High Haematocrit: Causes and What to Do

Haematocrit is the proportion of blood volume taken up by red blood cells. Lab Tests Online UK, which uses the name PCV, describes the test as one that helps assess anaemia, polycythaemia and dehydration. A high value means the blood is more concentrated with cells than expected, whether because there are more cells or because there is less fluid.

That distinction is the heart of interpretation. The NHS separates apparent erythrocytosis, where the red cell count is normal but plasma is reduced, from absolute erythrocytosis, where the body really produces too many red cells. Lab Tests Online UK uses a parallel term, relative polycythaemia, for the dehydration pattern.

Haematocrit is read with the red blood cell count and haemoglobin. If all three are raised and stay raised, the picture is more convincing than one isolated high value.

Lab Tests Online UK says the most common cause of an increased PCV is dehydration. When fluid intake is adequate, it returns to normal. After that, it lists primary polycythaemia, where the bone marrow overproduces red cells, and secondary polycythaemia, where the cause lies outside the bone marrow.

Testosterone is named by Lab Tests Online UK as one of the drugs that can cause secondary polycythaemia, alongside growth hormone, EPO and diuretics. This is why a raised haematocrit is a recognised finding to watch for in anyone taking these medicines, and why the prescriber will ask about them when the result is flagged.

The NHS says apparent erythrocytosis is often caused by being overweight, smoking, drinking too much alcohol or taking certain medicines including diuretics, and that it may improve if the underlying cause is identified and managed. Stopping smoking or reducing alcohol are the examples it gives.

It is not advisable to stop a prescribed medicine on your own because of a number. Raise the result with whoever prescribed it so they can decide whether to adjust the dose, the schedule or the monitoring.

Lab Tests Online UK notes that if a high PCV is detected you may have an ultrasound scan of the kidneys and possibly blood tests called EPO and JAK2, and that in polycythaemia vera the white cells and platelets may also be raised. The NHS says a GP may refer you to a haematologist to confirm the diagnosis and determine the underlying cause.

The NHS adds that erythrocytosis is sometimes discovered only on a routine blood test done for another reason, so many people have no symptoms when it is found. When symptoms are present, they include headaches, blurred vision, red skin, dizziness and itching after a bath or shower.

• Drink normally and avoid testing straight after a period of poor fluid intake if you can.

• Take a list of every medicine and supplement, including testosterone or other hormone treatments.

• Mention smoking, alcohol, sleep problems and any lung or heart condition.

• Ask whether the full blood count will be repeated and when.

The NHS says to seek medical help immediately for signs of a clot in the leg or lung: pain, swelling, redness and tenderness in one leg, breathlessness, chest or upper back pain, coughing up blood, or feeling faint. It says to seek emergency medical help for suspected heart attack or stroke.

Treatment, when it is needed, is aimed at reducing the risk of clots and dealing with the cause. The NHS names venesection, medicines such as hydroxycarbamide or interferon in polycythaemia vera, and daily low-dose aspirin in some people.