Screen for hyperviscosity syndrome, acute arterial or venous thrombosis, and severe erythrocytosis requiring urgent assessment.
Severe erythrocytosis (Hct ≥0.60 in men or ≥0.56 in women) increases blood viscosity and thrombotic risk and needs prompt clinical assessment. In BSH diagnostic guidance this level allows true erythrocytosis to be assumed — it is not an automatic emergency-venesection threshold. Urgency depends on symptoms and clinical context; venesection decisions are specialist-led.
Hyperviscosity symptoms: headache, visual disturbance, dizziness, confusion; retinal vein engorgement may be visible on fundoscopy. Symptomatic patients need same-day specialist discussion; acute neurological or cardiac events go via emergency pathways.
Polycythaemia vera (PV) is a clonal myeloproliferative neoplasm; JAK2 V617F is present in most, but not all, cases (approximately 95%). Arterial and venous thrombosis are major complications, including unusual-site thrombosis such as Budd-Chiari syndrome (hepatic vein thrombosis), presenting with ascites + hepatomegaly + abdominal pain.
Aquagenic pruritus (itching after bathing) is a useful clue to PV but is not pathognomonic — it can occur in other haematological and dermatological conditions. Gout, plethoric facies and conjunctival injection are likewise supportive, not diagnostic.
Confirm persistent erythrocytosis — raised venous Hb and/or Hct requiring confirmation and investigation — and distinguish probable relative from absolute erythrocytosis clinically. A single raised Hct may be dehydration or lab error. PV is a specific clonal myeloproliferative neoplasm and is never diagnosed from a raised Hct alone.
Relative erythrocytosis (pseudopolycythaemia) is common, caused by reduced plasma volume (dehydration, diuretics, obesity). It is usually a clinical judgement in primary care — red-cell mass is rarely measured, and a negative JAK2 does not prove it. Does NOT require venesection; treat hypertension and cardiovascular risk factors, and investigate if erythrocytosis persists unexplained.
Smoking causes pseudopolycythaemia via two mechanisms: (1) carboxyhaemoglobin (CO bound to Hb) falsely elevates measured Hb; (2) chronic hypoxia stimulates erythropoietin. Smoking cessation may improve Hct. Consider carboxyhaemoglobin measurement where carbon monoxide exposure is relevant, for example heavy smoking, occupational exposure or faulty domestic heating (BSH). Normal pulse oximetry does not exclude smoking-related carboxyhaemoglobinaemia or nocturnal hypoxia. OSA should be assessed clinically and via the local sleep pathway — not diagnosed from erythrocytosis alone.
Persistent haematocrit >0.52 (men) or >0.48 (women) warrants investigation (BSH) — it indicates erythrocytosis, not PV. A single elevated Hct can reflect dehydration, acute illness or laboratory error, so repeat to confirm persistence; the interval is set locally.
Red cell mass measurement is not routinely required in primary care and is rarely available; haematology may still consider it in selected cases of diagnostic uncertainty, such as suspected PV with borderline results.
Classify polycythaemia as primary (PV, JAK2+ myeloproliferative neoplasm) or secondary (erythropoietin-driven). This determines investigation pathway and treatment.
Polycythaemia vera (PV) is a JAK2-driven myeloproliferative neoplasm with uncontrolled red cell, and often platelet and white cell, production. JAK2 V617F is present in approximately 95%. Thrombosis is a major cause of morbidity and mortality, and a minority progress to myelofibrosis or acute leukaemia over many years.
Secondary polycythaemia is reactive, driven by erythropoietin (EPO). Chronic hypoxia (COPD, OSA, altitude) causes physiological EPO elevation. Renal disease and tumours cause inappropriate EPO secretion. Treating the underlying cause is the mainstay of management.
EPO levels help but are not diagnostic. A low EPO supports a primary/clonal process but is not diagnostic; a raised EPO supports secondary erythrocytosis but does not identify the cause. EPO may be normal in secondary erythrocytosis and low or normal in PV, and is affected by timing, hypoxia, renal disease and assay variation. Interpret JAK2 alongside EPO and the clinical picture; if JAK2 is negative with low EPO or high suspicion, refer to haematology, who decide on further testing (exon 12, bone marrow).
Thrombocytosis + leucocytosis alongside polycythaemia strongly suggests PV (trilineage proliferation). Secondary polycythaemia affects only red cells; platelets and WCC are normal. Splenomegaly favours PV over secondary erythrocytosis.
