The FTC and DOJ’s Proposed Statement of Antitrust Enforcement Policy Regarding Accountable Care Organizations Participating in the Medicare Shared Savings Program explains how the antitrust agencies will screen accountable care organizations (ACOs) to identify ones that are potentially anticompetitive and therefore require further review. In contrast to the antitrust screens the agencies have set forth in earlier Statements or Guidelines which are based on market shares in relevant markets, the Proposed Statement introduces a screen based on market shares in Primary Service Areas (PSAs). By examining several numerical examples, the proposed ACO screen is compared to some alternatives, including the review thresholds set forth in the 2010 Horizontal Merger Guidelines. The results indicate that the proposed ACO screen produces results qualitatively similar to other screens in some instances, but can be more stringent or more lax in others. It is therefore at best premature to conclude that PSAs are a useful tool for screening out potentially anticompetitive ACOs.