Diagnosis: making the best use of medical data.

To take the best possible care of patients, physicians must understand the basic principles of diagnostic test interpretation. Pretest probability is an important factor in interpreting test results. Some tests are useful for ruling in disease when positive or ruling out disease when negative, but not necessarily both. Many tests are of little value for diagnosing disease, and tests should be ordered only when the results are likely to lead to improved patient-oriented outcomes.