosterone. 1
• Therapeutic phlebotomy is indicated ONLY if hemoglobin >20 g/dL AND hematocrit >65% with documented hyperviscosity symptoms after excluding dehydration. 1
• Avoid routine phlebotomy: it causes iron depletion, decreased oxygen-carrying capacity, and paradoxically increases stroke risk. 1, 5
• If iron deficiency confirmed, cautious oral iron supplementation with close hemoglobin monitoring is necessary, as rapid increases in red cell mass can occur. 1