High Red Blood Cell Count (Polycythaemia)
What a raised RBC, haemoglobin, or haematocrit means — from dehydration and smoking to polycythaemia vera — and the tests your GP will run next.
The Quick Answer
Polycythaemia (also spelt polycytemia) — or more precisely, erythrocytosis — means you have more red blood cells than normal. On your blood test, this appears as an elevated RBC count, raised haemoglobin (Hb), and/or high haematocrit (HCT or PCV). Australian normal ranges are approximately:
The critical question is whether the polycythaemia is relative (the red cells are normal in number but concentrated due to dehydration), secondary (driven by an external signal such as low oxygen or high EPO), or primary (a bone marrow disorder such as polycythaemia vera producing cells independently of normal controls).
How Red Cell Production Is Normally Controlled
Red cell production (erythropoiesis) is regulated by a feedback loop involving oxygen sensing in the kidneys. When oxygen levels fall — due to altitude, lung disease, anaemia, or poor blood flow — specialised kidney cells detect this and produce more erythropoietin (EPO). EPO travels to the bone marrow and signals it to produce more red cells, restoring oxygen delivery. When oxygen levels are adequate, EPO production falls and red cell output returns to normal.
In secondary polycythaemia, this feedback loop is working correctly — it is simply responding to a real or perceived oxygen deficit (from lung disease, sleep apnoea, smoking, or an EPO-producing tumour). The bone marrow itself is normal.
In polycythaemia vera, the JAK2 mutation makes the bone marrow stem cells permanently activated and insensitive to normal feedback. They produce excessive red cells (and often white cells and platelets) even when EPO is suppressed to almost undetectable levels. This uncontrolled overproduction is what makes PV a myeloproliferative neoplasm.
Causes of a High Red Blood Cell Count
Causes are grouped as relative (concentrated but not actually increased), secondary (EPO-driven in response to a cause), or primary (bone marrow overproduction). Most cases seen in Australian general practice are relative or secondary.
Dehydration
The most common cause of a mildly raised haemoglobin and RBC in routine blood tests. When plasma volume falls, red cells become more concentrated. Hot weather, inadequate fluid intake, vomiting, diarrhoea, and diuretic medication all cause relative polycythaemia. Corrects when hydration is restored.
Smoking (cigarette and cigar)
Carbon monoxide displaces oxygen from haemoglobin, triggering EPO-driven red cell production. Heavy smokers can have haematocrit values several percentage points above normal. Cessation leads to gradual normalisation over months.
Obstructive sleep apnoea
Repeated nocturnal oxygen desaturation stimulates EPO production, raising red cell count. An important and underdiagnosed cause in overweight or obese middle-aged men. CPAP therapy normalises haematocrit within months in most cases.
Chronic lung disease (COPD, emphysema)
Chronic hypoxia from impaired gas exchange triggers a compensatory rise in EPO. Haemoglobin can reach very high levels in advanced COPD, making the blood viscous and increasing clot risk.
Living at high altitude
Lower atmospheric oxygen at altitude stimulates EPO production. This is a normal physiological adaptation. People permanently residing at high altitude (such as the Andes or Himalayan regions) have constitutionally higher red cell counts. Usually not clinically significant for Australians who visit briefly.
Testosterone therapy / anabolic steroids
Testosterone directly stimulates red cell production and is a common cause of significant polycythaemia in men on TRT or those using anabolic steroids. Regular haematocrit monitoring is mandatory during testosterone therapy. Dose adjustment or temporary cessation normalises the count.
Polycythaemia vera (PV)
A clonal myeloproliferative neoplasm caused by JAK2 mutation. The bone marrow overproduces red cells (and often white cells and platelets) independent of EPO. Diagnosed by the WHO criteria: elevated haemoglobin or haematocrit, JAK2 V617F mutation, and characteristic bone marrow changes. Increases thrombosis risk substantially.
EPO-secreting tumours
Renal cell carcinoma, hepatocellular carcinoma, cerebellar haemangioblastoma, uterine fibroid, and phaeochromocytoma can all produce EPO autonomously. If EPO is elevated and there is no obvious respiratory or other cause, imaging of the kidneys and liver is warranted.
Congenital erythrocytosis (EPO receptor mutations, high-affinity haemoglobin)
Inherited conditions affecting EPO signalling or haemoglobin oxygen release. Typically presents in younger patients with lifelong polycythaemia without other myeloproliferative features and a negative JAK2. Specialist haematology involvement is required.
