Background Diagnostic imaging (DI) is used several ways in patient management, and the evidence required for each of these roles is somewhat different. This presentation will focus on the evidence needed to develop guidelines for the use of DI in primary diagnosis.

Context GRADE states that randomised control trials of patient outcomes are the highest level of evidence for assessing diagnostic tests but also that accuracy can be used as a proxy for outcomes. DI guidelines provide two basic types of information: whether DI is indicated in a particular clinical situation and what is the best DI modality to use. In choosing a modality the accuracy of different DI modalities is important. However, the question of whether DI is indicated in a given clinical situation is at least as important, and in determining this, accuracy is less important.

Best Practice The type of evidence which is needed for this question relates to whether DI will affect the management of the patient. If the information that DI provides is not relevant to the management of the patient then DI is not indicated. If the pre-test probability of the diagnosis is very low or very high then DI is also not indicated.

Lessons When developing guidelines for DI first consider whether the type of information DI can provide is important in patient management. If it is, clinical decision rules are important in assessing whether the pre-test probability justifies its use. Accuracy only becomes important in determining which imaging modality to recommend.