Study Data & Methods
The study used the NAACOS Cost and Quality Data Warehouse which contains 100 percent of ACO assigned beneficiaries in addition to 80 percent of those eligible for assignment, but not assigned from 2011 to 2015. Data elements include all institutional and provider bills for those included in the sample. The assigned beneficiary count, assigned beneficiary overlap, risk score and PBPY medical expenditures were studied for the 2015 performance year, during which 392 ACOs participated in the program. Four ACOs were eliminated from this study because they did not have complete data in the NAACOS Data Warehouse.
Furthermore, validation checks were used to compare actual retrospective assigned beneficiary counts from the MSSP 2015 ACO public use file (PUF) with calculated retrospective assigned beneficiary counts and PBPY medical expenditures. This resulted in the exclusion of an additional 66 ACOs that exceeded a 20 percent validation threshold. Variances between the NACOS Data Warehouse and PUF could have been caused by substance abuse claims contained within the PUF and not the NACOS Data Warehouse, large claims included in the NAACOS Data Warehouse but not the PUF, or provider participant list changes from 2013 through 2015. The final sample includes 322 ACOs.
Claims-based prospectively and retrospectively assigned populations were calculated using CMS published rules for the 322 ACOs in 2015. Since the NAACOS Data Warehouse is based on assignable beneficiaries, 2013 and 2014 claims data were available to calculate the October 1, 2013 through September 30, 2014 prospective assignment window for the 2015 performance year as if the retrospective assignment ACO were instead based on prospective assignment.
ACOs frequently change provider participant composition between performance years. Changes in provider participants from the study period October 1, 2013 through December 31, 2015 were handled by only considering the provider tax identification numbers (TINs) that were continuous participants during the entire study period. This methodology allows individual providers to change participation within the study period and is consistent with CMS policy. This caused discrepancies between the PUF and the NAACOS Data Warehouse because the PUF includes all participant TINs as they are added or removed throughout the 2013, 2014 and 2015 performance years.
Six metrics of interest were calculated under prospective and retrospective populations for each ACO to quantify the 2015 impact of prospective versus retrospective assignment. These metrics include total assigned beneficiary count, assigned beneficiary overlap or match percentage, risk score, PBPY medical expenditures, risk-adjusted PBPY medical expenditures, and assigned beneficiary death rate. The assigned beneficiary match percentage is defined as the percentage of prospectively assigned beneficiaries who were also retrospectively assigned as a proportion of the total number of retrospectively assigned beneficiaries.