High haemoglobin (polycythaemia or erythrocytosis) means a haemoglobin above 16.5 g/dL in men or 16 g/dL in women, or a haematocrit above 49% in men and 48% in women [2]. Usually the cause lies outside the bone marrow, such as smoking, testosterone, sleep apnoea, lung or heart disease, living at high altitude, or dehydration, which only makes the number look high [2, 3]. Less often the cause is polycythaemia vera: a slow-growing bone marrow disease with a change in the JAK2 gene that raises the risk of blood clots and is controlled with regular venesection, low-dose aspirin and, when needed, medicines [1].
Key points
- A high haemoglobin or haematocrit is usually "secondary", meaning the body's response to a lack of oxygen or the effect of a medicine or hormone; examples are smoking, testosterone, sleep apnoea, lung or heart disease and living at high altitude [2, 3].
- Dehydration, diarrhoea and vomiting or water tablets (diuretics) only reduce the fluid part of the blood and make the haemoglobin look higher than it really is [2].
- Polycythaemia vera is a slow-growing bone marrow disease; almost all patients have a change in the JAK2 gene, and its main risk is blood clots [5, 6].
- The basic treatment of polycythaemia vera is regular venesection to keep the haematocrit below 45% and low-dose aspirin; medicines are added for people at higher risk [1, 8].
- Most patients live for many years with polycythaemia vera [1].
- Sudden weakness or numbness on one side of the body, trouble speaking, chest pain or sudden shortness of breath is an emergency [18, 19, 20].
What is polycythaemia?
Red blood cells carry oxygen to all the tissues of the body [15]. Haemoglobin (the substance in red cells that carries oxygen) and haematocrit (the share of the blood volume made up of red cells) show how many red cells the blood contains. When the haemoglobin is above 16.5 g/dL in men or 16 g/dL in women, or the haematocrit is above 49% in men or 48% in women, it is called polycythaemia or erythrocytosis [2]. There are three main kinds:
- Relative (apparent) polycythaemia: the number of red cells has not gone up; only the fluid part of the blood has gone down, for example with dehydration, diarrhoea and vomiting, or water tablets (diuretics) [2, 3].
- Secondary polycythaemia: the bone marrow is healthy, but the hormone erythropoietin (a hormone that increases red cell production) is raised [4]. Causes: lack of oxygen from lung or heart disease, sleep apnoea (pauses in breathing during sleep), smoking and living at high altitude; medicines such as testosterone and some diabetes medicines (the SGLT2 group, such as empagliflozin); and rarely tumours that make this hormone, such as kidney or liver cancer [2, 3, 4]. This kind is more common than polycythaemia vera [2].
- Primary polycythaemia (polycythaemia vera): the bone marrow itself makes too many red cells, without needing this hormone [1, 2].
If only the "red cell count" (RBC) is high on your report but the haemoglobin and haematocrit are not, this is not polycythaemia [2]. A high number of small red cells is common in carriers of thalassaemia [21]; see the page Thalassaemia minor.
What is polycythaemia vera?
Polycythaemia vera is one of the "myeloproliferative neoplasms": long-term bone marrow diseases in which too many blood cells are made [1, 15]. Almost all patients have a change in the JAK2 gene [6]; in this disease too many red cells are made, and in about half of patients the white cell and platelet counts are high as well [6].
In the World Health Organization classification, polycythaemia vera is one of the blood cancers (myeloid neoplasms) [14], but it progresses very slowly: most patients live with it for many years, and in people diagnosed at a younger age this time is much longer [1]. It is mostly found in middle and older age; in a large study only 1 in 10 patients was younger than 40 [6].
The main risk of this disease is blood clots, and preventing them is the main aim of treatment; clots in the arteries (such as stroke or heart attack) are more common than clots in the veins [5]. In a large study, before or at diagnosis, about 16 in every 100 patients had had a clot in an artery and 7 a clot in a vein [6]. After many years, in a small number of patients the bone marrow becomes scarred (myelofibrosis), and rarely the disease turns into acute leukaemia: over 20 years, about 1 in 6 patients develop myelofibrosis and about 1 in 25 develop acute leukaemia [1]. Read more: What are myeloproliferative neoplasms?
