Patti Sietz-Honig, a video editor at Fox 5 in New York, filed a complaint in 2022. The cost of seeing a specialist for chronic back pain had spiked, and she faced roughly $60,000 in bills.

Ms. Sietz-Honig pressed for updates about her complaint and sent articles critical of MultiPlan from Capitol Forum, a site focused on antitrust and regulatory news. Last March, the agency emailed her that her employer and her insurer, Aetna, had agreed to a “temporary exception” and made additional payments.

“Unfortunately,” the agency wrote, the law “does not prohibit the use of third-party vendors” to calculate payments.

Meanwhile, her longtime pain specialist started requiring payment upfront. To save money, Ms. Sietz-Honig spaced out her appointments.

“I’ve been in a lot of pain lately,” she said, “so I’ve been going — and paying.”

‘Not a Real Negotiation’

As MultiPlan became deeply embedded with major insurers, it pitched new tools and techniques that yielded even higher fees, and in some instances told insurers what unnamed competitors were doing, documents and interviews show.

After meeting in 2019 with a MultiPlan executive, a UnitedHealthcare senior vice president wrote in an internal email that other insurers were using MultiPlan’s aggressive pricing options more broadly, and that UnitedHealthcare could catch up.

“Dale did not specifically name competitors but from what he did say we were able to glean who was who,” the executive, Lisa McDonnel, wrote, referring to Dale White, then an executive vice president at MultiPlan. She described how Cigna, Aetna and some Blue Cross Blue Shield plans were apparently using MultiPlan.