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

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.

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.

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).

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.