In combination with a physical exam, your medical history and imaging studies, a liver function test can help your doctor diagnose liver disorders. Patients are normally asked to fast before a liver function test to prevent food from interfering with blood levels of the compounds being measured.
Liver function tests may measure the levels of several compounds found in the blood, some of which can be affected by substances found in food. For example, ascorbic acid and vitamin A can both increase blood levels of bilirubin, a product of the breakdown of haemoglobin. Ascorbic acid can also lower blood levels of lactate dehydrogenase (LDH). Fasting before the test ensures that your blood levels have not been temporarily altered by something you recently ate.
The length of time you need to fast before the test varies, depending on which liver function tests your doctor has ordered. Your doctor will inform you how long you should fast beforehand.
In addition to food, some medications, such as aspirin, antibiotics, oral contraceptives and narcotics, can also affect your liver function test results. Your doctor will tell you which medications to avoid based on which liver tests you will be taking.