For the analysis of large employer plans, we analyzed a sample of medical claims obtained from the 2018 IBM Health Analytics MarketScan Commercial Claims and Encounters Database, which contains claims information provided by large employer plans. We only included claims for people under the age of 65, as people over the age of 65 are typically on Medicare. This analysis used claims for almost 18 million people representing about 22% of the 82 million people in the large group market in 2018. Seventy percent of larger group enrollees who were enrolled for more than six months had at least one prescription drug claim in the year. Weights were applied to match counts in the Current Population Survey for enrollees at firms of 1,000 or more workers by sex, age and state. Weights were trimmed at eight times the interquartile range.

For the analysis of Medicare Part D, we used the 2018 Medicare Part D prescription drug event (PDE) claims data from the Centers for Medicare & Medicaid Services (CMS) Chronic Conditions Data Warehouse (CCW) for a 20 percent sample of Medicare beneficiaries. The analysis was limited to enrollees who filled a prescription in 2018, which equaled 42.9 million enrollees out of 46.1 million total (93.1%).

For both datasets, MarketScan’s Red Book was used to classify drugs by generic id and the therapeutic/pharmacologic category of the drug product. Drug spending paid for by someone other than an enrollee’s insurer, drugs administered in an inpatient setting, or not classified under the controlled substance act were excluded. Each prescription drug claim was counted as a single prescription regardless of the quantity or strength of that prescription. Drugs were grouped by the generic drug name, which may include multiple brands, but treats combination products separately.

To identify prescriptions filled by a mail-order pharmacy, we used the field indicating the type of pharmacy that filled the prescription. It is not possible to determine the method by which the prescription was subsequently mailed, and thus the totals here reflect prescriptions delivered via the U.S. Postal Service, as well as those delivered by other services, such as FedEx or UPS. In the Part D claims, specialty pharmacy claims are reported separately from mail-order pharmacy claims, although in some cases, specialty pharmacies may ship directly to patients; our analysis does not count these prescriptions as mail order because we are unable to identify them as such.

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