Redefining Overuse to Include Costs: A Decision Analysis for Computed Tomography in Minor Head Injury

A study was conducted to (1) determine the testing threshold for head computed tomography (CT) in minor head injury in the emergency department using decision analysis with and without costs included in the analysis, (2) to determine which variables have significant impact on the testing threshold, and (3) to compare this calculated testing threshold to the pretest risk estimate previously reported when the Canadian CT Head Rule (CCHR) was applied. It was hypothesized that the CCHR might not identify all patients above the testing threshold.

A decision analytic model was constructed using commercially available software and data from published literature. Outcomes were assigned values on the basis of quality-adjusted life-years (QALYs) and cost. Two testing thresholds were calculated, the first based only on the effectiveness of either strategy, the second on the overall net monetary benefit. Two-way sensitivity analyses were performed to determine which variables most affected the testing threshold.

When only effectiveness (QALYs) was considered, the testing threshold for obtaining head CT was 0.039%. This threshold increased to 0.421% when the net monetary benefit was considered in lieu of QALYs. Age, probability of lesion on CT requiring neurosurgery, and cost of CT were the main drivers of the model.

If only effectiveness is considered, current clinical decision rules might not provide a sufficient degree of certainty to ensure identification of all patients for whom the benefits of CT outweigh its risks. However, inclusion of cost in the analysis increases the testing threshold by an order of magnitude and well outside the range of uncertainty of current clinical decision rules. These results suggest that the term overuse should be redefined to include the provision of medical services with no benefits or for which harms including cost outweigh benefits.