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 OR DRINK BEFORE A BLOOD TEST?
It depends on the type of blood test you're having. The healthcare professional who arranges your blood test will tell you whether there are any specific instructions you need to follow before your test.
You can eat and drink as normal before some blood tests. In other cases, you will be instructed not to eat or drink (other than water) before your test. This is known as a fasting blood test. You may also be told not to drink alcohol or not to smoke before your test.
If you have any questions about your blood test, ask a healthcare professional, such as a GP or nurse, for advice.
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, you should also follow any instructions from your healthcare professional.
Fasting blood glucose test
Do not eat or drink anything except water for 8 to 10 hours before a fasting blood glucose test. These are used to diagnose diabetes, a condition caused by too much glucose (sugar) in the blood.
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)
You may be asked not to eat anything and only drink water for 9 to 12 hours before having blood cholesterol tests (lipid profile).
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)
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).
Gamma-glutamyl transferase (GGT) test
A GGT test is used to help diagnose liver disease. Your GGT levels may be affected if you drink alcohol in the 24 hours before the test. Smoking can also affect the test results. Your healthcare professional will advise you about not drinking and smoking before the test and how long for.