Examine for signs of PV (splenomegaly, plethora, thrombosis) and secondary causes (cyanosis, clubbing); measure blood pressure for vascular risk.
Splenomegaly is a useful sign favouring PV over secondary erythrocytosis. Spleen size correlates with disease burden, and massive or enlarging splenomegaly may suggest transformation to post-PV myelofibrosis.
Plethora (ruddy complexion) is due to raised Hct. Red palms, injected conjunctivae, facial flushing. More pronounced in PV than secondary polycythaemia (higher Hct in PV typically). Plethora reduces post-venesection as Hct falls to target (<0.45).
Low SaO₂ on room air indicates chronic hypoxia, a common driver of secondary erythrocytosis (COPD, altitude, cyanotic heart disease). Normal daytime saturation does not exclude nocturnal hypoxia — consider overnight oximetry or sleep assessment if OSA is suspected. In smokers, carboxyhaemoglobin can make oximetry misleading; BSH recommends considering carboxyhaemoglobin measurement.
Aquagenic pruritus (itching after hot bath/shower) increases suspicion of PV but is not pathognomonic. Management of troublesome pruritus is guided by haematology.
Investigate to differentiate PV from secondary polycythaemia. A positive JAK2 V617F is highly suggestive of a myeloproliferative neoplasm but does not by itself confirm PV; interpret with EPO and the clinical picture.
JAK2 V617F is present in approximately 95% of PV and strongly supports a myeloproliferative neoplasm, but does not by itself establish PV. Diagnosis requires specialist correlation of Hb/Hct or red-cell mass, JAK2 status, EPO, exclusion of secondary causes and, where required, bone-marrow morphology. Exon 12 mutations account for a small proportion of JAK2 V617F-negative cases (specialist testing).
EPO level helps discriminate: PV usually has low/normal EPO because red cell production is autonomous, whereas secondary erythrocytosis often has normal or raised EPO. EPO is affected by timing, hypoxia, renal disease and assay variation — interpret against the local laboratory reference range; a low EPO with JAK2-negative erythrocytosis needs haematology referral.
Overnight oximetry or sleep assessment helps diagnose obstructive sleep apnoea (OSA) when suspected (snoring, witnessed apnoeas, daytime somnolence). Nocturnal desaturation stimulates EPO; treating OSA may improve the erythrocytosis.
Renal ultrasound is considered when EPO is raised, renal disease is suspected, there is haematuria, an abdominal/renal abnormality, renal transplantation, or no other cause is identified. It detects polycystic kidney disease, renal cell carcinoma (paraneoplastic EPO secretion), post-transplant erythrocytosis (native kidneys secrete EPO), and renal artery stenosis (ischaemia → EPO).
Refer based on symptoms, thrombosis, JAK2/EPO results, other blood count abnormalities and underlying cause — not on a rigid Hct cut-off alone. PV requires specialist management; secondary polycythaemia may be managed in primary care if mild.
Severe erythrocytosis (Hct ≥0.60 in men or ≥0.56 in women) requires prompt clinical assessment; same-day specialist discussion is appropriate if accompanied by headache, visual disturbance, bleeding or thrombosis; focal neurology, confusion, seizure, or chest pain/breathlessness suggesting ACS or PE → 999 first. Urgency depends on symptoms, thrombosis, rate of rise, other count abnormalities, splenomegaly, pregnancy and underlying hypoxic disease.
JAK2 V617F positive is strongly suggestive of a clonal myeloproliferative neoplasm (usually PV) and warrants urgent haematology referral (BSH). Haematology confirms or refutes the diagnosis (WHO criteria, including bone marrow where required), risk-stratifies (age, thrombosis history), and initiates venesection + aspirin ± cytoreduction.
Probable relative erythrocytosis does not require venesection. Address reversible factors and cardiovascular risk (individualised weight management, BP control, smoking cessation). If erythrocytosis persists unexplained, proceed to EPO/JAK2 or haematology advice rather than labelling it Gaisböck syndrome.
Secondary erythrocytosis that is asymptomatic with an identified cause can usually be managed in primary care by treating the cause. For hypoxic pulmonary disease, BSH recommends respiratory assessment; venesection is not routine and is considered only for symptomatic hyperviscosity or selected patients with very high Hct, usually in specialist care, with a target around 0.50–0.52 rather than the PV target of <0.45 — excessive venesection may worsen tissue oxygen delivery. For congenital, idiopathic and other secondary erythrocytosis, targets are individualised and not copied from PV. Refer if the cause is unclear or symptoms develop.
