A blood test is a laboratory analysis performed on a blood sample extracted from the vein via a needle. By testing the levels of specific compounds in the blood, doctors can diagnose a medical problem or presage the risk of a future medical problem. Many blood tests require a fast before the sample is extracted.
The blood is the liquid tissue circulatory system of the body. Its purpose is to transport important molecules such as lipids, glucose, hormones, waste, amino acids and oxygen to the cells or organs. Physicians can learn a lot about the state of the body by testing the blood, but it must be free of any influence before the test.
In order to prevent nutrients from interfering with the test, it's a good idea to fast for at least 12 hours beforehand. You can consume water in your diet but no other foods or drinks. Your doctor should inform you whether a fast is required. You should also consult your doctor if you are taking medication, which can interfere with the results. Only a physician can tell you whether to stop taking medication for the purpose of a blood test.
The human body contains at least 60 percent water, depending upon age and gender. Water is used as a solvent and a facilitator of chemical reactions, so it's important to keep hydrated during the fast. Fortunately, water is almost completely free of nutrients that can affect blood levels, but it must be consumed plain. Coffee, tea, soda, fruit drinks and sports drinks can all interfere with a blood test.
In some cases, the consumption of food or drinks up to 12 hours before a test partially obscures the results. According to the American Heart Association, for example, this is especially true for a cholesterol test. LDL cholesterol and triglyceride levels in the blood will be affected if you consume anything but water; only the HDL and total cholesterol results will be usable. Other tests, such as the glucose and glycohemoglobin, may be negated entirely by the consumption of foods or drinks beforehand.
According to Dr. Jennifer Ashton, a medical correspondent for CBS News, a newer diabetes test known as an A1C test does not require fasting. Other diabetes tests may only require a three hour fast. If you are going to a public screening, you need to prepare for the fast beforehand in order to be ready for the test.