High haemoglobin: causes and what a raised result can mean

Haemoglobin is measured as part of a full blood count. Lab Tests Online UK explains that it is the protein in red blood cells that carries oxygen around the body, and that a higher-than-normal level usually means there are too many red blood cells, a state it calls polycythaemia. The NHS uses the term erythrocytosis, sometimes called polycythaemia, for a high concentration of red blood cells in the blood.

Normal ranges vary with age, sex and, to some degree, ethnic background, and your laboratory report will show the range that applies to you. Lab Tests Online UK stresses that a result is always interpreted in the context of symptoms, medical history and other results, not on the number alone. It also says haematocrit, which is the proportion of blood made up of red cells, is more often used than haemoglobin to check for polycythaemia.

Lab Tests Online UK lists the common causes: dehydration, which resolves once you are well hydrated; smoking, because heavy smokers have higher haemoglobin than non-smokers; living at high altitude, where the body makes more red cells to compensate for lower oxygen; severe lung disease, which reduces oxygen supply and triggers more red cell production; and excess production in the bone marrow, such as polycythaemia vera.

The NHS describes three groupings. In apparent erythrocytosis the red cell count is normal but there is less plasma, making the blood thicker; it is often linked to being overweight, smoking, drinking too much alcohol or taking medicines such as diuretics. Relative polycythaemia is similar and can result from dehydration. In absolute erythrocytosis the body makes too many red blood cells, either because of a primary problem in the bone marrow (most commonly polycythaemia vera) or secondary to an underlying condition that raises erythropoietin, the hormone from the kidneys that stimulates red cell production. The NHS gives COPD and kidney problems such as a kidney cancer or narrowing of the arteries supplying the kidneys as examples.

In some cases, the NHS notes, an underlying cause cannot be identified.

• Dehydration, including from heat, illness or not drinking enough before the test.

• Smoking.

• Living at altitude.

• Lung disease such as COPD, where tissues do not receive enough oxygen.

• Kidney problems that increase erythropoietin.

• Polycythaemia vera, a rare slow-growing blood cancer of the bone marrow.

Not everyone with erythrocytosis has symptoms, but the NHS says many do. It lists headaches, blurred vision, red skin particularly on the face, hands and feet, tiredness, high blood pressure, dizziness, tummy discomfort, confusion, bleeding problems such as nosebleeds and bruising, gout and itchy skin, especially after a bath or shower. It advises seeing a GP if you have persistent symptoms.

Seek immediate medical advice for signs of a blood clot or heart attack or stroke. The NHS lists pain, swelling, redness and tenderness in one leg as possible DVT, and breathlessness, chest or upper back pain, coughing up blood, feeling lightheaded or fainting as possible pulmonary embolism. Call 999 or go to A&E if you think someone is having a heart attack or stroke.

A mildly raised result may simply be repeated. Lab Tests Online UK says your doctor will consider whether dehydration or lifestyle factors are responsible, and may request further tests if the cause is not straightforward. The NHS says erythrocytosis is diagnosed by a blood test looking at the red cell count and haematocrit, is sometimes found by chance on a routine test, and that a GP may refer you to a haematologist.

The NHS says specialist tests may include a blood test for the changed JAK2 gene and an ultrasound scan of the tummy to look for kidney problems. A doctor will also ask about smoking, alcohol, medicines and lung problems, as these can all raise red cell numbers.

Treatment aims to prevent symptoms and complications such as blood clots and to treat any underlying cause. For apparent erythrocytosis, the NHS says improving things such as stopping smoking or cutting alcohol may help. Venesection, which is removing about a pint of blood at a time, is described as the simplest and quickest way of reducing red cells and may be recommended for polycythaemia vera, previous clots or symptoms of thick blood. In polycythaemia vera, daily low-dose aspirin may be prescribed to help prevent clots, and medicines such as hydroxycarbamide or interferon may be used to slow red cell production.

The NHS says the outlook depends largely on the cause: many cases are mild, but polycythaemia vera can require long-term treatment and carries a small risk of complications such as myelofibrosis or, rarely, acute myeloid leukaemia. It also notes that healthy lifestyle changes, such as stopping smoking and managing weight and blood pressure, reduce the clot risk in all types.

High haemoglobin is usually found through a GP blood test, and repeat testing and first-line checks are done in primary care. Referral to NHS haematology follows when the result stays raised without an obvious cause. Private haematology reviews and blood tests are also available in the UK and follow the same principles.