Sen. John Rockefeller IV is looking more closely at how health plans pay for out-of-network care.

Rockefeller, chairman of the U.S. Senate Commerce, Science and Transportation Committee, has sent letters to 18 insurers asking them about the data they use to determine reimbursement rates.

The recipients of the letters include several large Blue Cross and Blue Shield companies along with health insurers outside the Blue Cross and Blue Shield system, such as Humana Inc., Louisville, Ky., and Kaiser Permanente, Oakland, Calif., according to Commerce Committee officials.

The list also includes companies not commonly thought of as being health insurers, such as American International Group Inc., New York; MetLife Inc., New York; and Unum Group Corp., Chattanooga, Tenn.

The list does not include large health carriers such as Aetna Inc., Hartford, or WellPoint Inc., Indianapolis.

Rockefeller has held two hearings on out-of-network reimbursement practices in recent weeks. Witnesses have included representatives from Ingenix, a health care data firm that supplies pricing data to many health insurers, and its parent, UnitedHealth Group Inc., Minnetonka, Minn.

UnitedHealth President Stephen Helmsley defended the integrity of the pricing data at a hearing March 31.

New York Attorney General Andrew Cuomo has persuaded UnitedHealth to transfer the Ingenix pricing data to a new, nonprofit service, and he has persuaded several large insurers to help invest in launching the new service.

UnitedHealth agreed to the settlement because it believes in increasing public access to and confidence in the pricing data, Helmsley said.

In the letter to the health insurers, Rockefeller asks recipients whether they have used the Ingenix pricing data; how they disclosed use of the data to enrollees; and how they they will calculate out-of-network reimbursement when the new, nonprofit pricing service replaces Ingenix.