This is an archived version of the Handbook. For the current version, please go to or search for this chapter here.  Differential diagnostic activity

Outcome assessments can be biased despite effective blinding. In particular, increased diagnostic activity could lead to increased diagnosis of true but harmless cases of disease. For example, many stomach ulcers give no symptoms and have no clinical relevance, but such cases could be detected more frequently on gastroscopy in patients who receive a drug that causes unspecific stomach discomfort and therefore leads to more gastroscopies. Similarly, if a drug causes diarrhoea, this could lead to more digital rectal examinations, and, therefore, also to the detection of more harmless cases of prostatic cancer. Obviously, assessment of beneficial effects can also become biased through such a mechanism. Interventions may also lead to different diagnostic activity, for example if the experimental intervention is a nurse visiting a patient at home, and the control intervention is no visit.