Objective: To establish current practice in the use of Read codes for diabetes.

Design: Cross sectional study.

Setting: 17 practices in the Battersea primary care group in southwest London.

Data sources: Computerised medical records.

Main outcome measures: Number of codes in use in all practices; variation in the use of codes between practices; and prevalence of Read code use in diabetic patients.

Results: At least 9 separate Read code groupings and 25 individual diabetes codes were in use in the 17 general practices. Only one Read code (C10, diabetes mellitus) and its subcodes was being used in all 17 practices, but its use varied from 14% to 98% of patients with diabetes. The use of other key Read codes for monitoring the care of patients with diabetes also varied widely between practices; for example, < 20% of practices used the code for the location of care. Less than half of patients (45%) with diabetes had their type of diabetes coded, and even fewer (21%) had measures such as the examination of the retina coded.

Conclusions: The use of Read codes for diabetes needs to be standardised and coding levels improved if valid diabetic registers are to be constructed and the quality of care is to be monitored effectively. Until all patients with diabetes have the C10 Read code recorded, clinicians will have to use a wide range of Read codes and prescribing data to ensure that diabetes registers are complete.