Which laboratory tests should be obtained to evaluate the causes of an elevated red blood cell count, hemoglobin, and hematocrit?

Laboratory Evaluation for Elevated RBC, Hemoglobin, and Hematocrit

Order a complete blood count with differential, reticulocyte count, serum ferritin, transferrin saturation, and JAK2 mutation testing to distinguish between primary polycythemia vera and secondary causes of erythrocytosis. 1, 2

Initial Laboratory Workup

The diagnostic approach requires systematic testing to differentiate true polycythemia from relative erythrocytosis and to identify the underlying cause:

Essential First-Line Tests

• Complete blood count with red cell indices provides hemoglobin, hematocrit, RBC count, MCV, MCH, MCHC, and RDW—the foundation for distinguishing polycythemia patterns 3, 1
• Reticulocyte count assesses bone marrow erythropoietic activity and helps differentiate primary from secondary causes 2
• Peripheral blood smear identifies red cell morphology abnormalities, Rouleaux formation, and other cellular changes 3, 2
• Serum ferritin and transferrin saturation detect coexisting iron deficiency, which frequently accompanies erythrocytosis and requires opposite management (iron supplementation rather than phlebotomy) 1, 2

Confirming True vs. Relative Polycythemia

• Repeat CBC after 48-72 hours of adequate hydration to exclude relative polycythemia from dehydration or plasma volume depletion 1, 4
• Hemoglobin >18.5 g/dL (men) or >16.5 g/dL (women) and hematocrit >52% (men) or >48% (women) define true erythrocytosis requiring further evaluation 3, 2

Testing for Primary Polycythemia Vera

JAK2 Mutation Analysis

• JAK2 V617F mutation testing (exon 14) and JAK2 exon 12 mutation testing detect mutations in up to 97% of polycythemia vera cases, making this the cornerstone diagnostic test 3, 1, 2
• JAK2 testing should be ordered immediately when hemoglobin/hematocrit exceed diagnostic thresholds after excluding dehydration 2

WHO Diagnostic Criteria Application

The 2016 WHO criteria require specific laboratory confirmation 3:

Major Criteria:

• Hemoglobin >16.5 g/dL (women) or >18.5 g/dL (men), OR hematocrit >48% (women) or >49% (men) 3
• Presence of JAK2 mutation 3
• Bone marrow biopsy showing hypercellularity with trilineage myeloproliferation 3

Minor Criteria:

• Serum erythropoietin level (subnormal values support PV diagnosis) 3, 2

• Diagnosis requires either all three major criteria OR the first two major criteria plus one minor criterion 3, 2

Evaluation for Secondary Causes

If JAK2 mutation is negative, systematically evaluate secondary causes 2:

Hypoxia-Driven Causes

• Arterial oxygen saturation and arterial blood gas to detect hypoxemia (SaO2 <92% suggests secondary polycythemia) 1, 2
• Sleep study if obstructive sleep apnea is suspected based on clinical history 1, 2
• Pulmonary function tests and chest imaging to evaluate for chronic obstructive pulmonary disease or other pulmonary disease 1, 2
• Echocardiography to assess for cyanotic congenital heart disease with right-to-left shunting 1, 2

Non-Hypoxic Secondary Causes

• Renal imaging (ultrasound or CT) to exclude renal cell carcinoma, hydronephrosis, or polycystic kidney disease that produce erythropoietin 2
• Medication review for testosterone use (prescribed or unprescribed), erythropoietin therapy, or diuretics 1, 2
• Liver imaging if hepatocellular carcinoma is suspected 2

Additional Baseline Studies

• Serum creatinine and blood urea nitrogen assess renal function 3
• Lactate dehydrogenase (LDH) reflects tumor cell characteristics and tissue hypoxia 3
• C-reactive protein (CRP) evaluates for inflammatory conditions 3
• Platelet count and white blood cell count with differential identify concurrent thrombocytosis or leukocytosis suggesting myeloproliferative disorder 3

Bone Marrow Evaluation

• Unilateral bone marrow aspirate and biopsy is required if JAK2 mutation is positive to confirm PV diagnosis and assess for trilineage myeloproliferation 3, 2
• Bone marrow biopsy should be considered if diagnosis remains unclear after initial workup to exclude other myeloid neoplasms 2
• Immunohistochemistry and flow cytometry can confirm clonal populations 3

Common Diagnostic Pitfalls

• Don't rely on hematocrit alone—hemoglobin is more accurate because hematocrit can falsely increase by 2-4% with prolonged sample storage and is affected by hyperglycemia, while hemoglobin remains stable 1, 2, 4
• Don't overlook iron deficiency—MCV is unreliable for screening iron deficiency in erythrocytosis; serum ferritin and transferrin saturation are required 1, 2
• Don't use standard PV diagnostic thresholds at high altitude—physiologic adaptation can increase hemoglobin by 0.2-4.5 g/dL depending on elevation (1000-4500 meters) 2
• Don't order therapeutic phlebotomy before completing diagnostic workup—this can cause iron depletion and worsen outcomes 1, 2

When to Refer to Hematology

Immediate hematology referral is indicated for 2:

• Positive JAK2 mutation
• Hemoglobin >20 g/dL with symptoms of hyperviscosity
• Unexplained splenomegaly
• Diagnosis remains unclear after initial workup
• Concurrent cytopenias in other cell lines