Apparent polycythaemia (stress / Gaisböck syndrome)
Reduced plasma volume due to chronic hypertension, diuretic use, obesity, and stress — the red cell mass is actually normal, but haematocrit appears elevated because less fluid is present. Common in middle-aged men. Improving hydration, reducing alcohol and body weight, and addressing hypertension often resolves it.
Symptoms of Polycythaemia
Symptoms depend on the severity and cause. Mild relative polycythaemia from dehydration usually causes no symptoms at all. Polycythaemia vera can produce a characteristic array of symptoms related to blood hyperviscosity and bone marrow overactivity.
Headache and dizziness
Caused by increased blood viscosity and the resulting reduction in cerebral blood flow. Often described as a persistent heaviness or pressure in the head, particularly in people with polycythaemia vera.
Facial flushing and ruddy complexion
A plethoric (congested, reddish) facial appearance is a classic feature of polycythaemia vera and severe secondary polycythaemia. The palms, lips, and mucous membranes may also appear deep red.
Itching after a hot shower (aquagenic pruritus)
A distinctive symptom of polycythaemia vera — intense itching without a visible rash triggered by contact with warm or hot water. Thought to be related to mast cell activation. Highly specific for PV when present.
Visual disturbances
Blurring, flickering lights, or transient visual loss due to sluggish blood flow in retinal vessels. Should prompt urgent assessment — can be an early warning of thrombotic complications.
Blood clots (DVT, PE, stroke, heart attack)
The most serious complication of polycythaemia vera and severe secondary polycythaemia. Thick, viscous blood moves more slowly, predisposing to clot formation. PV is associated with unusually sited clots including portal vein and hepatic vein thrombosis.
Fatigue and weakness
Paradoxically, polycythaemia causes fatigue despite high oxygen-carrying capacity — thought to be related to hyperviscosity, splenomegaly, and inflammatory mediators in PV.
Splenomegaly (enlarged spleen)
The spleen enlarges to help process the excess red cells in polycythaemia vera. Can cause left-sided abdominal discomfort and early satiety (feeling full quickly when eating).
No symptoms (incidental finding)
Mild secondary polycythaemia — from dehydration, smoking, or mild sleep apnoea — often produces no symptoms and is found on a routine blood test. The underlying cause may also be asymptomatic at first.
Red Flags — When to See Your GP Promptly
Haematocrit above 52% in men or 48% in women
These are the diagnostic thresholds used in Australian haematology practice. Values this high significantly increase blood viscosity and thrombotic risk. Prompt GP referral for investigation is required.
New blood clot (DVT, PE, stroke, TIA)
A blood clot in the context of polycythaemia is a medical emergency. Polycythaemia — particularly PV — should always be considered as a possible contributing cause in any unexplained clotting event, especially in younger patients.
Positive JAK2 mutation result
A positive JAK2 result confirms a clonal blood disorder (usually polycythaemia vera) and requires urgent haematology referral for formal diagnosis, risk stratification, and initiation of treatment to prevent thrombosis.
Aquagenic pruritus (itch after hot shower) without another explanation
This distinctive symptom, in the context of elevated haemoglobin, is highly suggestive of polycythaemia vera and should trigger JAK2 testing.
Haemoglobin rising on serial blood tests
A steadily climbing haemoglobin over months, without a clear secondary cause such as starting testosterone therapy, warrants investigation for a primary bone marrow cause.
Splenomegaly found on examination
An enlarged spleen alongside polycythaemia strongly suggests a myeloproliferative neoplasm such as PV. Urgent haematology referral is required.
The Diagnostic Workup
Investigating polycythaemia follows a logical sequence designed to separate common, benign causes from the rarer but important primary bone marrow disorders.
Assess hydration and repeat the FBC
The first step is always to determine whether the patient is well hydrated at the time of testing. A repeat FBC after drinking adequate fluids can resolve apparent polycythaemia from dehydration without any further investigation.
History of smoking, alcohol, and sleep apnoea
A careful lifestyle and symptom history is informative. Loud snoring, witnessed apnoeas, excessive daytime sleepiness, or collar size above 43 cm in men should prompt a referral for sleep studies. Smoking history is always relevant.
Oxygen saturation (pulse oximetry)
A simple, painless measure of blood oxygen levels at the fingertip. A consistently low reading (below 94%) suggests a respiratory cause for EPO-driven polycythaemia. Overnight pulse oximetry can detect nocturnal desaturation from sleep apnoea.
