Overview. Blood tests are very common. When you have routine checkups, your doctor may recommend blood tests to see how your body is working. Many blood tests don't require any special preparations. For some, you may need to fast (not eat any food) for 8 to 12 hours before the test.verview. Blood tests are very common. When you have routine checkups, your doctor may recommend blood tests to see how your body is working. Many blood tests don't require any special preparations. For some, you may need to fast (not eat any food) for 8 to 12 hours before the test.