• epidemiology
• arthritis
• rheumatoid
• outcome assessment
• health care

We take the following three main messages from Porter and colleagues’ thoughtful viewpoint article.1

1. Although disease classification criteria are developed for research purposes, they are commonly used for diagnosis in the clinic. This common practice is wrong since the prior probabilities for the targeted disease are always higher in the research and lower in clinical practice settings. Therefore, the use of the same criteria for diagnosis in general practice would cause an unacceptably high number of false positives in that setting.

2. The use of classification criteria for research purposes, the usual setting being enroling patients into clinical trials, is also problematic. Such criteria can not only cause exclusion of patients who would benefit from the drug used in the study, but also the inappropriate inclusion of patients with no disease into the formal study. The authors give suitable examples for both scenarios.

3. The authors conclude that since the current disease criteria represent much hard work by experts in the field …