Symptoms
Polycythaemia vera often causes no symptoms at first and is found on a routine blood test [15]. Possible symptoms [15]:
- Headache, dizziness or weakness
- Double vision, or seeing dark or blind spots that come and go
- Itching all over the body, especially after a bath or a warm shower
- A red face that looks like a blush or sunburn
- A feeling of pressure or fullness below the ribs on the left side (from an enlarged spleen)
- Weight loss for no known reason
Itching after contact with water is common in polycythaemia vera: in one study, about 2 in 3 patients had it, and in most of them it had started on average about 3 years before diagnosis [7]. In another large study, an enlarged spleen and itching were each seen in about one-third of patients [6]. If you have this kind of itching together with a high haemoglobin, tell your doctor.
Which tests?
The doctor usually goes step by step [2, 3]:
- Repeating the CBC and comparing it with earlier results: has the haemoglobin always been high, or has it risen recently [3]?
- Asking about secondary causes: smoking, medicines (testosterone, water tablets, diabetes medicines of the SGLT2 group), lung or heart disease, sleep apnoea and living at high altitude [2, 4].
- A JAK2 gene test on a blood sample [2, 3].
- Erythropoietin level in the blood: it is below normal in most patients with polycythaemia vera [6]; secondary polycythaemia is due to a rise in this hormone [4], although the level does not always help with the diagnosis [3].
- Other tests: a blood film, kidney and liver tests and ferritin (iron stores) [2]; if needed, tests for sleep apnoea or lung disease [5], and sometimes a bone marrow sample [1].
If the erythropoietin level is low or normal and no secondary cause has been found, referral to a haematologist is needed [2]. To read the rest of your results, see Understanding your CBC results.
For physicians
| Criterion | Guideline wording |
|---|---|
| Major criterion 1 (ICC 2022) | Haemoglobin above 16.5 g/dL in men and above 16.0 g/dL in women, or haematocrit above 49% in men and above 48% in women, or red cell mass more than 25% above the mean normal predicted value [13] |
| Major criterion 2 | JAK2 V617F or JAK2 exon 12 mutation [13] |
| Major criterion 3 | Bone marrow biopsy showing age-adjusted hypercellularity with trilineage proliferation (panmyelosis), including prominent erythroid and granulocytic proliferation and increased pleomorphic, mature megakaryocytes without atypia [13] |
| Minor criterion | Subnormal serum erythropoietin [13] |
| Diagnostic rule | All 3 major criteria, or major criteria 1 and 2 plus the minor criterion [13] |
| Biopsy exception | In sustained absolute erythrocytosis (haemoglobin above 18.5 g/dL in men or above 16.5 g/dL in women and haematocrit above 55.5% or 49.5%) with a JAK2 V617F or exon 12 mutation, a biopsy may not be required [13] |
| WHO-HAEM5 | Major criteria: raised haemoglobin and/or haematocrit, panmyelosis with pleomorphic mature megakaryocytes in the bone marrow, and JAK2 V617F or exon 12 mutation; measurement of red cell mass with chromium-51 has been removed from the criteria [14] |
| Working diagnosis (Tefferi and Barbui) | JAK2 mutation with haemoglobin/haematocrit above 16.5 g/dL/49% in men or 16 g/dL/48% in women; bone marrow examination is advised but not mandated [1] |
| Findings in 1,545 patients | Median age 61 years; JAK2 V617F 95% and other JAK2 mutations 3%; subnormal erythropoietin 81%; leucocytosis (above 10.5 × 10⁹/L) 49%; thrombocytosis (450 × 10⁹/L or more) 53%; palpable spleen 36%; pruritus 36%; arterial thrombosis before or at diagnosis 16% and venous 7.4% [6] |
| Erythrocytosis without a JAK2 mutation | First exclude PV with JAK2 mutation screening (exons 12 to 15) and consider apparent or relative polycythaemia; review the history of Hb/Hct; EPO levels are variably informative; acquired causes: central (cardiopulmonary disease) or peripheral (renal artery stenosis) hypoxia, EPO-producing tumours (renal cell carcinoma) and drugs (testosterone, SGLT2 inhibitors, ESAs); hereditary in long-standing erythrocytosis with a family history [3] |
| Initial assessment (CMAJ 2020) | CBC, blood film, kidney and liver tests, ferritin, serum EPO and JAK2 V617F, followed by JAK2 exon 12 if negative; refer when EPO is low or normal and the work-up for secondary causes is negative [2] |
Treatment
Treatment regimens and doses: index5 regimens
Secondary polycythaemia
The main treatment is treating the cause, and venesection is not routinely recommended [2]:
- Stop smoking [2, 5].
- Sleep apnoea should be treated with a night-time breathing machine (CPAP); this treatment reduces the polycythaemia [5].
- In lung disease, a chest specialist decides about oxygen therapy; in these patients, lowering the haematocrit too far is harmful [5].
