Blood tests are used for various reasons—they can help your doctor diagnose disease, assess your risk factors for heart disease, determine how well your blood clots, or evaluate how well medicines are working. Many blood tests don’t require special preparation, but some tests require that you fast for 8 to 12 hours prior to the test. During the test, a small amount of blood is drawn from your body with a needle, usually from a vein in your arm. This takes just a few minutes and can cause mild discomfort in some people, though this is short-term. The sample is then sent to a laboratory to be tested, and your doctor will review the results with you.