Blood tests: What happens during a blood test and what can the results show?

Man having a blood test

Blood tests can be a simple way of getting more information about what’s going on inside your body. But what’s involved? Senior Cardiac Nurse Philippa Hobson explains.

Why would I have a blood test?

Blood tests are usually done to check how your body copes with illness, injury, inflammation, infection and some types of medication. Blood needs a very precise balance in order for your body to work well. If blood test results are abnormal, it gives the doctors a good indication of how to treat you or prevent problems occurring in the future.

Where would I have a blood test?

This may be done in hospital or by your GP or practice nurse. If you’re having an NHS health check, your blood test is usually a quick finger prick test that looks at the blood’s glucose and cholesterol levels. If the results are higher than recommended, you may be asked to go to your hospital for a full blood test.

How will my blood be taken?

A rubber strap called a tourniquet is wrapped tightly around your upper arm – this helps to bring the vein that runs inside your elbow to the surface, so that it is easier to find.

The skin around the vein is cleaned with a sterile alcohol wipe. A small needle is inserted into the vein and a small blood bottle is then attached.

The blood will flow into the bottle and when the bottle (or bottles) is full the needle is removed. If your veins are fragile or difficult to find, sometimes a different type of needle called a ‘butterfly’ (because it has plastic ‘wings’ on either side) is inserted into a small vein in the back of your hand.

After the needle is removed, a piece of gauze will be put over the puncture site and you will be asked to hold it there for a few minutes. Bleeding from a vein usually stops very quickly, but may take a few minutes if you take blood-thinning medication.

What happens to my blood after the test?

Usually, the blood bottles contain small amounts of a chemical to stop your blood clotting in the tube, so it can be measured accurately in the lab. Each bottle is labelled with your name, date of birth and hospital number, when the blood was taken, and has a different coloured top according to the type of test. Then they are taken to be analysed.

Empty vials for collecting blood during a blood test

What will my blood be tested for?

Blood tests can be used for many different things, including to check cholesterol and blood glucose levels. These help monitor your risk of heart and circulatory diseases and diabetes, or how your condition is being managed.

Tests for different chemicals and proteins can indicate how your liver or kidneys are working. A test for troponin can help to diagnose a heart attack, and a test for brain natriuretic peptide (BNP) can help diagnose heart failure.

If you take warfarin, your INR level (a measure of how quickly your blood will clot) will be tested regularly to make sure that you are prescribed the correct dose.

Do I need to do anything to prepare for a blood test?

Occasionally you may need to fast (not eat) before your blood test, but your doctor will let you know if this is required. Unless you have been told not to, have a drink of water before your test. If you’re dehydrated, it can be more difficult to find your vein.

How long does it take to get blood test results?

This partly depends on the urgency of the test. Most tests done at your GP surgery are ‘routine’, meaning there is no urgency, so it may take a few days to get the results. If the people reporting on the blood results see anything they are worried about, they will contact your doctor or nurse and the surgery will get in touch.

Will I need other tests too?

Sometimes the results of blood tests mean that the doctor may want to do other tests, such as a scan on your heart or kidneys, or an electrocardiogram (ECG).

I am scared of having a blood test. What can I do?

Many people feel like this and it is nothing to be ashamed of. Tell the person who is doing your blood test – this will help them to look after you better.

You could also ask someone to come with you and distract you during the test. Some people also find breathing for relaxation helpful. If the problem is that you faint at the sight of blood, this is caused by a fall in blood pressure: you can learn techniques to stop this from happening.

Some clinics can use a cream that numbs the area where the needle goes in. You should let them know in advance if this would help, as the cream may need to be applied at home in order to work properly. Unfortunately, there is no ‘needle-free’ way of obtaining blood.

More useful information