BACKGROUND: Among patients presenting with low back pain (LBP), GPs have to identify those with serious, treatable conditions. However, excluding these conditions in every patient with LPB is time consuming and of low yield. We have suggested that identifying those patients where these serious conditions need to be considered can be made more efficient through asking patient if they feel their LBP is new or unfamiliar in some way. OBJECTIVE: To evaluate the diagnostic validity of a simple heuristic based on the patient's view of the familiarity of LBP. METHODS: Cross-sectional diagnostic study with delayed-type reference standard, nested within a three-arm randomized trial of quality improvement for LBP. A total of 1378 patients presenting, with LBP, to one of 126 participating GPs were included. They were asked whether their LBP was familiar or not (index test). At 1 year, patients were interviewed with regard to relevant conditions that in hindsight might explain their LBP. Reviewers deciding on disease status (reference standard) were blinded to the results of the index test. RESULTS: Totally 1190 patients answered the index test question and were available for interview at 1 year. Only four of these had a serious cause of their LBP. Two of these were identified by the familiarity heuristic, resulting in low sensitivity. CONCLUSION: The number of diseased patients was too small to obtain a reliable estimate of sensitivity. Low prevalence of serious disease in primary care poses difficulties for diagnostic research. In hindsight we would question whether an RCT-setting emphasizing non-specific LBP is suitable for this kind of research. At present, the familiarity heuristic cannot be recommended for patients presenting with LBP.
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