UnitedHealth Group collected far more than any other Medicare health insurer in risk-adjustment payments in 2017 that were based on questionable billing practices, a federal agency disclosed Tuesday.

The finding puts the Minnetonka-based health care giant at the center of a controversy over whether health insurers use data from patient chart reviews and health risk assessments in ways that grant them improper payments from the federal Medicare program.

Investigators at the Office of Inspector General (OIG) at the U.S. Department of Health and Human Services have said they're concerned that insurers with Medicare Advantage health plans might be using the risk assessments, in particular, to game the system.

Whistleblower lawsuits have alleged that UnitedHealth Group and others have wrongly used data from chart reviews to boost Medicare payments — an industry-wide concern also raised previously by OIG.

In September, an OIG report found 162 Medicare Advantage health insurers collectively received $9.2 billion in risk-adjustment payments based on two questionable methods companies use to document the health status of enrollees.

Of all the insurers, investigators found that one company stood out from the rest — a company that covered 22% of all beneficiaries enrolled in the health plans at the time, yet received a disproportionately high $3.7 billion, or 40% of the total payments based on these methods.

OIG did not identify the insurance company in its September report, but named UnitedHealth Group on Tuesday in response to a data request from the Star Tribune.

In a statement, UnitedHealth Group said the report was "based on old data and is inaccurate and misleading — a disservice to seniors and an attack on the [federal government's] payment system."

Risk-adjustment payments are directed to companies that run Medicare Advantage health plans, an increasingly popular option for seniors who want to receive their government benefits through private health insurance companies.

The federal government pays more money to Medicare Advantage (MA) insurers that enroll patients with conditions requiring more costly medical services. The goal is to compensate health plans that cover people with complex medical needs, so insurers aren't motivated to cherry pick just healthy enrollees.

Risk-adjustment payments generally are based on records from when beneficiaries receive medical care for conditions, but insurers also can review medical charts and administer health risk assessments (HRAs) to find other diagnoses that health care providers might not have reported for some reason. The report focused on payments based solely on diagnoses using these two methods, investigators said, because patients might never have received care for those conditions.

"Companies should receive appropriate compensation for providing care to beneficiaries with serious and chronic health conditions," the report concluded. "However, mechanisms such as chart reviews and HRAs should not be misused to collect diagnoses that inappropriately increase payments to MA companies and do not result in improved care for MA beneficiaries."

Investigators said they are particularly concerned about payments based on health risk assessments (HRAs) performed in patient homes, since Medicare Advantage plans themselves or vendors hired by insurers often conduct the assessments.

The practice is concerning, OIG says, because information from those assessments might not be communicated to the patient's primary care givers, or because the diagnoses reported based solely on an HRA might not be valid.

The report found UnitedHealth Group received 58% of all payments based on HRAs, or $1.5 billion out of $2.6 billion overall paid using that controversial method. UnitedHealth Group accounted for two-thirds of all risk-adjusted payments resulting from diagnoses reported only on in-home HRAs and no other service records.

UnitedHealth Group defended the use of in-home health risk assessments, which are common in Medicare Advantage plans from the company's health insurance division UnitedHealthcare.

"In-home clinical care programs and chart reviews are needed for appropriate senior care and payment," the company said. "UnitedHealthcare's status as an early clinical home provider is not only appropriate, it's best practice."

In previous investigations, OIG has determined that chart reviews and HRAs have an impact on risk-adjustment payments, Jacqualine Reid, a research analyst who led the September report, said in an interview.

The September report "did not determine whether the companies were engaging in fraud," Reid said. Even so, it raises important questions about payment integrity, she said, since health plans shouldn't be paid based on inaccurate diagnoses.

"For this report, we wanted to determine if some companies had a disproportionate share of payments from chart review and HRAs," she said. "We determined that 20 companies had a disproportionate share, which could not be explained by their enrollment size. And then, additionally, one company really stood out."

Based on the findings, investigators recommended that the federal Centers for Medicare and Medicaid Services (CMS) take action to determine the appropriateness of payments to UnitedHealth Group. The agency did not respond to a request for comment.