The goal of diagnostic testing is to identify patients with a particular disease. Often, it is just as important that the test not mistakenly identify healthy persons as having disease. For example, a new test for the acquired immunodeficiency syndrome (AIDS) might identify 99.99% of all patients infected with the human immunodeficiency virus (HIV). However, if the test is positive in large numbers of persons who are not infected, its usefulness may be overshadowed by the harm caused by false positive results.