Pretest probability: should we care?

t [ i a d t t o o c b s o h c i Pretest probability is a term most radiologists may be at least vaguely familiar with from lectures they may have attended on evidencebased medicine either as residents or at radiology meetings. However, although the term is reasonably self-explanatory, it is probably not a concept most radiologists understand very clearly or consciously apply on a regular basis in their daily practice. Nevertheless, it is in fact a concept we use or should use more frequently than we realize. Pretest probability is defined as “the probability of the target disorder before a diagnostic test result is known” [1], that is, the likelihood that the patient actually has the disease or condition the physician is ordering the imaging study to diagnose. Pretest probability is important to radiologists for two reasons. First, it helps determine whether a diagnostic imaging study should be done in the first place, and second, it can assist a radiologist in interpreting the results of an imaging study. Pretest probability is calculated “as the proportion of patients with the target disorder, out of all the patients with the symptom(s), both those with and without the disorder” [1]. Although the pretest probability for an individual patient is related to the local disease prevalence, it is not the same as the local disease prevalence [2] because it is also influenced by other factors peculiar to the individual patient, such as symptoms, lifestyle, and comorbidities. In fact, the pretest probability of a disease in an individual patient cannot be calculated precisely; it can only be estimated [2]. Some studies have suggested that physicians’ estimates of the pretest probability of disease for the e same clinical scenarios vary widely and, even among specialists, are often inaccurate [3,4]. However, Stiell et al [5] showed that in an ctual clinical situation, physicians’ stimates of pretest probability can e quite accurate. Imaging is indicated for diagnois only if there is a reasonable preest probability that the patient has disease but not enough certainty o commit to therapy. The threshld approach quantifies the range f pretest probability that justifies erforming a diagnostic test [6]. If it is virtually clinically certain hat a patient has a particular diagosis, imaging is not usually indiated. For instance, if a child has a alpable clavicular fracture, an xay will not help in the manageent of the fracture [7] and need ot be obtained. On the other and, if the likelihood of a diagnois is very low, imaging to make that iagnosis is not indicated. As an exmple, in a patient with a typical hronic tension headache, the likeihood of a brain tumor or some ther significant intracranial abnorality is so low that imaging should ot be done [8]. The pretest probability or likeliood of a disease can also assist in he interpretation of imaging findngs, particularly if the findings are quivocal or unexpected. That is, it an also be used to help determine he posttest probability of a disease. he posttest probability is “the ikelihood that your patient has the isease, condition, or injury you are esting for at the moment that the esult of the test you (or someone) rdered is delivered to you” [9]. If he findings are equivocal and the retest probability, or likelihood hat the patient actually has the disase, is very low, the radiologist d