Because of the way normal/abnormal thresholds for lab tests are set they are generally terrible screening tests. Doing the test without a specific reason will more often steer you wrong than right, and do more harm than good. There is tremendous *normal* variation for many of these tests, and so-called abnormal values frequently are not an indication of disease. Even when there is a problem it can be time consuming and dangerous to run it down before it becomes clear enough to offer some decent clues, and there’s often little or no benefit to getting the diagnosis earlier.