Evaluation of Erythrocytosis

Initial Confirmation and Laboratory Assessment

Begin by confirming true erythrocytosis with repeat hemoglobin and hematocrit measurements using an automated cell counter, as a single measurement is unreliable. 1

Diagnostic Thresholds

Men: Hemoglobin >18.5 g/dL or hematocrit >52% (WHO threshold >49%) 1, 2
Women: Hemoglobin >16.5 g/dL or hematocrit >48% 1, 2
Hemoglobin is more reliable than hematocrit because hematocrit can falsely increase by 2–4% with prolonged sample storage, whereas hemoglobin remains stable 1

Essential First-Line Laboratory Tests

Order the following tests immediately when erythrocytosis is confirmed: 1

Complete blood count with red cell indices (MCV, MCH, MCHC, RDW) 1
Reticulocyte count 1
Peripheral blood smear review 1
Serum ferritin and transferrin saturation 1
C-reactive protein (CRP) 1
White blood cell differential and platelet count 1

High RDW with normal or low MCV suggests coexisting iron deficiency, which frequently accompanies erythrocytosis and requires opposite management (iron supplementation rather than phlebotomy). 1

Distinguish Primary from Secondary Erythrocytosis

Step 1: JAK2 Mutation Testing (First-Line Molecular Test)

JAK2 V617F testing is mandatory as the first-line molecular assay, detecting >90–95% of polycythemia vera (PV) cases. 1, 2, 3

If JAK2 V617F is negative, immediately test for JAK2 exon 12 mutations, which account for an additional 2–3% of PV cases 1, 2
Together, these two assays capture >97% of PV patients when combined with elevated hemoglobin/hematocrit 1, 2
JAK2 testing can be performed even during iron deficiency or while receiving IV iron—do not delay testing 2

Step 2: Serum Erythropoietin (EPO) Level

Measure serum EPO after confirming adequate hydration to differentiate primary from secondary causes: 1, 3

Low or subnormal EPO (below reference range): >90% specific for polycythemia vera but sensitivity <70%, so normal EPO does not exclude PV 1, 2, 3
Normal EPO: Present in up to 30% of confirmed PV patients; proceed with JAK2 testing and consider bone marrow biopsy if diagnosis unclear 2
Elevated EPO: Indicates secondary erythrocytosis; pursue evaluation for hypoxic and non-hypoxic causes 1, 3

WHO Diagnostic Criteria for Polycythemia Vera

PV diagnosis requires EITHER (a) both major criteria plus ≥1 minor criterion, OR (b) the first major criterion plus ≥2 minor criteria: 1, 2

Major Criteria

Elevated hemoglobin/hematocrit:
- Men: Hb ≥18.5 g/dL or Hct >49% (or sustained rise ≥2 g/dL reaching ≥17 g/dL) 2
- Women: Hb ≥16.5 g/dL or Hct >48% (or sustained rise ≥2 g/dL reaching ≥15 g/dL) 2
Presence of JAK2 mutation (V617F or exon 12) 1, 2
Bone marrow biopsy showing hypercellularity with trilineage (panmyelosis) growth and pleomorphic megakaryocytes 2

Minor Criteria

Subnormal serum erythropoietin level 1, 2
Endogenous erythroid colony (EEC) formation in vitro 2

When to Order Bone Marrow Biopsy

Bone marrow biopsy is indicated when: 1, 2

JAK2 testing is negative but clinical suspicion for PV remains high 2
Diagnosis remains equivocal after initial workup 1, 2
JAK2 is positive and confirmation of WHO criteria is needed 1

Evaluation for Secondary Erythrocytosis

If JAK2 is negative and EPO is normal or elevated, systematically evaluate for secondary causes: 1

Hypoxic Causes (Elevated EPO)

Sleep study for obstructive sleep apnea (nocturnal hypoxemia drives EPO production) 1
Pulmonary function tests and chest imaging for chronic obstructive pulmonary disease 1
Arterial oxygen saturation: <92% indicates hypoxemia-driven secondary erythrocytosis 2
Smoking history: Carbon monoxide exposure causes "smoker's polycythemia" (resolves with cessation) 1
Cyanotic congenital heart disease with right-to-left shunting (compensatory erythrocytosis to optimize oxygen transport) 1
High-altitude residence: Adjust hemoglobin thresholds by 0.2–4.5 g/dL depending on elevation (1,000–4,500 meters) 1

