What laboratory workup is indicated for a patient with elevated hemoglobin?

Laboratory Workup for Elevated Hemoglobin

For elevated hemoglobin, obtain a complete blood count with differential, JAK2 V617F mutation testing, serum erythropoietin level, and consider bone marrow biopsy if polycythemia vera (PV) is suspected based on initial results.

Initial Assessment and Thresholds

Elevated hemoglobin warrants investigation when levels exceed 16.5 g/dL in men or 16.0 g/dL in women 1. However, you should also pursue workup if there's a documented increase above an individual's baseline, even within normal range, or if PV-related features (thrombocytosis, leukocytosis, microcytosis, splenomegaly, aquagenic pruritus, unusual thrombosis) accompany borderline-high values 2.

Important caveat: The 2016 WHO criteria lowered diagnostic thresholds, but these overlap substantially with normal reference ranges. In one study, 5.99% of men with presumptively normal blood counts had Hb >165 g/L 3. Therefore, don't reflexively order extensive workups for isolated borderline elevations without supporting clinical features.

Core Laboratory Tests

First-Line Testing

Complete Blood Count with Indices:

• Red blood cell count (RBC >6.8 × 10⁶ for men or >5.9 × 10⁶ for women suggests PV) 4
• Mean corpuscular volume (MCV) - microcytosis from iron deficiency supports PV 2
• Platelet count (≥450 × 10³/μL rarely seen in secondary erythrocytosis) 4
• White blood cell count with differential
• Red cell distribution width (RDW) 5

Serum Erythropoietin (EPO):

• Low EPO is highly suggestive of PV (>90% specificity) but sensitivity is only ~70% 2
• Normal EPO doesn't exclude PV
• Elevated EPO points toward secondary erythrocytosis 2

JAK2 V617F Mutation:

• Present in up to 97% of PV cases 6
• Essential for diagnosis per WHO criteria 1
• Can be positive even with normal hemoglobin/hematocrit in "masked PV" 7

Additional Parameters

Iron Studies:

• Serum ferritin
• Transferrin saturation (TSAT)
• Low ferritin or microcytosis supports PV due to increased iron utilization 4

Inflammatory Markers:

• C-reactive protein (CRP) to assess for inflammatory causes 5

Reticulocyte Count:

• Helps distinguish regenerative vs. non-regenerative processes 5

Diagnostic Algorithm

Step 1: If Hb >16.5 g/dL (men) or >16.0 g/dL (women), or documented increase from baseline, or PV-related features present:

• Order CBC with differential, serum EPO, JAK2 V617F mutation

Step 2: Interpret results:

• Low or normal EPO + positive JAK2 → Proceed to bone marrow biopsy
• Low or normal EPO + negative JAK2 → Still consider bone marrow biopsy if clinical suspicion high
• Elevated EPO → Investigate secondary causes (see below)

Step 3: Bone marrow examination when indicated:

• Look for hypercellularity with trilineage growth (panmyelosis)
• Prominent erythroid, granulocytic, and megakaryocytic proliferation
• Pleomorphic, mature megakaryocytes
• Cytogenetic studies (abnormal in only 13-18% at diagnosis) 2

Secondary Causes to Exclude

When EPO is elevated or PV features absent, investigate:

Hypoxic Conditions:

• Oxygen saturation measurement
• Chest X-ray for pulmonary disease
• Consider sleep apnea evaluation

Renal Causes:

• Renal function tests (creatinine, urea)
• Abdominal ultrasound for renal masses or cysts 2

Other:

• Smoking history (strongly favors secondary erythrocytosis) 4
• Obesity assessment (BMI ≥30 kg/m² uncommon in PV) 4
• Liver function tests
• Consider altitude exposure (Hb increases ~0.2 g/dL per 1000m above sea level) 8

Critical Pitfalls

1. Don't assume age <40 rules out PV, but it's rare 4
2. Normal hemoglobin doesn't exclude PV - "masked PV" exists with JAK2 mutations and normal counts, often presenting with thrombosis 7
3. Don't order red cell mass (RCM) measurement routinely - it's costly, rarely changes management, and misses PV patients at the lower distribution tail 2
4. Current smoking strongly suggests secondary erythrocytosis over PV 4
5. Isolated borderline elevation without clinical features may warrant observation rather than immediate extensive workup 3

When to Consult Hematology

• Positive JAK2 mutation
• Equivocal bone marrow findings
• Unusual thrombosis with any hemoglobin elevation
• Persistent unexplained erythrocytosis after initial workup 5