Fasting prior to a blood test is often required to get an accurate test result. You will most likely need to fast for the following blood tests:

1. Blood glucose test
2. Triglyceride level test
3. Cholesterol test
4. Basic metabolic panel
5. Renal function panel

If someone does not fast prior to their blood test, the protein, vitamin, fat, carbohydrate, and mineral in the food or drink they consume may skew the accuracy of their blood test results.

For example, the fasting blood glucose test is done to diagnose prediabetes and diabetes. Eating or drinking before the test can make your blood sugar higher than what it’s supposed to be and may make the diagnosis inaccurate.

The length of fasting varies, depending on the test. Most tests will instruct you to consume only water for the eight to ten hours preceding the test. A 12-hour fast may be required for a few tests.

However, not all blood tests require fasting beforehand. Some tests, such as a fecal occult blood test, do not require fasting but do restrict certain foods such as red meat and broccoli, as well as some medications that can cause a false positive test.

If your doctor has ordered a new blood test for you and has not specified whether or not you should fast, ask them if fasting is necessary.