Non-Hypoxic Causes (Elevated EPO)

Renal imaging (ultrasound or CT) to exclude renal cell carcinoma, hydronephrosis, or cystic disease producing EPO 1
Hepatocellular carcinoma, pheochromocytoma, uterine leiomyoma, meningioma (EPO-producing tumors) 1
Testosterone use (prescribed or unprescribed)—common in young adults and causes erythrocytosis 1
Erythropoietin therapy 1

Congenital Causes (Consider if Lifelong Erythrocytosis)

High-oxygen-affinity hemoglobin variants 1, 3
Erythropoietin receptor mutations 3, 4
Chuvash polycythemia (von Hippel-Lindau gene mutation) 1, 3
NADH-cytochrome b5 reductase deficiency (hereditary methemoglobinemia) 2

Management Principles

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). 1, 5

Phlebotomy protocol: Remove 300–450 mL weekly or twice weekly until Hct <45%, then maintenance phlebotomy as needed 5
Always replace removed blood volume with equal fluid (dextrose or saline) to prevent hemoconcentration and stroke 1
Add low-dose aspirin (81–100 mg daily) as second cornerstone of therapy for thrombosis prevention 1, 5

Cytoreductive therapy is mandatory if: 5

Age ≥60 years or history of prior thrombosis 5
Poor phlebotomy tolerance, symptomatic splenomegaly, or severe symptoms 5
Platelet count >1,500 × 10⁹/L or leukocyte count >15 × 10⁹/L 5
First-line agents: hydroxyurea, interferon-α, or pegylated interferon 5

Secondary Erythrocytosis

Routine phlebotomy is contraindicated in secondary erythrocytosis because it causes iron depletion, decreased oxygen-carrying capacity, and paradoxically increases stroke risk. 1, 5

Phlebotomy is indicated ONLY when ALL of the following are met: 1, 5

Hemoglobin >20 g/dL AND hematocrit >65% 1, 5
Documented symptoms of hyperviscosity (headache, blurred vision, confusion) 1, 5
Patient is adequately hydrated (rehydrate with oral or IV normal saline first) 1, 5
Iron deficiency has been excluded (transferrin saturation ≥20%) 1, 5

Treatment of underlying condition is necessary: 1

Smoking cessation for smoker's polycythemia 1
CPAP therapy for obstructive sleep apnea 1
Management of COPD or chronic lung disease 1
Dose adjustment or discontinuation of testosterone if causative 1

Iron Deficiency Considerations

Iron deficiency frequently coexists with erythrocytosis and requires opposite management—iron supplementation, not phlebotomy. 1, 5

MCV is unreliable for screening iron deficiency in erythrocytosis; use serum ferritin, transferrin saturation, and iron levels 1
Iron-deficient red cells have reduced oxygen-carrying capacity and deformability, increasing stroke risk 1
If transferrin saturation <20%, initiate cautious oral iron supplementation with close hemoglobin monitoring until stores are replete 1, 5
Iron deficiency can mimic hyperviscosity symptoms but requires iron, not phlebotomy 5

Common Pitfalls to Avoid

Never perform routine or repeated phlebotomies in secondary erythrocytosis—this leads to iron depletion and paradoxically increases stroke risk 1, 5
Never perform phlebotomy without equal-volume fluid replacement—this raises hemoconcentration and stroke risk 1, 5
Do not overlook coexisting iron deficiency—it mimics hyperviscosity but requires iron supplementation 1, 5
Do not delay JAK2 testing until after iron repletion—the assay is independent of iron status 2
Do not diagnose PV solely on low EPO—WHO criteria require JAK2 mutation or bone marrow findings 2

Referral Indications

Refer immediately to hematology if: 1

JAK2 mutation is positive 1
Hemoglobin >20 g/dL with symptoms of hyperviscosity 1
Unexplained splenomegaly 1
Diagnosis remains unclear after initial workup 1
Unexplained cytopenias in the setting of erythrocytosis 1

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