Base-rate error in the interpretation of immunohistochemistry

Failure to appreciate the importance of the frequency of a disorder in the appropriate population (the base rate) may lead to the misinterpretation of the diagnostic significance of unexpected test results (unexpected test result defined in this context as a test result that is positive in a higher proportion of cases of an alternative diagnosis than in the diagnosis considered most likely before the test). This study aimed to determine whether pathologists are vulnerable to this error. Pathologists were asked to estimate the probability of tumour B in a scenario in which, prior to the immunostaining result, an experienced pathologist considers there to be a 99% chance that the patient has tumour A and a 1% chance that they have tumour B. Antibody X is positive in 80% of cases of tumour B and negative in 90% of cases of tumour A and is positive in the case described in the scenario. The estimates made by consultant pathologists ranged from 0 to 100% (mean 29.7%). The Bayesian answer would be 7.5%. These findings suggest that base-rate error may lead some pathologists to overestimate the implications for the likelihood of a diagnosis in the light of an unexpected immunohistochemical result.