Defining GP practice areas based on true service utilisation

Where patients who are registered with a general practice actually live (the service utilisation area) often differ from what GPs consider their practice boundaries or catchment area to be, as well as from administrative boundaries. A key aim of primary care commissioners is to allocate resources efficiently and to locate services in such a way that access is convenient for patients. To achieve this robust understanding of practice service utilisation areas and the overlap between practices and administrative areas are essential. We used kernel analysis of the postcodes of over 400,000 registered patients to define GP service utilisation areas. We estimated service utilisation for each of 64 practices for a period of five years (2002-2006) exploring the areas in which 99%, 98%, and 95% of registered patients were expected to live. These service utilisation areas were not coterminous with other practices or with administrative boundaries. We present a simple analytical method to define GP catchment areas that captures the true service utilisation area and identifies the extent of overlap. This is a practical tool that can assist health care commissioning.