- If you take testosterone or similar medicines, your doctor may change or stop it [2]; injections, higher doses and older age raise the risk [4].
- In people who stopped an SGLT2 diabetes medicine, the polycythaemia improved or went away [4]; but do not stop any medicine on your own, and leave the decision to your doctor.
Polycythaemia vera
Treatment of polycythaemia vera
- 1
Regular venesection (phlebotomy)
As with blood donation, some blood is taken each time so that the haematocrit stays below 45% [1, 11]. In a large study, in people whose haematocrit was kept below 45%, deaths from heart and blood vessel disease and major clots occurred about a quarter as often as in those whose haematocrit was 45 to 50% [8].
- 2
- 3
Medicines that reduce blood cell production
For people at higher risk, that is, over 60 or with a history of clots, a medicine is added: hydroxyurea capsules or interferon injections under the skin [1]. In people at lower risk a medicine is sometimes needed too; for example, when they cannot tolerate venesection or need it very often, the spleen gets bigger, the white cell or platelet count becomes very high, or symptoms are troublesome; for these people interferon is recommended [10]. If hydroxyurea does not work or causes side effects, interferon or ruxolitinib tablets are used instead [10, 12].
If itching bothers you, tell your doctor; controlling the blood counts usually helps, and in itching that has not improved with hydroxyurea, ruxolitinib has been effective [5].
For physicians
Risk stratification and basic treatment (Tefferi and Barbui, 2023). High risk: age over 60 years or a history of thrombosis; low risk: neither. Phlebotomy to a haematocrit below 45% plus aspirin 81 mg once or twice daily (absent contraindications) is the backbone of treatment in all risk groups; cytoreduction for high-risk disease, with hydroxyurea or pegylated interferon-α first line and busulfan or ruxolitinib second line; systemic anticoagulation with a history of venous thrombosis. Median survival about 15 years, and more than 35 years at age 40 or younger; 20-year risks of thrombosis, post-PV MF and AML about 26%, 16% and 4% [1].
CYTO-PV. 365 patients with JAK2-positive PV on phlebotomy, hydroxyurea or both: haematocrit target below 45% vs 45 to 50%; after a median follow-up of 31 months, cardiovascular death or major thrombosis in 2.7% (5 of 182) vs 9.8% (18 of 183); HR for the high-haematocrit group 3.91 (95% confidence interval 1.45 to 10.53; P=0.007) [8]. ECLAP. 518 patients, aspirin 100 mg daily vs placebo: composite of non-fatal myocardial infarction, non-fatal stroke, pulmonary embolism, major venous thrombosis or cardiovascular death RR 0.40 (0.18 to 0.91; P=0.03); major bleeding not significantly increased (RR 1.62; 0.27 to 9.71) [9]. BSH: low-dose aspirin (75, 81 or 100 mg) for all; a proton pump inhibitor over 75 years of age or with a high bleeding risk; screen for hypertension, hyperlipidaemia, diabetes and smoking [5].
ELN 2021 (cytoreduction). In patients younger than 60 without previous thrombosis, start cytoreduction with at least one of: strictly defined intolerance to phlebotomy, symptomatic progressive splenomegaly, persistent leucocytosis (above 15 × 109/L), progressive leucocytosis (at least a 100% increase with a baseline below 10 × 109/L or at least 50% with a baseline above 10 × 109/L), extreme thrombocytosis (above 1500 × 109/L), inadequate haematocrit control requiring phlebotomies, persistently high cardiovascular risk or persistently high symptom burden; recombinant interferon alfa (ropeginterferon alfa-2b or peginterferon alfa-2a) is the recommended cytoreductive treatment in this group; in patients on hydroxyurea who need a change of therapy, interferon alfa or ruxolitinib [10]. Low-PV (127 low-risk patients aged 18 to 60): ropeginterferon alfa-2b 100 µg every 2 weeks plus phlebotomy vs phlebotomy and aspirin; response (median haematocrit 45% or less without progression over 12 months) 84% (42 of 50) vs 60% (30 of 50); absolute difference 24% (7 to 41%; p=0.0075) [11]. RESPONSE (222 patients with an inadequate response to or intolerance of hydroxyurea, phlebotomy-dependent with splenomegaly): primary end point (haematocrit control to week 32 and at least a 35% reduction in spleen volume) 21% with ruxolitinib vs 1%; haematocrit control 60% vs 20%; complete haematological remission 24% vs 9%; at least a 50% reduction in total symptom score 49% vs 5%; herpes zoster 6% vs 0% [12].
