Physicians' understanding of CT probabilities in ED patients with acute abdominal pain

Objective: Computerized tomography (CT) is often employed to diagnose or rule out certain suspected abdominal pathologies. The aim of this study is to compare emergency physicians' estimated post-test disease probabilities to the probabilities obtained for similar diagnostic tests as reported in the literature.

Methods: Physicians were asked to estimate pre and posttest probabilities before and after CT scan results in patients with nontraumatic abdominal and pelvic pain. The actual post-test probability was calculated using published likelihood ratios and compared to physician judgment.

Results: 210 patient encounters were included. In the negative CT group, physicians' median pre-test probability was 40% with a post-test probability of 0%, while the actual post-test probability is 4.2% (p<0.001). Physicians' median pre-test probability for a positive CT was 70% with a post-test probability of 100%, while the actual post-test probability is 98% (p<0.001). The diverticulitis subgroup had no significant differences between physician and actual post-test probabilities. The post-op abscess subgroup had significant differences in post-test probabilities in both the negative CT (30% difference, p=0.028) and positive CT subgroups (-37% difference, p=0.003).

Conclusions: When applying the probability theory of disease, physicians tend to overestimate the power of CT scanning. The difference in physician and actual post-test probabilities may be small or not clinically significant in diseases with good positive and negative likelihood ratios such as in diverticulitis; however, this difference may be large and clinically significant in diseases with poor likelihood ratios such as in post-op abscess.