# Polycythaemia

> Polycythaemia for FMGE Medicine: relative versus absolute, JAK2 in polycythaemia vera, phlebotomy targets and hyperviscosity signs.

- Canonical URL: https://prepelephant.com/topics/fmge/medicine/polycythaemia-fmge
- Exam / course: FMGE · Subject: Medicine
- Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team
- First published: 2026-10-02
- Last updated: 2026-10-02
- How to cite: "Polycythaemia", PrepElephant, https://prepelephant.com/topics/fmge/medicine/polycythaemia-fmge

## Direct answer

Polycythaemia is an elevated red-cell mass — or, in relative (spurious) polycythaemia, a normal red-cell mass concentrated by reduced plasma volume (dehydration, diuretics, alcohol, "stress polycythaemia") — presenting with hyperviscosity symptoms: headache, dizziness, tinnitus, visual blurring, burning palms and soles (erythromelalgia), and thrombosis including Budd-Chiari syndrome in the young. Absolute polycythaemia divides into primary — polycythaemia vera, a myeloproliferative neoplasm in which a JAK2 V617F mutation (over 95 percent) drives autonomous erythropoiesis with suppressed erythropoietin — and secondary, with appropriately raised erythropoietin from chronic hypoxia (altitude, chronic lung disease, cyanotic heart disease, sleep apnoea, smoking) or erythropoietin-secreting tumours (renal cell, hepatocellular, cerebellar haemangioblastoma). Treatment of polycythaemia vera is phlebotomy to a haematocrit below 45 percent, with low-dose aspirin and cytoreduction (hydroxycarbamide) for high-risk patients.

## What you must remember

- **Classification in one breath:** relative (low plasma volume, normal red-cell mass — dehydration, diuretics, alcohol) versus absolute: primary polycythaemia vera with JAK2 and LOW erythropoietin; secondary with HIGH erythropoietin from hypoxia or tumour — the erythropoietin level is the fork.
- **Polycythaemia vera clinical set:** hyperviscosity symptoms, erythromelalgia (burning palms and soles, responsive to aspirin), aquagenic pruritus (itching after hot baths — histamine), gout from high cell turnover, splenomegaly, and thrombosis including hepatic vein (Budd-Chiari) — polycythaemia vera belongs in every young Budd-Chiari work-up.
- **Laboratory discriminants:** haemoglobin over 16.5 g/dL in men (16.0 in women, WHO 2016) or raised haematocrit, JAK2 V617F positivity in over 95 percent (exon 12 in most of the rest), LOW erythropoietin, often leucocytosis, thrombocytosis and basophilia.
- **Treatment targets and doses:** venesection 400-500 mL every few days initially, maintaining haematocrit BELOW 45 percent (42 considered for women in some guidance); low-dose aspirin 75-100 mg daily; hydroxycarbamide for high-risk patients (age over 60 or prior thrombosis), interferon-alpha preferred in young or pregnant patients.
- **Secondary polycythaemia causes:** chronic hypoxia — altitude (a real Indian entity in Ladakh), chronic obstructive pulmonary disease, sleep apnoea, cyanotic congenital heart disease; smoking; erythropoietin-secreting tumours — renal cell, hepatocellular, cerebellar haemangioblastoma, uterine fibroma; post-transplant erythrocytosis; exogenous erythropoietin and androgens.
- **Physiological adaptation:** high-altitude residents may carry haematocrits that would be disease at sea level — treat symptoms, not numbers; phlebotomy is reserved for symptomatic hyperviscosity.
- **Complication clock of untreated vera:** thrombosis early, and over years transformation to myelofibrosis (post-polycythaemic myelofibrosis) or acute myeloid leukaemia — the reason for lifelong haematological follow-up.
- **Emergency picture:** hyperviscosity with thrombosis or visual loss warrants urgent phlebotomy (with volume replacement) — and remember that repeated venesection can deplete iron stores, with iron replacement used cautiously (it drives erythropoiesis), with iron replacement used cautiously (it drives erythropoiesis).

## Working through a high haemoglobin

A 54-year-old man presents with three months of headache, itching after hot showers and burning feet; haemoglobin 19 g/dL, haematocrit 57 percent, white cells 13,000 with basophilia, platelets 620,000, and a palpable spleen tip. The constellation of leucocytosis, thrombocytosis and basophilia shouts myeloproliferative neoplasm rather than a relative rise: JAK2 V617F is positive and erythropoietin low — polycythaemia vera. Management opens with venesection 450 mL twice weekly until haematocrit is under 45 percent, daily aspirin 75 mg, allopurinol for hyperuricaemia, and risk stratification — his age puts him in the high-risk group, so hydroxycarbamide is added; he is counselled on transformation risk and lifelong surveillance.

The contrasting pathway belongs to a 46-year-old mountain-dwelling smoker with chronic lung disease, haemoglobin 18.5 g/dL, normal white cells and platelets, JAK2 negative and erythropoietin high — secondary polycythaemia, managed by treating the lungs and stopping smoking, with phlebotomy only for hyperviscosity symptoms. The third archetype is the anxious executive on diuretics with haemoglobin 17 but a normal red-cell mass — relative polycythaemia, needing reassurance and hydration, not venesection. Three haematocrits, three diseases, separated by erythropoietin, JAK2 and red-cell mass.

## Where students slip

The recurring slip is treating every high haemoglobin as one disease: the exam's architecture is the three-way split (relative, primary, secondary), bridged by the erythropoietin level plus JAK2 status. Second, the target number: below 45 percent is the trial-supported phlebotomy goal, and "normal range" is not the answer. Third, aquagenic pruritus and erythromelalgia are the two symptoms a generic summary underweights and NBE overtests; erythromelalgia responding to aspirin is stem-worthy. Fourth, Budd-Chiari in a young patient puts polycythaemia vera on the screen-first list. Fifth, physiological secondary polycythaemia at altitude is not routinely venesected — only symptomatic hyperviscosity justifies it. Finally, transformation to myelofibrosis or acute leukaemia supplies the follow-up questions.

## Frequently asked questions

### How is polycythaemia classified?

Relative (reduced plasma volume, normal red-cell mass), primary absolute (polycythaemia vera with JAK2 mutation and low erythropoietin), and secondary absolute (high erythropoietin from hypoxia or tumours).

### What is the haematocrit target in polycythaemia vera and why?

Below 45 percent, achieved by venesection, because controlled follow-up showed thrombosis rates rise significantly above this threshold.

### Which mutation and laboratory pattern confirm polycythaemia vera?

JAK2 V617F (over 95 percent, exon-12 variants in most others) with low serum erythropoietin, often with leucocytosis, thrombocytosis and basophilia.

### What is erythromelalgia in this context?

Burning pain and redness of palms and soles from platelet-mediated microvascular occlusion, characteristically relieved by low-dose aspirin.

### When is phlebotomy indicated in secondary polycythaemia?

Only for symptomatic hyperviscosity (headache, visual disturbance, thrombosis) in secondary or physiological polycythaemia; otherwise treat the underlying hypoxic cause.