Blood tests

People have blood tests so that their blood can be checked in a laboratory. The most common blood tests involve checking the cells in your blood including the red blood cells, white blood cells and platelets.

By checking the blood cells, medical professionals at Great Ormond Street Hospital (GOSH) will be able to see if you have an infection or certain conditions including anaemia or leukaemia.

People also have blood tests to check whether their medications are working or to identify problems with their blood chemistry. People who have diabetes often have to have blood tests to check the level of sugar in their blood.

How it works

A health professional will use a needle to take a small amount of blood. He or she will use an antiseptic wipe to clean the area first of all. The needle will be inserted into either the inside of your elbow, wrist or back of your hand, sometimes you may be offered cream or spray to anaesthetise the area, though it shouldn’t hurt much anyway.

After the blood has been taken you will be asked to put pressure on the area where the blood has been taken from. This is to stop bruising on your arm. He or she will then give you a plaster (or bandage if you are allergic) to put on your arm.

If you feel dizzy

If you feel faint, weak or dizzy during the blood test, tell whoever is taking the blood. Only a very small amount of blood is taken so it is very unlikely for you to get ill from loss of blood.

Eating before a blood test

Unless you have been told that you should not eat before your blood test you are able to eat and drink what you like. Sometimes you may be asked to not eat before the test, though they will often arrange this for the morning so that you do not get too hungry. Having to stay off food for a blood test is quite common for people with diabetes so that their blood sugar can be monitored

Further tests

Blood tests are not always accurate so you may have to have more than one test. If you have an ongoing condition you may have to have lots of blood tests to monitor whether your treatment is helping you.


Your doctor may ask you to come to visit when the results are back or sometimes you will be asked to phone the surgery. If you are worried about the results it may be a good idea to take a friend with you or a parent.

Last reviewed: February 2011

Compiled by:
Great Ormond Street Hospital