Common things are common, but what is common? A foundation for
The well-known clinical axiom stating that “common things are common”
attests to the pivotal role of probability in diagnosis. Despite the
popularity of this and related axioms, there is no operationalized
definition of a common disease, and no practicable way of incorporating
actual disease frequencies into differential diagnosis. In this
expository essay, we aim to reduce the ambiguity surrounding the
definition of a common (or rare) disease and show that incidence – not
prevalence – is the proper metric of disease frequency for diagnosis.
We explore how a numerical estimates of disease frequencies based on
incidence can be incorporated into differential diagnosis as well as the
inherent limitations of this method. These concepts have important
implications for diagnostic decision making and medical education, and
hold promise as a method to improve diagnostic accuracy.