Why are diagnostic tests not perfect?

Answer 1

A diagnostic test is never perfect. The fundamental problem lies between the sensitivity and the specificity of a test. These characteristics oppose each other, and choices in test design reflect the search for the optimal combination of sensitivity and specificity according to the clinical situation in which it is used.

For example, screening mammograms have to be very sensitive, to detect as many cancers as possible. Therefore the specificity is sacrificed so that we may identify as many early breast cancers as possible. Consequently, many screening mammograms which are interpreted as being abnormal do not, in the end, detect a real cancer. Many other abnormalities are found, and the diagnostic workup weeds these out so that the cancers are finally identified.

The flip side would be a very specific but not very sensitive test. Breast ultrasound, to determine the nature of an anomaly is very specific for simple cysts and certain cancers, but is not sensitive - hence it is not used in screening but rather in the workup of an abnormal screening mammogram.

Test design takes into account the two sides, and conscious decisions are made to favor either sensitivity or specificity.