Treatment depends on cause. Confirmed PV: haematology-led venesection to Hct <0.45, low-dose aspirin unless contraindicated ± cytoreduction. Secondary erythrocytosis: treat the cause; do not venesect in primary care or prescribe aspirin solely for a raised Hct. Relative erythrocytosis: address reversible factors and cardiovascular risk, no venesection.
PV treatment options (haematology-managed — order is specialist-led and depends on risk, response, tolerance and local commissioning):
Next steps: see Snoring and sleep apnoea · COPD · Smoking cessation · Haematological cancers
Venesection to Hct <0.45 is the cornerstone of confirmed PV treatment (BSH; CYTO-PV trial), for both men and women. Venesection removes iron, limiting erythropoiesis; iron deficiency may follow, but a low ferritin is not itself a treatment target and can cause symptoms — do not give iron replacement in PV without discussion with haematology.
Low-dose aspirin is recommended in confirmed PV unless contraindicated (BSH). Consider active bleeding, peptic ulcer disease, anticoagulant treatment, acquired von Willebrand syndrome (extreme thrombocytosis), aspirin allergy/intolerance and pregnancy (specialist advice). PPI co-prescription if GI risk.
Hydroxycarbamide (hydroxyurea) is a first-line cytoreductive agent for high-risk PV (defined principally by age and previous thrombosis). It inhibits DNA synthesis, reducing Hct, platelets and WCC; the main side effect is myelosuppression. It is initiated and stabilised by haematology, with GP prescribing only under the active local shared-care protocol after formal transfer (confirm the current version — the May 2024 version had a May 2026 review date).
Secondary polycythaemia treatment targets the underlying cause. Treating OSA and hypoxic lung disease may improve Hct. In hypoxic lung disease, specialist venesection is considered only for symptomatic hyperviscosity or selected patients with very high Hct, targeting around 0.50–0.52. Routine venesection in asymptomatic secondary polycythaemia does NOT improve outcomes and may worsen tissue hypoxia.
Cardiovascular risk-factor control and smoking cessation matter in both PV and secondary erythrocytosis; lifestyle advice supports, but does not replace, specialist treatment.
Avoiding dehydration limits further rises in blood viscosity. Advice should be individualised — fixed fluid volumes are inappropriate for people with heart failure, CKD, cirrhosis, hyponatraemia risk or fluid restriction.
Smoking cessation is critical in secondary polycythaemia. Smoking worsens COPD hypoxia (↑ EPO), raises carboxyhaemoglobin (falsely elevated Hb), and damages endothelium (↑ thrombosis). Quitting may improve Hct and reduces vascular risk.
Weight loss may improve relative erythrocytosis and OSA severity, lowering EPO drive. A Mediterranean-style diet supports cardiovascular risk reduction.
CPAP therapy for OSA normalizes nocturnal SaO₂, reducing EPO secretion. This may reduce EPO-driven erythrocytosis. Good CPAP adherence is essential. Side effects (nasal congestion, mask discomfort) improve with humidifier and mask re-fitting.
Monitor Hct trajectory, watch for thrombotic events, and re-refer if Hct rising or new symptoms. Chronic polycythaemia requires long-term surveillance.
Hct target <0.45 for both men and women with confirmed PV (BSH; CYTO-PV trial). This target does not apply to secondary erythrocytosis, where specialist venesection (if needed) in hypoxic lung disease generally targets 0.50–0.52.
Thrombotic events on treatment indicate inadequate control or high-risk disease. If stroke/MI/DVT occurs despite Hct <0.45 + aspirin, haematology escalates to hydroxycarbamide or ruxolitinib, adds anticoagulation, or intensifies venesection frequency. Primary care role is early recognition and urgent referral.
Transformation risk: a minority of PV evolves to post-PV myelofibrosis or acute myeloid leukaemia over many years. Signs: progressive splenomegaly, tear-drop cells, rising LDH, anaemia, thrombocytopenia. Blasts on the film suggest leukaemic transformation — refer urgently.
Regular cardiovascular review is important: PV carries increased cardiovascular risk, so control BP, lipids, diabetes, smoking and weight. QRISK underestimates risk in PV (does not include polycythaemia as variable).