The Clinical Practice Research Datalink (CPRD) is an ongoing primary care database of anonymised medical records from general practitioners, with coverage of over 11.3 million patients from 674 practices in the UK. With 4.4 million active (alive, currently registered) patients meeting quality criteria, approximately 6.9% of the UK population are included and patients are broadly representative of the UK general population in terms of age, sex and ethnicity. General practitioners are the gatekeepers of primary care and specialist referrals in the UK. The CPRD primary care database is therefore a rich source of health data for research, including data on demographics, symptoms, tests, diagnoses, therapies, health-related behaviours and referrals to secondary care. For over half of patients, linkage with datasets from secondary care, disease-specific cohorts and mortality records enhance the range of data available for research. The CPRD is very widely used internationally for epidemiological research and has been used to produce over 1000 research studies, published in peer-reviewed journals across a broad range of health outcomes. However, researchers must be aware of the complexity of routinely collected electronic health records, including ways to manage variable completeness, misclassification and development of disease definitions for research.

Atrial fibrillation (AF) carries an increased risk of ischaemic stroke, and oral anticoagulation with warfarin can reduce this risk. The objective of this study was to evaluate the association between time in therapeutic International Normalised Ratio (INR) range when receiving warfarin and the risk of stroke and mortality. The study cohort included AF patients aged 40 years and older included in the UK General Practice Research Database. For patients treated with warfarin we computed the percentage of follow-up time spent within therapeutic range. Cox regression was used to assess the association between INR and outcomes while controlling for patient demographics, health status and concomitant medication. The study population included 27,458 warfarin-treated (with at least 3 INR measurements) and 10,449 patients not treated with antithrombotic therapy. Overall the warfarin users spent 63% of their time within therapeutic range (TTR). This percentage did not vary substantially by age, sex and CHA2DS2-VASc score. Patients who spent at least 70% of time within therapeutic range had a 79% reduced risk of stroke compared to patients with ≤30% of time in range (adjusted relative rate of 0.21; 95% confidence interval 0.18-0.25). Mortality rates were also significantly lower with at least 70% of time spent within therapeutic range. In conclusion, good anticoagulation control was associated with a reduction in the risk of stroke.

To cite this article: Gallagher AM, Rietbrock S, Plumb J, van Staa TP. Initiation and persistence of warfarin or aspirin in patients with chronic atrial fibrillation in general practice: do the appropriate patients receive stroke prophylaxis? J Thromb Haemost 2008; 6: 1500-6. Summary. Background: Practice guidelines recommend longterm stroke prophylaxis in patients with chronic atrial fibrillation (cAF). Objectives: To examine treatment initiation and persistence and factors that influence the choice of cAF treatment. Patients/methods: This study used the General Practice Research Database, including computerized medical records of general practitioners in the UK. Patients aged 40+ years with cAF after 1 January 2000 were included. Cox proportional hazards regression models evaluated initiation and treatment continuation over time of warfarin and aspirin. Treatment discontinuation was defined as no repeat prescription within a three-month period after the expected end of the treatment course. Results: The study population included 41 910 cAF patients. Elderly patients (aged 85+) were less likely to start warfarin [relative rate (RR) = 0.16, 95% confidence interval (CI) 0.15-0.18] and more likely to start aspirin (RR = 1.66, 95% CI 1.47-1.88) than patients aged 40-64 years. A history of dementia (RR = 0.28, 95% CI 0.17-0.44) and falls (RR = 0.76, 95% CI 0.70-0.83) also reduced the likelihood of warfarin initiation. Adjusting for age and gender, higher stroke risk (CHADS 2 score) was not found to be associated with initiation of warfarin or aspirin contrary to current guidelines recommendations. One-year persistence was 70% for warfarin and 50% for aspirin. Treatment persistence was higher in elderly patients using warfarin and aspirin. A higher CHADS 2 score was associated with improved persistence only with warfarin. Conclusions: The low likelihood of patients with cAF in general practice remaining on treatment long-term indicates that not all benefits as observed in clinical trials may be achieved in usual clinical practice.

Although concordance levels were reasonably high, the findings from this study can be used to direct efforts for better recording in both datasets.