The number of accountable care organizations across the United States has grown 38 percent over the past six months, a new research report from health care business company Leavitt Partners finds.

The 221 ACOs across 45 states and the District of Columbia is a big jump from the 160 ACOs that were identified in November, spread across 40 states. ACO activity in the private sector outnumbers the government sector by a factor of four to one.

"The consistent growth in the number and variety of organizations adopting accountable care demonstrates the momentum of the ACO movement," says study author Andrew Croshaw. "Many providers appear to believe the accountable care model is an important component of the future of American health care."