Blood tests are commonly used to measure various components in the blood that can detect disease or disease risk. According to the National Heart, Lung, and Blood Institute, blood tests can be used to measure blood components such as hemoglobin, lipids, glucose or certain enzymes in the blood. Certain foods can affect the results of certain blood tests if eaten prior to testing, therefore, various fasting preparations and lists of food to avoid are given to patients depending on the blood test they are receiving.
Fecal Occult Blood Test
A fecal occult blood test is used to measure blood in the stool for detection of colon and intestinal problems. According to Harvard Health Publications, certain foods should be avoided several days prior to a fecal occult blood test. These include red meat, which can turn your test positive; foods such as radishes, turnips, cabbage and cauliflower that contain a chemical that can turn the test positive; and citrus fruits or vitamin C supplements that can turn your test falsely negative.
Lipid Blood Tests
Prior to a lipid blood test, a 12-hour fast is recommended, states the Cleveland Clinic. According to HealthCentral.com, this is due to the fact that triglycerides are unusually high after meals. Since LDL, or "bad cholesterol," is calculated using fasting triglyceride levels, eating prior to a lipid blood test could affect LDL cholesterol results.
Blood Glucose Test
Blood glucose tests are used to determine the amount of digested carbohydrate present in the blood. According to Medline Plus, an online resource of the National Institutes of Health, blood glucose tests are usually performed when an individual is thought to be at risk for diabetes. For a fasting blood glucose test, it is recommended that you not drink or eat anything at all eight hours before the test, unless the blood test is being done randomly, which would be specified by your physician.
Complete Blood Count
According to Medline Plus, a complete blood count, or CBC, is used to measure such blood components as hemoglobin, hematocrit, red blood cells and white blood cells. No fasting or avoidance of foods is necessary prior to this particular blood test.