Blood tests are commonly used to investigate the cause of symptoms, detect or monitor disease state, or predict disease risk. Advance preparation is not needed prior to most blood tests, as recent food intake does not significantly influence the majority of the components tested. However, some blood tests require that you avoid all foods and liquids -- except for water -- for up to 12 hours before the test.
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The traditional lipid panel requires a 12-hour fast, which means no food or drink, other than water, can be consumed 12 hours prior to having the blood drawn. This lipid test, also known as a cholesterol panel, checks for blood levels of total cholesterol, triglyceride, as well as LDL and HDL cholesterol -- commonly called the bad and good cholesterol. However, an emerging practice is to order cholesterol tests that do not require fasting, since the only component of the lipid test that is influenced by recent food intake is the triglyceride, or fat level in the blood.
Avoidance of all food and liquid, other than water, is also required at least 8 hours before a fasting blood glucose test -- used to screen for the presence of prediabetes and diabetes. Fasting is also required prior to an oral glucose tolerance test, which is used in pregnancy to screen for the presence of gestational diabetes. A blood glucose test can also be measured without fasting, but the fasting test is more commonly used for diagnostic purposes.
Blood levels of vitamins, minerals, electrolytes and other nutrients may be temporarily increased after a eating. Even alcohol and caffeine can alter several components in the blood. Even so, most lab tests do not require fasting for accurate results. However, your doctor may have a preference on whether you should avoid specific foods or nutrition supplements prior to having certain tests completed.
Improper test preparation can alter your test results, and may even lead to an incorrect diagnosis or unnecessary treatment. For accurate blood test results, be sure to understand and follow your preparation instructions.
Reviewed by Kay Peck, MPH RD