Insurance companies have long blamed private-equity-owned hospitals and physician groups for exorbitant billing that drives up health care costs. But a tool backed by private equity is helping insurers make billions of dollars and shift costs to patients.

The tool, Data iSight, is the premier offering of a cost-containment firm called MultiPlan that has attracted round after round of private equity investment since positioning itself as a central player in the lucrative medical payments field. Today Hellman & Friedman, the California-based private equity giant, and the Saudi Arabian government’s sovereign wealth fund are among the firm’s largest investors.