Polycythemia lab investigation values
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Polycythemia: Lab Investigation Values
Polycythemia (erythrocytosis) is defined as an abnormally high red cell mass, usually with elevated hemoglobin (Hb). It is classified as relative (hemoconcentration with normal red cell mass) or absolute (true increase in red cell mass, either primary or secondary).
Classification at a Glance
| Type | EPO Level | Example |
|---|---|---|
| Relative (spurious) | Normal | Dehydration, Gaisböck syndrome |
| Absolute - Primary (PV) | Low/suppressed | Polycythemia vera (JAK2 mutation) |
| Absolute - Secondary (compensatory) | High | High altitude, lung disease, cyanotic heart disease |
| Absolute - Secondary (paraneoplastic) | High | Renal cell carcinoma, hepatocellular carcinoma, cerebellar hemangioblastoma |
I. Polycythemia Vera (PV) - Lab Findings
CBC / Peripheral Blood
| Parameter | Finding in PV |
|---|---|
| Hemoglobin (Hb) | >18.5 g/dL (males), >16.5 g/dL (females) |
| Hematocrit (Hct) | >52% (males), >48% (females) |
| RBC count | 6-12 × 10¹²/L (markedly elevated) |
| MCV, MCH, MCHC | Normal or low (microcytic/hypochromic if iron-deficient from chronic blood loss) |
| RDW | May be elevated (due to associated iron deficiency) |
| Platelets | Elevated in ~2/3 of patients, often >1000 × 10⁹/L (thrombocytosis) |
| WBC | Neutrophilic leukocytosis: 10-30 × 10⁹/L |
| Basophils | Absolutely increased (characteristic) |
| Immature granulocytes | Present in ~50% of cases |
| ESR | Reduced (due to high RBC mass) |
| Blood viscosity | High (can make blood films difficult to prepare) |
Bone Marrow
- Moderately to markedly hypercellular
- Panmyelosis: prominent normoblastic erythroid hyperplasia + granulocytic + megakaryocytic proliferation
- Pleomorphic megakaryocytes (characteristic)
- Late disease: myelofibrosis (spent phase)
Biochemical Investigations
| Test | Result in PV |
|---|---|
| Serum EPO | Low/suppressed (key differentiator from secondary) |
| JAK2 V617F mutation | Positive in >95% of cases |
| JAK2 exon 12 mutation | Positive in most remaining ~5% |
| Neutrophil alkaline phosphatase (NAP/LAP score) | Markedly elevated in 80% of patients |
| Serum B12 and B12-binding capacity | Elevated (increased transcobalamin III from granulocytes) |
| Serum uric acid | Elevated (hyperuricemia) - due to increased nucleic acid turnover; can cause secondary gout or renal stones |
| Arterial O₂ saturation | Normal (distinguishes PV from secondary hypoxic polycythemia) |
| Platelet aggregation | Decreased response to ADP and epinephrine in 80% of untreated patients |
| Red cell mass | Increased (measured by ⁵¹Cr-labelled RBCs) |
| Plasma volume | May also be expanded (can mask true erythrocytosis - important caveat in PV) |
II. WHO Diagnostic Criteria for PV (2017)
Major Criteria
- Hb >16.5 g/dL (males) or >16.0 g/dL (females), or hematocrit elevated, or other evidence of increased red cell volume
- Bone marrow biopsy showing hypercellularity with panmyelosis (granulocytic + erythroid + megakaryocytic proliferation with pleomorphic megakaryocytes)
- Presence of JAK2 V617F or JAK2 exon 12 mutation
Minor Criterion
- Low serum erythropoietin (EPO) level
Diagnosis: Requires all 3 major criteria OR major criteria 1 + 3 + the minor criterion (bone marrow biopsy can be omitted if Hb >18.5 g/dL in males or >16.5 g/dL in females, JAK2 positive, and EPO low).
WHO-5 (latest update) removed red cell mass measurement as a formal diagnostic requirement.
III. Secondary Polycythemia - Lab Findings
| Test | Secondary (Hypoxic) | Secondary (Paraneoplastic) |
|---|---|---|
| Hb/Hct | Elevated | Elevated |
| Serum EPO | Elevated (appropriately) | Elevated (inappropriately) |
| JAK2 mutation | Absent | Absent |
| WBC | Usually normal | Usually normal |
| Platelets | Usually normal | Usually normal |
| NAP score | Normal | Normal |
| Arterial O₂ saturation | Decreased (<92%) | Normal |
| 2,3-DPG | Increased (response to hypoxia) | Normal |
| Bone marrow | Erythroid hyperplasia only | Erythroid hyperplasia only |
| B12 / B12-binding capacity | Normal | Normal |
| Uric acid | May be mildly elevated | Normal |
IV. Relative Polycythemia (Pseudopolycythemia / Gaisböck Syndrome)
| Test | Finding |
|---|---|
| Hb/Hct | Elevated |
| Red cell mass | Normal (or high-normal) |
| Plasma volume | Decreased (hemoconcentration) |
| EPO | Normal |
| JAK2 | Negative |
| WBC/Platelets | Normal |
| Clinical context | Dehydration, diuretics, burns, obesity, tobacco use, hypertension |
V. Differentiating Key Tests at a Glance
| Lab | PV | Secondary Absolute | Relative |
|---|---|---|---|
| EPO | Low | High | Normal |
| JAK2 V617F | + (>95%) | - | - |
| O₂ saturation | Normal | Low (if hypoxic) | Normal |
| NAP/LAP score | High (80%) | Normal | Normal |
| WBC | High | Normal | Normal |
| Platelets | High | Normal | Normal |
| Red cell mass | Increased | Increased | Normal |
| Serum B12 | Elevated | Normal | Normal |
| Uric acid | Elevated | Normal/mild | Normal |
| Bone marrow | Panmyelosis | Erythroid hyperplasia | Normal |
Sources:
- Henry's Clinical Diagnosis and Management by Laboratory Methods, pp. 717-718, 2933-2960
- Harrison's Principles of Internal Medicine 22E (2025), pp. 862-864
- Robbins & Cotran Pathologic Basis of Disease, Table 14.8
- Robbins & Kumar Basic Pathology, Table 10.5
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