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Undergoing venesection

Patient information A-Z

This leaflet explains what venesection is and describes the risks and benefits of having this treatment. It also tells you what to expect when you come to your appointment. You are welcome to bring someone with you to keep you company.

What is a venesection and what does it do?

Venesection is a procedure to reduce red blood cells. This is the simplest and quickest way of reducing the number of red cells in your blood. It will reduce the amount of blood in your body by removing about one pint (half a litre) of blood at a time. It is similar to the procedure used for donating blood.

Why is it necessary to have a venesection?

Having a venesection takes away cells and liquid. The body adds liquid blood faster than red blood cells so effectively thinning the blood and so reducing the risk of clots (thrombosis).

How often will this be needed?

This will be different for each person. Once your condition is under control, you may need it every six to 12 weeks. By having a blood test one week later, following the test, it will show how effective it has been.

How is a venesection done?

Before it is carried out your brief medical history will be taken. Then your blood pressure will be taken. If your blood pressure is too low we do not carry out the procedure. A tourniquet is applied to your arm in the same way as if you were having a blood sample taken. The needle, which is already fixed to the blood collection bag, is inserted into your vein. This is left fixed in place with tape throughout the collection, until being removed at the end of the procedure.

Before your appointment

Have your breakfast as usual and please have plenty of fluids to drink (not alcohol). You do not need to bring anything in particular with you. There is no reason why you should not drive or continue with normal activities before and after the test. Remember to bring sufficient money to cover the cost of parking and refreshments etc. There is a café just outside the clinic.

How to find us

The venesection room is situated in the blood testing clinic, in the Addenbrooke’s Treatment Centre (ATC) on the ground floor. Please visit the ATC reception if you need further guidance.

At your appointment

Your stay should be about half an hour in total. On arrival in the blood test clinic please sit outside room 4. You will then be called in to room 4 for the procedure to be carried out. The procedure itself lasts for about ten to fifteen minutes. When finished the needle is removed, pressure is applied for a few minutes and a small dressing put on. You will be offered a glass of water and asked to rest in the chair. Your blood pressure will be taken again to make sure you are safe to leave.

What are the risks and benefits?

When the needle is inserted it may feel uncomfortable for a few seconds. When the needle is removed it may bleed a little. Pressure will be applied by holding the site firmly for a few minutes. If you are taking tablets that thin the blood (like Warfarin), pressure will be applied for five to 10 minutes. We may ask you to stay a little longer to check there is no bleeding, once pressure has been removed. There may also be some bruising which may be slightly uncomfortable. It is advisable to rest for a few hours following the procedure. Having a venesection is a very safe, simple and quick treatment for removal of an excess of red blood cells.

Are there any side effects?

Most people carry on as normal after they have left the unit. Following the removal of the blood you may feel a little dizzy because your blood pressure may be low; this is not unusual and can be minimised by resting before you leave the department and having something to drink. Some people may feel a little washed out for a couple of days. You might experience some bruising at the site. Otherwise there are no side effects. Avoid smoking or drinking alcohol for at least an hour afterwards. For 24 hours do not carry anything heavy and avoid vigorous sport, exercise, or heavy work.

Following venesection

After your appointment please have a full blood count test one week later to check how effective the procedure has been. This can either be done at your GP surgery or you can come back to the blood testing clinic.

We are smoke-free

Smoking is not allowed anywhere on the hospital campus. For advice and support in quitting, contact your GP or the free NHS stop smoking helpline on 0800 169 0 169.

Other formats

Help accessing this information in other formats is available. To find out more about the services we provide, please visit our patient information help page (see link below) or telephone 01223 256998.

Contact us

Cambridge University Hospitals
NHS Foundation Trust
Hills Road, Cambridge

Telephone +44 (0)1223 245151