A review identifies and classifies reasons for ordering diagnostic tests.

OBJECTIVE To consider the reasons and context for test ordering by doctors when faced with an undiagnosed complaint in primary or secondary care. STUDY DESIGN AND SETTING We reviewed any study of any design that discussed factors that may affect a doctor's decision to order a test. Articles were located through searches of electronic databases, authors' files on diagnostic methodology, and reference lists of relevant studies. We extracted data on: study design, type of analysis, setting, topic area, and any factors reported to influence test ordering. RESULTS We included 37 studies. We carried out a thematic analysis to synthesize data. Five key groupings arose from this process: diagnostic factors, therapeutic and prognostic factors, patient-related factors, doctor-related factors, and policy and organization-related factors. To illustrate how the various factors identified may influence test ordering we considered the symptom low back pain and the diagnosis multiple sclerosis as examples. CONCLUSIONS A wide variety of factors influence a doctor's decision to order a test. These are integral to understanding diagnosis in clinical practice. Traditional diagnostic accuracy studies should be supplemented with research into the broader context in which doctors perform their work.