Laboratory Workup for Elevated Hematocrit and Hemoglobin
Begin with a repeat complete blood count using an automated cell counter to confirm the elevation, then systematically evaluate for primary versus secondary causes through serum erythropoietin level, JAK2 mutation testing, and iron studies. 1
Initial Confirmation and Baseline Testing
Repeat the CBC with differential using an automated analyzer to confirm persistent elevation and rule out laboratory error or transient changes. 1, 2 Hemoglobin is the preferred measurement over hematocrit because it remains stable at room temperature, whereas hematocrit can falsely increase by 2-4% with prolonged sample storage due to MCV changes. 3, 4
Define True Elevation Based on Sex and Altitude
For males at sea level, hemoglobin >16.5 g/dL or hematocrit >52% warrants investigation. 1, 2 For females at sea level, hemoglobin >16.0 g/dL or hematocrit >48-49% requires evaluation. 1, 2 Altitude adjustment is critical: hemoglobin increases approximately 0.9 g/dL per 1000 meters above sea level, and failure to adjust leads to overdiagnosis of polycythemia. 1, 5
Core Laboratory Panel
Order the following tests immediately to distinguish primary from secondary erythrocytosis:
Complete blood count with red cell indices (MCV, MCH, MCHC) to assess for microcytosis (suggesting iron deficiency or hemoglobinopathy) versus macrocytosis (suggesting B12/folate deficiency). 3, 2
Reticulocyte count to evaluate bone marrow response—an elevated count suggests active hemolysis or blood loss, while a normal count is typical of polycythemia vera. 3, 2
White blood cell count and platelet count from the automated analyzer to identify pancytosis, which suggests polycythemia vera rather than isolated erythrocytosis. 3
Serum erythropoietin (EPO) level: Low or inappropriately normal EPO suggests polycythemia vera, while elevated EPO indicates secondary polycythemia from hypoxia or other causes. 1, 2
Iron studies (serum ferritin, transferrin saturation, serum iron, TIBC) because iron deficiency must be corrected before accurate interpretation and can coexist with erythrocytosis. 3, 1, 2
C-reactive protein (CRP) to assess for inflammation that may affect iron studies interpretation. 2
Testing for Primary Erythrocytosis (Polycythemia Vera)
JAK2 V617F mutation testing should be performed immediately if hemoglobin is >18.5 g/dL in men or >16.5 g/dL in women, as this mutation is present in over 95% of polycythemia vera cases. 1, 2 If JAK2 V617F is negative but clinical suspicion remains high, test for JAK2 exon 12 mutations. 2
WHO Diagnostic Criteria for Polycythemia Vera
Diagnosis requires both major criteria plus one minor criterion, OR the first major criterion plus two minor criteria: 2
Major criteria:
- Hemoglobin >16.5 g/dL in men or >16.0 g/dL in women (adjusted for altitude)
- Presence of JAK2 mutation (V617F or exon 12)
Minor criteria:
- Bone marrow hypercellularity with trilineage growth
- Subnormal serum erythropoietin level
- Endogenous erythroid colony formation
Bone marrow biopsy is required if JAK2 mutation is positive to confirm diagnosis and assess for trilineage myeloproliferation. 2
Evaluation for Secondary Causes
If JAK2 is negative and EPO is elevated, systematically evaluate for secondary causes:
Hypoxia-Driven Causes
Smoking history and carbon monoxide exposure: "Smoker's polycythemia" results from chronic tissue hypoxia stimulating erythropoietin production. 2
Sleep study if nocturnal hypoxemia is suspected—obstructive sleep apnea produces nocturnal hypoxemia that drives EPO production. 1, 2
Pulmonary function testing to evaluate for chronic obstructive pulmonary disease or other chronic lung disease. 2
Echocardiography to assess for cyanotic congenital heart disease with right-to-left shunting causing compensatory erythrocytosis. 2
Non-Hypoxic Secondary Causes
Renal imaging (ultrasound or CT) to evaluate for renal cell carcinoma, polycystic kidney disease, or other renal masses that produce erythropoietin. 2
Testosterone use assessment (prescribed or unprescribed), as testosterone therapy commonly causes erythrocytosis. 2
Screening for other EPO-producing tumors if clinically indicated: hepatocellular carcinoma, pheochromocytoma, uterine leiomyoma, meningioma. 2
Relative Polycythemia Assessment
Evaluate for plasma volume depletion causes:
- Hydration status assessment and review of diuretic use. 2
- Consider stress polycythemia (Gaisböck syndrome) in hypertensive, obese patients with normal EPO and negative JAK2. 2
Peripheral Blood Smear Review
A peripheral blood smear reviewed by a qualified hematologist is essential to identify abnormal red cell morphology, assess for hemoglobinopathies, and evaluate white blood cell and platelet morphology. 2
Common Pitfalls to Avoid
Never use unadjusted reference ranges at altitude—this leads to overdiagnosis of polycythemia and underdiagnosis of anemia. 1, 5
Don't rely on the 3:1 hematocrit-to-hemoglobin ratio for diagnosis, as this relationship varies with age, disease states, and is unreliable in many conditions. 6, 7
Don't overlook coexisting iron deficiency in patients with erythrocytosis—iron-deficient red cells have reduced oxygen-carrying capacity and deformability, increasing stroke risk. 2
Mean corpuscular volume (MCV) is unreliable for screening iron deficiency in erythrocytosis—serum ferritin and transferrin saturation are required for accurate diagnosis. 2
Hyperglycemia falsely elevates MCV and calculated hematocrit but does not affect hemoglobin measurement, making hemoglobin the more reliable parameter. 3
Referral Indications
Refer immediately to hematology if: