The end of diagnosis: when to stop testing.

Defining the end of diagnosis and deciding when to stop further testing represent crucial features of many medical pursuits. The aim of this article is to analyze the principles that govern the decision to discontinue a chain of consecutive diagnostic tests. Using Bayes formula and threshold analysis, four common "stop rules" of medical diagnostics can be derived. The first rule relates to a lack of therapeutic consequence associated with continued testing. The second rule concerns empirical therapy, that is, the use of a benign, effective, and specific type of therapy to cap a diagnostic chain and use therapeutic success as confirmation for a diagnostic suspicion. The third rule states that the benefit of a suspected diagnosis should stay higher than the costs of a test invested in confirming its presence. Lastly, as the fourth rule, the cumulative test costs should not exceed the expected risks of a missed diagnosis. Although in clinical practice the ambiguities and variations among patients may compromise the calculation of exact stop values for each rule, knowledge of these underlying general principles may be sufficiently helpful in managing the individual patient.