• Sumant R Ranji, Kaveh G Shojania, Robert L Trowbridge, and Andrew D Auerbach.
• Department of Medicine, University of California San Francisco, San Francisco, California 94143-0131, USA. sumantr@medicine.ucsf.edu
• J Hosp Med. 2006 Mar 1; 1 (2): 81-7.

BackgroundSpiral computed tomographic pulmonary angiography (CTPA) has become the primary test used to investigate suspected pulmonary embolism (PE) at many institutions, despite uncertainty regarding its sensitivity and specificity. Although CTPA-based diagnostic algorithms focus on minimizing the false-negative rate, we hypothesized that increasing use of CTPA also might lead to false-positive diagnoses.ObjectiveDetermine the frequency of possible false-positive diagnoses of PE when CTPA is the primary diagnostic test.DesignRetrospective cohort study.SettingTwo academic teaching hospitals.Participants322 patients with suspected PE evaluated with CTPA.MeasurementsWe used a validated prediction rule to determine the pretest probability of PE in each patient. We combined these pretest probabilities with published estimates of CTPA test characteristics to generate expected posttest probabilities of PE. We compared these posttest probabilities to actual treatment decisions to determine the rate of false-positive diagnoses of PE.ResultsAmong 322 patients investigated for PE, 37 (12%) had high pretest probability, 101 (32%) moderate, and 184 (57%) low. CT scans were interpreted as positive for PE in 57 patients (17.8%). Regardless of the pretest probability of PE, 96.5% of patients with a positive CTPA were treated with anticoagulants. Even under an optimistic assumption of CTPA test characteristics, as many as 25.4% of these patients may have been treated unnecessarily as a result of a false-positive diagnosis. Most of these patients had a low pretest probability of PE.ConclusionsFailure to utilize Bayesian reasoning when interpreting CTPA may lead to false-positive diagnoses of pulmonary embolism in a substantial proportion of patients.(c) 2006 Society of Hospital Medicine.

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