A blood test is when a sample of blood is taken for testing in a laboratory. Blood tests have a wide range of uses and are one of the most common types of medical test.
For example, a blood test can be used to:
assess your general state of health
confirm the presence of a bacterial or viral infection
see how well certain organs, such as the liver and kidneys, are functioning
screen for certain genetic conditions such as cystic fibrosis.
What happens during a blood test?
Most blood tests are carried out at your GP surgery or your local hospital under the supervision of a nurse, or in some cases, a doctor.
A test usually involves placing a needle attached to a syringe into one of the blood vessels in the inside of your elbow or wrist. You will feel a sharp prick as the needle goes in but this isn’t particularly painful.
A sample of blood is then taken and the needle is removed. You will be given a cotton-wool pad to put pressure on the the site of the injection, which stops any bleeding and should prevent bruising.
Most blood tests only take a few minutes to complete.
Read more about how a blood test is performed.
Only a small amount of blood is taken during the test so you shouldn’t feel any significant after-effects.
However, some people do feel dizzy and faint during and after the test. If this happens to you, tell the person carrying out the test so they can help you feel more comfortable.
After a blood test, you may have a small bruised area on your skin where the needle went in. Occasionally, a larger area of bruising may appear. This can be because there was a lack of pressure at the site of the jab or the blood vessel was damaged by the needle.
Bruises can be painful but are usually harmless. However, tell your GP if you frequently get bruises after having a blood test.