Using multiple random index dates with the reverse waiting time distribution improves precision of estimated prescription durations.

PURPOSE: To improve the precision of prescription duration estimates when using the reverse waiting time distribution (rWTD). METHODS: For each patient we uniformly sampled multiple random index dates within a sampling window of length δ . For each index date, we identified the last preceding prescription redemption, if any, within distance δ . Based on all pairs of last prescription and index date, we estimated prescription durations using the rWTD with robust variance estimation. In simulation studies with increasing misspecification we investigated bias, root mean square error (RMSE) and coverage probability of the rWTD using multiple index dates (1, 5, 10, and 20). We applied the method to Danish data on warfarin prescriptions from 2013 to 2014 stratifying by and adjusting for sex and age. RESULTS: In simulation scenarios without misspecification, the relative bias was negligible (-0.04% to 0.01%) and nominal coverage probabilities almost retained (93.8%-95.4%). RMSE decreased with the number of random index dates (e.g., from 1.3 with 1 index date to 0.6 days with 5). With misspecification, the relative bias was higher irrespective of the number of index dates. Precision increased with the number of index dates, and hence coverage probabilities decreased. When estimating durations of warfarin prescriptions in Denmark, precision increased with number of index dates, in particular in strata with few patients (e.g., men 90+ years: width of 95% confidence interval was 16.2 days with 5 index dates versus 35.4 with 1). CONCLUSIONS: Increasing the number of random index dates used with the rWTD improved precision without affecting bias.

Prevalence and incidence rates of atrial fibrillation in Norway 2004-2014.

OBJECTIVE: To study time trends in incidence of atrial fibrillation (AF) in the entire Norwegian population from 2004 to 2014, by age and sex, and to estimate the prevalence of AF at the end of the study period. METHODS: A national cohort of patients with AF (≥18 years) was identified from inpatient admissions with AF and deaths with AF as underlying cause (1994-2014), and AF outpatient visits (2008-2014) in the Cardiovascular Disease in Norway (CVDNOR) project. AF admissions or out-of-hospital death from AF, with no AF admission the previous 10 years defined incident AF. Age-standardised incidence rates (IR) and incidence rate ratios (IRR) were calculated. All AF cases identified through inpatient admissions and outpatient visits and alive as of 31 December 2014 defined AF prevalence. RESULTS: We identified 175 979 incident AF cases (30% primary diagnosis, 69% secondary diagnosis, 0.6% out-of-hospital deaths). AF IRs (95% confidence intervals) per 100 000 person years were stable from 2004 (433 (426-440)) to 2014 (440 (433-447)). IRs were stable or declining across strata of sex and age with the exception of an average yearly increase of 2.4% in 18-44 year-olds: IRR 1.024 (1.014-1.034). In 2014, the prevalence of AF in the adult population was 3.4%. CONCLUSIONS: We found overall stable IRs of AF for the adult Norwegian population from 2004 to 2014. The prevalence of AF was 3.4% at the end of 2014, which is higher than reported in previous studies. Signs of an increasing incidence of early-onset AF (<45 years) are worrying and need further investigation.