What is a blood test?

Blood tests are one of the most common types of medical test. They have many uses, including assessing your general health, checking if you have an infection, seeing how well specific organs are functioning and screening for certain genetic conditions.

What is a blood test?

A blood test is when a sample of blood is taken out of a vein in your arm. This is done by a nurse at your doctor's practice, or in a laboratory by a phlebotomist. A phlebotomist is a medical laboratory technician trained to collect blood and samples from people for testing in a laboratory.

How is a blood test done?

First, a band is put around your upper arm to make the veins below expand. This makes it easier to draw blood from them. Your arm may feel tight for the short time the band is applied.

The injection site is then cleaned with an alcohol swab.

You may feel a brief sting or pricking as the needle is inserted. A tube will be attached to the needle for the blood to be collected in. Depending on how many blood tests you are having, you may need more than one vial of blood to be collected while the needle and tube are in place.

Image credit: Canva

What if I feel nervous about getting a blood test? 

Some people feel nervous about having blood taken. However, the person taking the sample is trained in this procedure, and it is nearly always straightforward.

If you feel squeamish or nervous, it can be better not to watch while your blood is being taken. If you feel faint or lightheaded before, during or after the blood test, tell the nurse or phlebotomist. There should be the option for you to lie down.

If you have had this sort of reaction in the past, or if you have any other concerns, tell the person taking the blood sample before they start, if they don't ask you about this.

What happens after my blood test?

After the blood sample is taken, a cotton swab is put on the site. You may be told to hold the swab and apply pressure there for at least 3 minutes. This will stop bleeding. It also reduces the chance of a bruise developing. A plaster is applied, which can be removed after about half an hour.

After your blood test, wait for several hours before using the arm the test was done on for any heavy lifting or anything else that uses a lot of effort.

Do people have any reactions to a blood test?

Some people develop a bruise at the injection site. Any small bruise should fade after a few days.

Less often, there may be bleeding or a lump (called a haematoma) may develop at the injection site. If this happens, cover the area with a clean cotton swab or tissue and apply firm pressure to the area for 10–15 minutes. A cold pack (eg, ice cubes in a plastic bag or wrapped in a cloth) can also reduce swelling.

The injection site may be tender and a more extensive bruise may develop from a haematoma. The bruise can take up to 10 days to go away, but there are no long-term side effects from this.

If you have any ongoing concerns after the test, contact your doctor.

Learn more

Common laboratory tests – a patient's guide Family Doctor NZ

Credits: Health Navigator Editorial Team. Reviewed By: Gwenda Lawrence, medical laboratory scientist, Auckland Last reviewed: 25 Aug 2020