Many blood tests require fasting for a certain amount of time -- anywhere from six to 14 hours, depending on your doctor's instructions -- before you have your blood drawn. What you should eat before starting your fast varies according to the type of test. For most common blood tests, you can eat whatever you want before starting your fast and you may drink water during the fast period. Follow your doctor's instructions on what to eat or drink before your test. Not even diet drinks or chewing gum are normally allowed before fasting blood tests.
Fasting Glucose Tests
For most glucose, or blood sugar tests, you do no need to follow any particular diet. In some cases, your doctor will suggest a three day carbohydrate-loading diet before your test. For the two or three hour Glucose Tolerance Test, the Central Sydney GP Network suggests eating more than 150 g of carbohydrate per day for three days, or 10 carbohydrate servings. After the fast, the lab technician draws blood and you eat or drink 75-g of glucose, followed by several blood draws an hour apart. Do not do a carbohydrate-loading diet unless your doctor tells you to.
Cholesterol Tests
It's not necessary to follow any dietary fasts if you're having only a total cholesterol level done. However, if you're having a lipid profile including triglyceride levels, abstain from any type of alcohol for 24 hours before the test, the Harvard Health Publications reports.
Catecholamine test
Catecholamines are hormones produced by the adrenal glands. For between one to three days before a catecholamine test, depending on your physician, avoid alcohol, coffee, bananas, chocolate and tea, then fast for 12 hours before the test. You may have water during the fasting period. The same rules apply to serum tests for metanephrines, metabolites of catecholamines.
GGT Test
Gamma-glutamyl transferase, also called the GGT, is a test used to assess liver function. In addition to fasting before the test, avoid alcohol for 24 hours before the blood test.
