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
A blood test usually involves the phlebotomist taking a blood sample from a blood vessel in your arm. and the usual place for a sample is the inside of the elbow or wrist, where the veins are relatively close to the surface. Blood samples from children are most commonly taken from the back of the hand. The child's hand will be anaesthetised (numbed) with a special cream before the sample is taken.
You can find out more about blood tests, their purpose and the way they are performed on the NHS Choices website.
Can I eat and drink before having a blood test?
It depends on the type of blood test you’re having.
If you have any questions about your blood test, ask your healthcare professional for advice.
Preparations for blood tests
You can eat and drink as normal before some blood tests. However, for others you may be given specific instructions so that the test results aren’t affected, for example, not to eat and sometimes not to drink before your test (known as a fasting blood test).
Below are some examples of different blood tests and what you may need to do to prepare for them, including how long you may need to fast. However, it is important you speak to your healthcare professional first about what type of blood test you are going to have and follow their advice carefully.
Fasting blood glucose test
Do not eat or drink anything except water for 8-10 hours before a fasting blood glucose test. These can be used to diagnose diabetes, a condition caused by too much glucose (sugar) in the blood.
Check with your healthcare professional first as it is not always necessary to fast before a blood test.
Iron blood test
Iron blood tests are usually taken in the morning before you eat anything. You should also avoid taking iron pills or tablets for 24 hours before your test. Your body absorbs iron very quickly from food or pills, so this can raise your iron levels and affect the test results.
Iron blood tests help diagnose conditions such as iron deficiency anaemia (lack of red blood cells caused by low iron levels).
Blood cholesterol tests (lipid profile)
Do not eat anything and only drink water for 10-12 hours before having blood cholesterol tests (lipid profile).
Cholesterol is a fatty substance that can build up in your arteries and increase your risk of conditions such as heart disease. There are several different cholesterol tests. When these are done together it’s called a lipid profile. A lipid profile tests the levels of:
- ‘good’ (HDL) cholesterol
- 'bad’ (LDL) cholesterol
- total cholesterol
- triglycerides (other fatty substances)
Some laboratories can calculate your LDL cholesterol level from your total cholesterol, triglycerides and HDL levels. However, you will still need to fast for these tests. If you do not need to fast, your healthcare professional will let you know.
If you’re just having a triglycerides test, do not drink alcohol for 24 hours before the test (you’ll also need to fast as explained above).
The Health A-Z has more information about diagnosing high cholesterol.
GGT (gamma-glutamyl transferase) test
A GGT test is used to help diagnose liver disease. Do not eat for at least eight hours before the test and do not drink alcohol for 24 hours before the test. Smoking can also affect the test results. Your healthcare professional will tell you if you need to stop smoking before the test and how long for.