BSH 2019: specific situations and secondary erythrocytosis. Acute thrombosis: treat according to current guidelines, cytoreduction to control the counts and venesection to a haematocrit below 0.45 if not controlled; indefinite anticoagulation for unprovoked venous thrombosis (depending on bleeding risk). Surgery: pre-operative planning with a haematologist and standard thromboprophylaxis. COPD: erythrocytosis (usually haematocrit above 0.55) in 6 to 8% and OSA in 1.7 to 8%; refer for oxygen therapy or CPAP; with hyperviscosity symptoms or a haematocrit above 0.56, venesection to 0.50 to 0.52 (a haematocrit below 0.45 is harmful in these patients). Post-renal transplant erythrocytosis (5 to 20%): ACE inhibitor or ARB; venesection only for persistent symptoms, target 0.50. Idiopathic erythrocytosis: cytoreduction is not indicated; venesection in selected cases with an arbitrary target below 0.55 (below 0.45 if thrombosis is related) [5]. Drugs: erythrocytosis in up to 66.7% of men on testosterone (more with intramuscular forms, higher doses and older age) and thromboembolic events in up to 2.7%; with SGLT2 inhibitors 2.1 to 22%, with improvement after stopping the drug [4].
Treatment of polycythaemia vera: doses
Phlebotomy + low-dose aspirin
All patients with PV (low and high risk), in the absence of a contraindication to aspirin
All patients with PV (low and high risk), in the absence of a contraindication to aspirin
| Drug | Dose | Route | Schedule |
|---|---|---|---|
| Phlebotomy | 300 mL per session (in the Low-PV trial) | Until the haematocrit is below 45% and to keep it there | |
| Aspirin | 81 mg once or twice a day (ECLAP: 100 mg once a day) | Oral | Daily |
Haematocrit target below 45% [8]; aspirin 81 mg once or twice daily is the backbone of treatment in all risk groups [1]; doses of 75, 81 or 100 mg are used in different countries, and a proton pump inhibitor should be given over 75 years of age or with a high bleeding risk [5]. The volume of each phlebotomy in Low-PV was 300 mL [11].
Hydroxyurea
High-risk PV (age over 60 years or history of thrombosis): first-line cytoreductionCycle length: Continuous
High-risk PV (age over 60 years or history of thrombosis): first-line cytoreduction
| Drug | Dose | Route | Schedule |
|---|---|---|---|
| Hydroxyurea | Oral | Daily |
Peginterferon alfa-2a
First-line cytoreduction, especially in patients younger than 60; an alternative to hydroxyurea
First-line cytoreduction, especially in patients younger than 60; an alternative to hydroxyurea
| Drug | Dose | Route | Schedule |
|---|---|---|---|
| Peginterferon alfa-2a | Subcutaneous |
ELN 2021: recombinant interferon alfa (ropeginterferon alfa-2b or peginterferon alfa-2a) is the recommended cytoreductive treatment in patients younger than 60 without previous thrombosis who meet a criterion for starting cytoreduction [10]; in first-line treatment of high-risk PV it is also one of the two drugs of choice [1]. Dose details in the full text of the article.
Ropeginterferon alfa-2b
Adults with PV (FDA label)Cycle length: Every 2 weeks
Adults with PV (FDA label)
| Drug | Dose | Route | Schedule |
|---|---|---|---|
| Not previously on hydroxyurea | |||
| Ropeginterferon alfa-2b | Start 100 µg; increase by 50 µg every 2 weeks up to a maximum of 500 µg | Subcutaneous | Every 2 weeks |
| Switching from hydroxyurea | |||
| Ropeginterferon alfa-2b | Start 50 µg together with hydroxyurea; increase by 50 µg every 2 weeks up to a maximum of 500 µg | Subcutaneous | Every 2 weeks; reduce hydroxyurea by 20 to 40% every 2 weeks during weeks 3 to 12 and stop it by week 13 |
CBC every 2 weeks during dose titration and every 3 to 6 months during maintenance; rescue phlebotomy may be needed during titration. Boxed warning: interferon alfa products may cause or aggravate fatal or life-threatening neuropsychiatric, autoimmune, ischaemic and infectious disorders; monitor closely with periodic clinical and laboratory evaluations [16]. Low-PV: fixed dose of 100 µg every 2 weeks plus phlebotomy in low-risk patients [11].
