Seventy-three 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 today by the American Medical Association. In 91 percent of the 380 metropolitan statistical areas studied, at least one insurer had a commercial market share of 30 percent or more, and in 46 percent of MSAs a single insurer’s share was at least 50 percent. “Our findings should prompt federal and state antitrust authorities to vigorously examine the competitive effects of proposed mergers between health insurers,” the report concludes. The findings are based on 2017 data captured from commercial enrollment in fully and self-insured health maintenance organizations, preferred provider organizations and point-of-service plans, consumer-driven health plans and public health exchanges.