Using probabilistic reasoning

Diagnostic tests—whether clinical signs, imaging, or laboratory tests—are imperfect: there is always a possibility that test results are inaccurate and our diagnosis is wrong. However, we need to make decisions about whether to treat or not to treat patients, and so we need to feel confident that our diagnosis is above a certain threshold before we decide to treat a patient and below a certain threshold if we decide to withhold treatment. The threshold depends on the disease and the potential harms and benefits of treating or not treating patients. Unless we have clear strategies to cope with the uncertainties of testing, false positive results mislead us to treat some patients unnecessarily and false negative results lead us to fail to treat some patients adequately or in time.