Ruxolitinib
PV with an inadequate response to or intolerance of hydroxyureaCycle length: Continuous
PV with an inadequate response to or intolerance of hydroxyurea
| Drug | Dose | Route | Schedule |
|---|---|---|---|
| Ruxolitinib | 10 mg twice a day (extended-release form: 22 mg once a day) | Oral | Daily |
Doses follow the trial protocol, guideline or official drug label (source given in the notes of each regimen). Before prescribing, check blood counts, renal and hepatic function, drug interactions and the patient’s condition, and adjust the dose according to the drug label.
Pregnancy and everyday life
- Heart and blood vessel risk factors: keep your blood pressure, cholesterol and blood sugar under control and do not smoke; your doctor checks these factors carefully in polycythaemia vera [5].
- Iron supplements: take iron supplements only on the advice of a haematologist [5].
- Surgery: before any operation, tell the surgeon that you have polycythaemia vera; in this disease both bleeding and clotting are more likely during surgery, and a haematologist should control the blood counts and plan the care beforehand [5].
- Pregnancy: plan a pregnancy with a haematologist and an obstetrician. Hydroxyurea should be stopped at least 3 months before conception (in both women and men); if a medicine is needed in pregnancy, interferon is considered, and low-dose aspirin is continued throughout pregnancy and after delivery [5].
When should I see a doctor urgently?
Call your local emergency number straight away if you have signs of a stroke, a heart attack or a clot in the lungs:
- Sudden numbness or weakness of the face, arm or leg, especially on one side of the body; sudden confusion or trouble speaking or understanding speech; sudden trouble seeing; dizziness or loss of balance; or a sudden severe headache with no known cause [18]
- Chest pain or discomfort, shortness of breath, feeling weak or faint, or pain in the jaw, neck, back or arm [19]
- Shortness of breath, a fast or irregular heartbeat, chest pain that gets worse with a deep breath or coughing, or coughing up blood [20]
See a doctor as soon as possible if an arm or leg is swollen, painful, warm or red; this may be a clot in a vein [20]. Read more about blood clots: Blood clots in the leg and lungs.
See a doctor in the next few days if your haemoglobin or haematocrit has been high on two tests, especially with itching after a bath, a red face or headaches [2, 15].
Frequently asked questions
Is a haemoglobin of 17 or 18 dangerous?
This is above the normal range in both men and women, but how dangerous it is depends on the cause [2]. The doctor usually repeats the test, looks for causes such as smoking, medicines, sleep apnoea or lung disease, and if needed asks for a JAK2 test and an erythropoietin level [2, 3]. If the cause is polycythaemia vera, keeping the haematocrit low reduces the risk of clots [8].
Is giving blood a good way to lower haemoglobin?
Therapeutic venesection should be prescribed and supervised by a doctor. In polycythaemia vera it is a main part of treatment [1], but in secondary polycythaemia it is not routinely recommended [2], and in lung disease lowering the haematocrit too far is even harmful [5]. So first find the cause with your doctor.
Does smoking raise haemoglobin?
Do testosterone injections raise haemoglobin?
Yes. In studies, up to 2 in 3 men taking testosterone developed polycythaemia; the risk was higher with injections, higher doses and older age [4]. Other androgens (male hormones, such as some muscle-building drugs) were also studied [4]. If you take testosterone and your haemoglobin is high, your doctor decides whether to change or stop it [2].
Is polycythaemia vera cancer? How long can you live with it?
In the World Health Organization classification, polycythaemia vera is one of the bone marrow neoplasms (blood cancers) [14], but it progresses very slowly. Most patients live for many years, and in people diagnosed at age 40 or younger this time is usually more than 35 years [1]. Controlling the haematocrit and taking aspirin reduce the main risk, blood clots [8, 9].
Do high altitude or dehydration also raise haemoglobin?
Yes. Living at high altitude is one of the causes of secondary polycythaemia [2]. Dehydration, diarrhoea and vomiting or water tablets also reduce the fluid part of the blood and make the haemoglobin look higher than it really is (relative polycythaemia) [2]; the doctor considers this before specialised tests [3].
Related links
Related blood disorders
References
21 referencesShow all references, 1 with a study cardClose the reference list
Numbers in the text, such as [1], refer to these sources. Sources with a “Study card” button are summarised in the site’s evidence library.
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- 7Aquagenic pruritus in polycythemia vera: characteristics and influence on quality of life in 441 patients
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- 16BESREMi (ropeginterferon alfa-2b-njft) injection — Prescribing Information
- 17JAKAFI (ruxolitinib) tablets and JAKAFI XR extended-release tablets — Prescribing Information
- 18
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- 21Evaluation of microcytosis
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