Blood tests are carried out for a number of reasons. For example, to diagnose or rule out certain conditions, to monitor an existing condition, or simply to find out what your
blood group is before you have a transfusion.
In most cases, you will be able to eat and drink as normal before having a blood test. However, there are times when you will not be able to eat before a blood test, and you may also be asked not to drink certain liquids, such as tea, coffee, alcohol and fruit juice. This is because eating or drinking certain foods and liquids may affect the accuracy of certain types of blood test. The healthcare professional who is treating you will tell you if there is anything that you need to do in preparation. You should ask them if you have any concerns.
Tests for autoimmune diseases
blood is tested to check for conditions affecting the
immune system, such as HIV, hepatitis A or B, or cancer. For this test, you may be told not to eat, or to avoid certain foods before the test. However, unless you are told otherwise, it's a good idea to have a snack before the test to stop yourself feeling faint.
Blood cholesterol level
A blood test is often used to check the amount and type of cholesterol in your body. Cholesterol is a fatty substance that builds up in the
artery walls and can lead to serious health problems, such as
disease. There are two types of cholesterol test and, depending on which type you have, you may be asked to fast beforehand.
A test known as the fasting lipoprotein profile may be used to check how much low density lipoprotein (LDL) or 'bad cholesterol' is contained in your
blood. Before having this test, you will be asked not to eat for up to 14 hours so that your body has enough time to digest the food in your digestive system, and to clear any fatty particles from your
It is also possible to have a cholesterol test without fasting, but this test can only be used to measure the total amount of cholesterol and the high density lipoprotein (HDL) or 'good cholesterol' in your
Blood glucose test
There are a number of different blood tests that are used to measure
glucose levels. They are often used to check for diabetes, or to find out how well organs such as the pancreas and
liver are processing
If you are having this test for the first time, you may be asked to continue eating as normal. However, if the results from the test are abnormal, you may need to have a second test, when you will be asked not to eat beforehand. This second test is to check whether your original abnormal
glucose test result was caused by the food you ate.
Blood tests are sometimes used to measure iron levels in the
blood, which can help to diagnose conditions such as anemia. Iron tests are usually taken in the morning, before you have had anything to eat. You should also avoid taking iron pills or tablets for 24 hours before the test. This is because iron is absorbed very quickly from food or pills, and can make your
blood iron levels higher than normal.