Analysis of primary care prescribing--a "constructive" coding system for drugs.

The introduction of a computerized medical information system as part of a project assessing quality in primary care required the coding of drugs prescribed. The development of a simple four-digit system is described, coding drug, therapeutic class, and route of administration. The system allows easy measurement of the use of combined preparations.The drugs used for insomnia and acute bronchitis have been analysed. The results show marked variation with the age of the patient.The potential for medical audit is discussed as it is now possible to link prescribing figures in routine primary care with relevant clinical information.