Seventy percent of U.S. health insurance markets are highly concentrated, based on guidelines used by the Department of Justice and Federal Trade Commission to assess market competition, according to a study released yesterday by the American Medical Association. In 89% of the 388 metropolitan areas studied, at least one insurer had a commercial market share of 30% or more, and in 14 states, a single insurer’s share was at least 50%. According to the study, the combined impact of proposed mergers among four of the nation’s largest health insurance companies would exceed federal antitrust guidelines designed to preserve competition in as many as 97 metropolitan areas within 17 states. The findings are based on 2013 data on commercial enrollment in fully and self-insured health plans, including consumer-driven health plans. For more information, see the AMA news release. The AHA recently submitted separate comment letters to the Department of Justice on the competitive impact of Aetna’s proposed acquisition of Humana and Anthem’s proposed acquisition of Cigna.