Patient registries have become an important instrument in epidemiological studies in chronic disease. There are several types of registry (table 1). Administrative registries have been developed to identify patients with particular conditions, and these are then used in epidemiological studies to identify prevalence of certain conditions. For administrative registries, a limited amount of primarily demographic information is required as the purpose is only to identify patients rather than to learn as much as possible about an individual patient. Such registries exist in Europe, where patient information is recorded in common registries at each encounter with a physician or a healthcare facility.^1–3 Such registries have been used to identify associations between the primary disease and other complications such as the development of cancer among patients with psoriasis.⁴

Clinical registries have been developed specifically for drug trials. Some registries have been set up to identify patients who might be suitable for drug trials, whereas others have been developed for the management of the drug trial itself. Registries have also been set up as a surveillance mechanism to follow patients treated with particular medications.⁵ For the latter type of registry, more than simple demographic information, such as previous medications and level of disease activity, is gathered for individual patients.⁶

For a longitudinal observational cohort much more detailed information is required. Here …