WASHINGTON — Roughly 700,000 of the more than 8 million people who enrolled in marketplace health insurance plans have lost their coverage, Medicare administrator Marilyn Tavenner testified Thursday before Congress.

Her surprise disclosure came during a House Oversight and Government Reform Committee hearing in which Republicans blasted Tavenner about a lack of transparency under the Affordable Care Act.

After mentioning in Thursday’s testimony that 7.3 million Americans were covered by marketplace plans, committee chairman Darrell Issa, R-Calif., asked Tavenner what happened to the other 700,000.

“Individuals may have either gotten employer-sponsored insurance, they may have found that they were eligible for Medicaid instead of the marketplace, and some individuals may have decided not to go forward and pay,” said Tavenner.