SELECTION OF CONTROLS IN DATABASE CASE-CONTROL STUDIES - GLUCOCORTICOIDS AND THE RISK OF GLAUCOMA

In case-control studies conducted using computerized databases, contro ls are often selected as a random sample from the base population. Thi s representative choice of controls is intended to guard against selec tion bias. We show, using data from a database case-control study, tha t such a definition of controls may also lead to selection bias under two conditions: (1) if the target disease has a prolonged asymptomatic clinical course with its detection depending on a specific physical e xamination and (2) if exposed patients have a higher likelihood of hav ing the disease detected than unexposed patients. The extent of the bi as that could result from the use of randomly selected controls was in vestigated in the context of a case-control study of the risk of ocula r hypertension or glaucoma associated with the use of glucocorticoids, conducted using the Quebec universal health insurance computerized da tabases. This article also illustrates that a computerized database ca n be useful to empirically explore opportunities for bias. (C) 1998 El sevier Science Inc.