Prevalence of abnormalities influences cytologists' error rates in screening for cervical cancer

Medical screening tasks are often difficult, visual searches with low target prevalence (low rates of disease). Under laboratory conditions, when targets are rare, nonexpert searchers show decreases in false-positive results and increases in false-negative results compared with results when targets are common. This prevalence effect is not due to vigilance failures or target unfamiliarity.

To determine whether prevalence effects could be a source of elevated false-negative errors in medical experts.

We studied 2 groups of cytologists involved in cervical cancer screening (Boston, Massachusetts, and South Wales, UK). Cytologists evaluated photomicrographs of cells at low (2% or 5%) or higher (50%) rates of abnormality prevalence. Two versions of the experiment were performed. The Boston, Massachusetts, group made decisions of normal or abnormal findings using a 4-point rating scale. Additionally, the group from South Wales localized apparent abnormalities.

In both groups, there is evidence for prevalence effects. False-negative errors were 17% (higher prevalence), rising to 30% (low prevalence) in the Boston, Massachusetts, group. The error rate was 27% (higher prevalence), rising to 42% (low prevalence) in the South Wales group. (Comparisons between the 2 groups are not meaningful because the stimulus sets were different.)

These results provide the first evidence, to our knowledge, that experts are not immune to the effects of prevalence even with stimuli from their domain of expertise. Prevalence is a factor to consider in screening for disease by human observers and has significant implications for cytology-based cervical cancer screening in the post-human papillomavirus vaccine era, when prevalence rates of high-grade lesions in the population are expected to decline.