Sensitivity and specificity
The quality of a screening test is described in terms of:
- How well the screening test tells who truly has a disease (sensitivity)
- How well the screening test tells who truly does not have a disease (specificity)
The goal of any screening test is to correctly identify everyone who has a certain disease (100 percent sensitivity). An ideal screening test would also be able to correctly identify everyone who does not have the disease (100 percent specificity). A perfect test would be able to correctly identify everyone with no mistakes. There would be no false negatives (when people who have the disease are missed by the test) and no false positives (when healthy people are incorrectly shown to have the disease).
The trade-off between sensitivity and specificity
No screening test has perfect (100 percent) sensitivity and perfect (100 percent) specificity. There is always a trade-off between the two. A test that is very sensitive will pick up even the slightest abnormal finding. This means it will miss few cases of the disease, but it will also mistake some healthy people as having the disease when they don't. These false positive findings can lead to further testing and some anxious moments for a person who does not have the disease.
A test that is very specific, on the other hand, will have few false positive results, but may miss more cases of the disease. This balance between sensitivity and specificity is important for all screening tests, including mammography and clinical breast exam.
Learn more about the accuracy of mammography.
Learn more about the accuracy of clinical breast exam.