Patient information

Blood tests

Find out what to expect from your blood test appointment at Charing Cross, Hammersmith or St Mary's hospitals, or Willesden Community Diagnostic Centre.

What is a blood test?

A blood test is when a sample of blood is taken to be tested 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

What happens during a blood test?

A blood test usually involves placing a needle attached to a stylet into one of the blood vessels in the inside of your elbow or the back of your hand. You will feel a sharp scratch as the needle goes in but this isn't particularly painful.

Most blood tests only take a few minutes to complete.

When you have your blood test there is a very low risk that bruising could occur in conditions which include:

drugs such as warfarin or aspirin
conditions such as a bleeding disorder or low platelets (thrombocytopenia)
when it is hard to get a vein, for example if the arm is swollen

After your blood test

If you were asked to fast before your test, it is possible you will feel light-headed afterwards, so please have a drink and something to eat afterwards.

The time scale for the results varies depending on the type of test. The requesting clinician for the blood test will be able to advise on approximate time scale for the test result.