Test Sensitivity and Specificity

How sensitive and specific a diagnostic test is determines how reliable it can be.

We like to think that any test ordered by a doctor is perfect. If it can reliably detect the presence of a certain disease and it's always right, it's something to be thankful for. In reality, no test is 100% perfect. Good tests are sensitive and specific, though not absolutely.

The following are quick statements about the sensitivity and specificity of tests.

• Sensitivity means how well the test finds that someone with a disease indeed has it. Ideally, all patients with a disease are tested positive for it. Otherwise, there would be false negatives from failed detection.
• Specificity means how much the test result depends on the presence or absence of that disease. Ideally, all patients without the disease are tested negative for it. Otherwise, there would be false positives from the presence of other diseases.
• A test that is sensitive but not specific is problematic, because a positive test result could come from many different things.
• A test that is specific but not sensitive is problematic, because the condition the test is for may not be detected.
• Although nothing is 100% sensitive and specific, tests that are standard of care still have enough sensitivity and specificity to be worth using, and it sometimes helps to use tests in combination.

Keep these things in mind when your doctor discusses test results. Also, if you wonder why a certain diagnostic test isn't used, ask yourself if it's more sensitive and/or specific than the existing one.