Quality of Screening Tests

Sensitivity and specificity

The quality of a screening test is described in terms of:

  • Sensitivity – how well the screening test shows who truly has a disease
  • Specificity – how well the screening test shows who truly does not have a disease

The goals of any screening test are:

  • To correctly identify everyone who has a certain disease (100 percent sensitivity)
  • To correctly identify everyone who does not have the disease (100 percent specificity)

A perfect test would correctly identify everyone with no mistakes. There would be no:

  • False negatives (when people who have the disease are missed by the test) 
  • 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’s always a trade-off between the two.

A test that’s very sensitive may pick up even the slightest abnormal finding. This means it will miss few cases of the disease, but it may also mistake some people as having the disease when they don’t.

These false positive findings can lead to further testing and some anxious moments for people who don’t have the disease. 

A test that’s very specific, on the other hand, may have few false positive results, but may miss more cases of the disease.

The balance between sensitivity and specificity is important for all screening tests, including mammography.

Learn more about the accuracy of mammography.