reports

Adjusting for patients’ clinical case mix explains variation in general practice prescribing

Research report no. 291, Department of Statistical Science. London, UK: University College London.

University College London, London, UK

BACKGROUND: The potential of using patient-level measures of clinical case mix based on patient morbidity to explain variation in general practice prescribing has not previously been examined in the UK.

AIM: To examine whether patients’ clinical case mix explains variations in prescribing in general practice.

DESIGN OF STUDY: Retrospective study of a cohort of patients followed for one year.

SETTING: 129 general practices, with a total list size of 1,032,072 contributing data to the General Practice Research Database.

METHODS: Each patient was assigned a morbidity group based on their diagnoses, age, and gender using the Johns Hopkins ACG case mix system. The variability in prescribing explained by age, gender and morbidity was examined using multilevel regression models.

RESULTS: The annual median percentage of patients receiving a prescription in a practice, was 65% (range: 11% to 75%). The percentage of patients receiving prescriptions increased with age and morbidity. Age and gender only explained 10% of the total variation in prescribing. Including morbidity explained 80% of the total variation. When variation in prescribing was split into between and within practices, most of the variation was found to be within practices and morbidity explained both variations well.

CONCLUSIONS: This study shows that inclusion of a diagnosis based patient morbidity measure into prescribing models can explain a large amount of variability both between and within practices. The use of patient-based case mix systems should be explored further when examining variation in prescribing patterns between practices in the UK, in particular for specific prescribing categories, and may prove useful in fairer allocation of budgets.