Introduction

On the rare occasions when infections are acquired following transfusion, a unique opportunity can arise to study the natural history of the infectious agent in question. These studies are particularly useful when little is known about the outcome of the infection, for example when the agent is newly discovered or if it is rare. When recipients of potentially infected blood are traced during a look-back exercise, large groups of patients who may have acquired the infection are identified (HCV Look-back Collation Collaborators, 2002). Patients who are traced in this way can be invited to participate in natural history studies because the precise dates when they acquired their infections are usually known. Information about transfusion recipients who test negative for the agent in question can be equally useful as they can sometimes provide control data for comparison. Because the date of acquisition of infection is usually known, it is possible to study the natural history of infections that have a relatively benign early course from the precise date of acquisition. Under normal circumstances such infections may not be detected until much later, when patients present with symptoms. If no symptoms develop then the infections may never be detected.

The recruitment and subsequent collection of regional or national data into a central database, or register, is a prerequisite for undertaking such natural history studies. An example of such a database is the UK hepatitis C (HCV) national register.