What investigations are used to diagnose polycythaemia?

Investigations for Polycythaemia

The diagnosis of polycythaemia requires a complete blood count (CBC) with differential, JAK2 mutation testing, and serum erythropoietin level as first-line investigations, followed by bone marrow biopsy for confirmation in most cases. 1

Initial Diagnostic Approach

1. Complete Blood Count (CBC) with differential:

   • Confirms erythrocytosis: Hemoglobin >16.5 g/dL or hematocrit >49% in men, or >16 g/dL or >48% in women 2
   • Assesses other cell lines (leukocytosis, thrombocytosis)
   • Includes peripheral blood smear examination

2. Serum Erythropoietin Level:

   • Critical for distinguishing between primary and secondary polycythaemia
   • Subnormal EPO is a minor diagnostic criterion for polycythaemia vera (PV)
   • Normal or elevated EPO suggests secondary polycythaemia

3. JAK2 Mutation Testing:

   • Test for JAK2 V617F or exon 12 mutations
   • Present in >95% of PV cases 1, 2
   • Essential for distinguishing primary from secondary causes

Confirmatory Testing

1. Bone Marrow Aspirate and Biopsy:

   • Required for definitive diagnosis, especially when:
     • JAK2 mutation testing is negative but clinical suspicion remains high
     • Distinguishing between different myeloproliferative neoplasms is necessary
   • Characteristic findings in PV include:
     • Hypercellularity
     • Increased megakaryocytes with cluster formation
     • Giant megakaryocytes
     • Pleomorphic megakaryocyte morphology
     • Decreased iron stores
     • Cytogenetic abnormalities (present in 13-18% of patients) 1

2. Red Cell Mass and Plasma Volume Measurement:

   • May be required in ambiguous cases to differentiate primary, secondary, and combined polycythaemia 3
   • Less commonly performed since the advent of JAK2 testing

Additional Investigations for Secondary Causes

If JAK2 mutation is negative and/or EPO is normal/elevated, investigate for secondary causes:

1. Chest X-ray:

   • To evaluate for cardiopulmonary disease causing hypoxia 3

2. Arterial Blood Gas Analysis:

   • To assess for hypoxemia

3. Sleep Study:

   • If obstructive sleep apnea is suspected

4. Abdominal Ultrasound:

   • To evaluate for renal or hepatic pathology
   • To assess for splenomegaly

5. Genetic Testing for Congenital Causes:

   • High-oxygen-affinity hemoglobins
   • Congenital erythrocytosis

Diagnostic Algorithm

1. Initial Suspicion:

   • Elevated hemoglobin/hematocrit above the 95th percentile adjusted for sex and race
   • Documented increase in hemoglobin/hematocrit above patient's baseline

2. First-line Testing:

   • CBC with peripheral blood smear
   • JAK2 V617F mutation testing
   • Serum erythropoietin level

3. Interpretation:

   • If JAK2 mutation positive + low EPO: Likely PV, proceed to bone marrow biopsy
   • If JAK2 mutation negative + low EPO: Consider JAK2 exon 12 testing, then bone marrow biopsy
   • If JAK2 mutation negative + normal/high EPO: Investigate secondary causes

Common Pitfalls and Caveats

• Relative Polycythaemia: Can occur due to dehydration or diuretic use, causing hemoconcentration. Always ensure proper hydration before testing.

• Iron Deficiency: Repeated phlebotomies without diagnosis can lead to iron deficiency, decreased oxygen-carrying capacity, and increased stroke risk 1. Always check iron studies.

• Masked Polycythaemia: Concurrent iron deficiency can mask true polycythaemia. Consider this in patients with borderline hemoglobin/hematocrit values.

• Secondary vs. Primary: Never assume secondary polycythaemia without appropriate testing, as PV can coexist with conditions that typically cause secondary polycythaemia.

• Bone Marrow Biopsy Timing: While guidelines recommend bone marrow biopsy for confirmation, in practice, this may be deferred in cases with clear JAK2 mutation positivity, low EPO, and typical blood count findings 2, 4.