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Blood tests are a common way for your doctor to monitor your health and nutritional status, check for the presence of health conditions and assess your risk for disease. While the level of most blood components will not change rapidly from consuming nutrients, some of the substances in your blood will be affected by eating or drinking in the hours prior to the test. Before having blood tests completed, be sure to check with your doctor so you know if you can eat or need to fast prior to the test.

Blood Tests That Require Fasting

Because blood glucose and blood fats can be elevated for several hours after eating, fasting glucose, triglyceride or cholesterol are the most common tests that require fasting. Certain hormone measurements, such as testosterone and cortisol, may require an early morning blood sample but will not require fasting. Your doctor may even ask you to avoid alcohol, certain nutritional supplements or medications prior to a test, but as a rule you can take medications and supplements according to your usual schedule -- even when fasting.

Blood Tests That Do Not Require Fasting

Most blood tests do not require any special preparation. For example, you will not need to fast prior to a pregnancy blood test. Fasting is also not required for many routine tests of blood chemistry -- for example, tests that measure kidney, liver or thyroid function can be completed any time of the day. Even the test that measures a 2 to 3 month average blood glucose -- the A1c test -- does not require fasting. Additionally, some cholesterol measures can be accurately measured without fasting if your doctor prefers to order these instead of fasting tests.

How to Fast

If you are required to fast prior to your blood test, you should receive specific instructions. Most cholesterol and triglyceride tests require fasting for 12 hours prior to the blood draw, while fasting blood glucose typically requires at least an 8 hour fast. Most people have fasting lab tests completed early in the morning, so if they need to fast for 12 hours, they can be sleeping for most of the fast. Fasting means no food or drink, other than water, for the period specified. These restrictions include beverages including coffee, tea and even gum. You can eat right after having your fasting test completed, so bring a snack with you if you are particularly hungry.

Warnings and Precautions

Check with your doctor to ensure you understand the directions for your blood tests. If you don’t properly fast, your test results could lead to an incorrect diagnosis and the need to repeat the blood test. However, some people find it difficult or even unsafe to do fasting lab tests. For example, people taking insulin to manage their diabetes may be at risk for low blood sugar if meals are delayed or missed. If you have any difficulties fasting due to transportation or health issues, ask your doctor about alternative tests that do not require fasting.

Reviewed by: Kay Peck, MPH, RD