Serum EPO level
Erythropoietin is the hormone that drives red cell production. In polycythaemia vera, EPO is typically suppressed (the bone marrow is producing cells autonomously). In secondary polycythaemia, EPO is normal or elevated. This is a key test in distinguishing primary from secondary erythrocytosis.
JAK2 V617F mutation testing
This blood test detects the genetic mutation present in more than 95% of people with polycythaemia vera. It is the most important test when PV is suspected. A positive result combined with elevated haematocrit and characteristic bone marrow changes meets WHO criteria for PV diagnosis.
Red cell mass studies (nuclear medicine)
When it is unclear whether polycythaemia is true (absolute) or apparent (relative), formal red cell mass measurement using radioactively labelled red cells gives the definitive answer. Available at major Australian hospitals.
Imaging and haematology referral
Abdominal ultrasound checks for splenomegaly and renal or hepatic masses. CT of the abdomen and pelvis may be needed if an EPO-secreting tumour is suspected. Any patient with confirmed absolute polycythaemia or positive JAK2 is referred to a haematologist for formal management.
Treatment — Matched to the Cause
Treating the secondary cause
For secondary polycythaemia, treating the underlying cause is the primary goal. CPAP therapy for sleep apnoea, smoking cessation, adjusting testosterone dose, treating COPD, or addressing the EPO-producing tumour all lead to gradual normalisation of the red cell count over weeks to months. Regular monitoring FBCs confirm improvement.
Venesection (phlebotomy)
The cornerstone of treatment for polycythaemia vera. Removing 450–500 mL of blood (a unit donation) at regular intervals rapidly reduces haematocrit. In PV, the target is haematocrit below 45% for men and below 42% for women — these thresholds have been shown to substantially reduce thrombotic events. Initially weekly or fortnightly, then monthly for maintenance. The procedure takes about 30 minutes in a day hospital or haematology clinic.
Low-dose aspirin
Aspirin 100 mg daily is recommended for most patients with polycythaemia vera to reduce the risk of blood clots. It reduces platelet activation and microvascular complications. Aspirin is not indicated for secondary polycythaemia in the absence of other indications, unless thrombotic risk is otherwise elevated.
Cytoreductive therapy
For high-risk PV patients (age above 60 or prior thrombotic events), medications that reduce bone marrow activity are used alongside venesection. In Australia, hydroxyurea (hydroxycarbamide) is the most commonly used agent. Interferon-alfa is preferred in younger patients and those who are pregnant. Ruxolitinib (a JAK inhibitor) is available for patients who do not respond to or cannot tolerate hydroxyurea.
Lifestyle modifications
For all forms of polycythaemia: maintain adequate hydration (especially in hot weather), stop smoking, achieve and maintain a healthy weight, use CPAP if sleep apnoea is present, limit alcohol, and stay active. These measures reduce overall cardiovascular and thrombotic risk regardless of the specific cause.
Practical Prevention and Monitoring Tips
Stay well hydrated
Drink at least 2 litres of water daily and more during hot weather or exercise. Dehydration is the most common cause of an artificially high haematocrit reading on blood tests.
Stop smoking
Smoking is the most modifiable common cause of true polycythaemia. Every cigarette raises carbon monoxide levels and drives EPO production. Cessation support is available through your GP, Quitline (13 78 48), and nicotine replacement therapy.
Treat sleep apnoea
If you snore heavily or wake unrefreshed, discuss a sleep study with your GP. Effective CPAP use eliminates nocturnal hypoxia and often normalises haemoglobin within 3–6 months.
Monitor testosterone therapy
Men on TRT should have their haematocrit checked every 3–6 months. If haematocrit exceeds 54%, dose reduction or a therapeutic venesection is usually recommended.
Know your clot symptoms
If you have polycythaemia vera, learn the signs of DVT (swollen, painful calf), PE (sudden breathlessness, chest pain), TIA (facial droop, arm weakness, speech change). Seek emergency care immediately.
Avoid iron supplementation in PV
Iron supplementation stimulates red cell production and can worsen polycythaemia vera. Most PV patients become iron deficient through repeated venesection — this is intentional and is managed by the haematologist.
Related Reading
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This page provides general educational information about high red blood cell count and polycythaemia. It is not a substitute for professional medical advice, diagnosis, or treatment. Always consult your GP about abnormal blood test results — they have access to your full medical history and can interpret your results in context. SmarterBlood does not provide medical care.