Diminishing returns on the road to diagnostic certainty.

In the quest for diagnostic certainty, one can be led into a false sense of accomplishment by the results of sensitive, specific, and well-executed diagnostic tests that provide little or no diagnostic information. This is a consequence of the fact that as one approaches diagnostic certainty the useful information returned by diagnostic tests and observations approaches zero. This can be demonstrated by calculating the quantity of useful information provided by a test in a given clinical environment as one approaches certainty of diagnosis.