1. Medication possession ratio associated with statins from EHR prescribing data relative to pharmacy dispensing data in the OptumLabs™ Data Warehouse Objective Background Methods 1 Partnership for Health Analytic Research (PHAR), Beverly Hills, CA USA 2 Rutgers Biomedical and Health Sciences, Newark, NJ, USA 3 American Medical Group Association, Alexandria, VA, USA 4 Weill Cornell Medical College, Healthcare Policy and Research, NY, NY, USA 5 University of Pennsylvania, Perelman School of Medicine, Philadelphia, PA, USA 6 Visiting Fellow at OptumLabs, Cambridge, MA, USA Medication possession ratios (MPR) are commonly used by researchers and healthcare providers (HCPs) to reflect medication exposure and adherence. MPR is the percentage of time a patient possesses a supply of a prescribed medication. MPR is commonly derived using pharmacy claims data. MPRs are difficult to determine with EHR prescribing (Rx) data alone due to limited ascertainment of prescription fills and no dispensing or discontinuation data. The linkage between EHR Rx and pharmacy claims data provides a unique opportunity to study MPRs following an EHR Rx. Data from this research initiative is intended to provide guidance to researchers using EHR Rx data in the absence of pharmacy dispensing data. We conducted a retrospective cohort study within the OptumLabs Data Warehouse, which contains de-identified administrative claims and EHR data. We included patients who had both (linked) claims (with a pharmacy benefit) and EHR data. Among patients with an ambulatory statin EHR Rx, we determined MPRs using pharmacy claims within 12 months following the last statin EHR Rx in 2012. We conducted stratified analyses by the number of EHR Rxs in the 12 month follow-up period and by patients with an incident vs. prevalent statin EHR Rx. Christopher G Rowan, PhD 1,2,6, Nikita Stempniewicz, BS 3,6, James Flory, MD, MSCE 4, Tobias Gerhard, PhD 2, James D Lewis, MD, MSCE 5, John Cuddeback, MD, PhD 3,6 and Sean Hennessy, PharmD, PhD 5 Affiliations The purpose was to describe the MPR for statins following a statin EHR Rx. Results 16795, 25309, and 6744 patients had an EHR Rx for atorvastatin, simvastatin, and rosuvastatin, respectively. For all patients with a statin EHR Rx, approximately 60% had an EHR Rx in the prior year (prevalent cohort). MPR is defined as the percentage of days covered (out of 365 days) by pharmacy dispensing data. The median MPR at 12 months was 70%, 72%, and 57% for patients with an atorvastatin, simvastatin, and rosuvastatin EHR Rx, respectively. Figure 1 shows the percentage of patients achieving each MPR threshold within 365 days of the first statin EHR Rx during the study period. Figure 2 shows data stratified by the number of EHR Rxs in the 12 month follow-up period. MPRs were approximately 10% lower for patients with no prior statin EHR Rx, compared to the prevalent cohort. Conclusion 40%, 41%, and 33% of patients prescribed atorvastatin, simvastatin, and rosuvastatin, respectively, had a MPR of 80% or more during the 12 month follow-up period. With an increasing number of statin EHR Rxs in the 12 month exposure period, the % of patients achieving an 80% MPR increased. Given the potential for exposure misclassification, researchers using EHR data should be aware of MPRs relative to EHR Rx and dispensing data and the impact of improving classification by increased the number of EHR Rxs. Researchers should also be aware of MPR differences for incident vs. prevalent prescribing.