Wrongly estimated pretest probability can be harmful.

By necessity, a wrongly estimated pretest probability leads to a wrongly estimated posttest probability, even for correctly interpreted quantitative diagnostic test results. The effect of wrongly estimated pretest probability on the clinical utility of quantitative test results is unknown. We simulated test results from one real and two imaginary, quantitative diagnostic tests to study this issue for specific values of benefit and cost of treatment. We found that a wrongly estimated pretest probability reduces the clinical utility of a correctly interpreted quantitative diagnostic test result and may, in extreme cases, cause harm to the patient.