Before taking a blood test, doctors will often tell their patients to go a certain amount of time without eating. This is because the food a person eats in this time period can affect the results of a test, creating a false outcome. While these fasting periods are only required for some blood tests, it is important to heed them when they are ordered in order to ensure accurate results.
According to the American Heart Association, the typical fasting time before a cholesterol blood test is between nine and 12 hours. There are two types of cholesterol. High-density lipoprotein (HDL) cholesterol is considered “good” cholesterol as it can help to reduce the risk of heart attack. Low-density lipoprotein (LDL) cholesterol is the “bad” cholesterol, which can contribute to the thickening of the artery walls when its levels are high.
A cholesterol blood test can examine for LDL cholesterol, HDL cholesterol, and total cholesterol. Eating before a cholesterol blood test will not render useless the outcomes of the HDL or total cholesterol results. However, if food is consumed during the specified time period before the test, the LDL cholesterol results can be significantly distorted.
The U.K. National Health Service reports that patients are usually instructed not to eat or take any supplements before a mchc blood test for iron. Iron deficiency anemia is the most common form of anemia and tests for it can yield misleading results if the pretest eating plan is not followed. Fasting before a blood test for iron is important because iron absorbs into the bloodstream shortly after it is consumed; not fasting before a blood test for iron will cause a higher reading of iron.
Blood glucose testing may or may not require fasting, depending on the circumstances. For an initial glucose test, a patient will generally be given no dietary changes, per the U.K. National Health Service. However, for supplemental procedures, pre-test fasting is frequently required. When this is the case, the U.S. National Institutes of Health states that patients should generally avoid all foods and drinks for six hours.
When a doctor recommends fasting before a blood test, it is because eating beforehand can cause the results to be skewed or unusable. Patients with any questions or concerns about what to do or eat before blood tests should talk to their doctors. Different tests require different procedures and while some may need no diet change at all, for others fasting before a blood test is crucial to producing accurate results.