PROJECT SUMMARY Type 1 diabetes (T1D) and type 2 diabetes (T2D) impact nearly 30 million Americans. For both T1D and T2D patients, tight glycemic control can reduce the risk of microvascular (e.g., blindness, lower extremity amputations, etc.) and macrovascular (e.g., myocardial infarction) complications, ultimately reducing lifetime health care costs and improving quality of life. To achieve tight glycemic control, patients rely on various medications and different formulations of insulin. For T1D and insulin-using T2D patients, list prices for insulin more than tripled from 2007 to 2018.1 While the list price is not ultimately the amount paid by an insurer or the amount received by a manufacturer, the list price is used to determine cost sharing amounts for patients. In response to the rising cost of insulin, Colorado passed legislation to cap the out-of-pocket cost of insulin at $100 per month. While Colorado was the first state to pass such a policy, more than a dozen states and a Medicare pilot program have followed suit, making it important to understand the effects of legislative/regulatory caps on insulin out-of-pocket costs. Through a partnership of faculty from the University of Colorado and University of Utah, we propose to evaluate the effects of Colorado's legislative cap for insulin out-of-pocket costs. We will focus on two key outcomes: insulin utilization (Aim 1) and glycemic control (Aim 2). Using linked electronic health record and all-payer claims data from Colorado and Utah, we will conduct a difference-in-differences analysis. With a difference-in-differences analysis, we will compare the change in utilization and glycemic control before and after the implementation of the out-of-pocket cap in Colorado to changes in utilization and glycemic control in a comparison group (Utah) that is experiencing the same general trajectory, but was not exposed to the policy change during the period of analysis. Our proposal will break new ground by combining electronic health record and all-payer claims data so that we can not only measure utilization, but also the downstream clinical outcome of glycemic control, a variable not available from claims data alone. In addition to generating fundamental new knowledge on the effect of a legislative cap for insulin out-of-pocket costs, this research will generate pilot data that will support future grant applications on the relationship between out-of-pocket caps and long-run health outcomes and spending, and separately, the budget impact of such a policy change for payers. Our team is particularly well-suited to conduct this analysis, as we bring together expertise in prescription drug payment policy, quasi- experimental methods, treatment for diabetes, and the Colorado and Utah linked electronic health record and claims databases.