Data sources

We conducted a retrospective observational study using data from the UK Clinical Practice Research Datalink (CPRD), formerly known as the General Practice Research Database. In the health care system in the UK (the National Health Service, NHS) general practitioners (GPs) play a pivotal role, providing primary health care for 98% of the population and referring patients for specialist consultations or hospital admissions. The medical records of the GPs contain prospective information on demographics, prescriptions and diagnoses made by GPs and diagnoses from specialist consultations, outpatient visits and hospitalizations (17). The CPRD is a widely used computerized database of clinical primary care records that from its inception in 1987 stores information on around 10 million patients registered with a general practice, with active patients covering 6.9% of the UK population. The geographic distribution of the participating GP-practices and the demographic distribution of the patients are broadly representative of the general UK population (17). Clinical data for each patient are captured and stored in CPRD using Read codes (18). Data quality assessments are performed at the practice level (17). Independent validation studies (19,20) and reviews (21–23) have reported that the clinical data in the CPRD are in general of high quality. We used the ethnicity classification as developed and tested by Mathur (24), for which a high level of concordance within and across NHS sources was found in an analysis of CPRD records of ethnicity. This research was conducted in accordance with the principles of the Helsinki declaration and the protocol for this study was approved by CPRD’s Independent Scientific Advisory Committee. All data on patients were stored anonymously in CPRD and, therefore, informed consent was not required for this study.