A top Republican in the U.S. Senate is questioning Medicare Advantage billing practices at UnitedHealth Group following a critical report last year from a federal watchdog agency and a Wall Street Journal article last week on a new investigation by the Justice Department.

The Wall Street Journal reported Friday that the Justice Department recently launched an investigation into billing practices at Eden Prairie-based UnitedHealth Group to see if the company fraudulently used diagnosis data to trigger billions in extra Medicare Advantage payments.

In October, the Office of Inspector General at the U.S. Department of Health and Human Services said UnitedHealth Group stood out from its peers in the use of this data to boost payments.

U.S. Sen. Charles Grassley, R-Iowa, sent a letter to the company Monday, saying he's long been a proponent of Medicare Advantage health plans even as he's pushed for Medicare to recover improper payments to health insurers that sell the coverage.

"Despite these oversight efforts, [Medicare Advantage organizations] continue to defraud the American taxpayer, costing them billions of dollars a year," Grassley wrote to UnitedHealth Group CEO Andrew Witty in a letter first reported Tuesday by the Wall Street Journal.

Grassley added: "UnitedHealth Group benefited financially more than any other [Medicare Advantage organization], which raises serious questions about its practices. The apparent fraud, waste and abuse at issue is simply unacceptable and harms not only Medicare beneficiaries, but also the American taxpayer."

UnitedHealth Group said it welcomed the chance to share information with Grassley, "especially given the ongoing misinformation campaign by the [Wall Street Journal]."

The company says it has called for CMS to conduct audits for every Medicare Advantage (MA) health plan, every year, and holds itself "to the highest ethical standards of quality and integrity in our practices."

"Medicare Advantage plans are doing exactly what the program was designed to do: meet the government's objective of delivering better health outcomes and lower costs for seniors and Medicare overall," the company said in a statement. "The MA program is highly regulated by the federal government, MA plans are regularly audited for accuracy and compliance, and we consistently perform at the industry's highest levels."

Medicare Advantage is a privatized version of the federal Medicare health insurance program for seniors in which the federal government hires private health insurers to manage care for patients.

Grassley gave UnitedHealth Group until March 10 to explain what steps the company has taken to review and verify the accuracy of all diagnosis data submitted to the federal government based solely on health risk assessments and chart reviews — two methods called into question by reporting from federal auditors and the Wall Street Journal.

The senator also wants the company to quantify the number and amount of inappropriate payments identified as stemming from these processes.

UnitedHealth Group should provide all company policies and procedures for obtaining diagnostic confirmation from an enrollee's primary care provider, Grassley wrote, to make sure patients are treated for diagnoses identified in health risk assessments or chart reviews.

The senator also asked for all records and audits related to the company's compliance program for monitoring the accuracy and appropriateness of submitted diagnosis data from 2019 to 2024. Grassley wants to see the company's training manuals and guidance documents for conducting health risk assessments and manual chart reviews, plus information on related software.

"Does UnitedHealth Group use artificial intelligence to conduct the aforementioned processes?" the senator asked in the letter. "Are all diagnoses identified by artificial intelligence confirmed by a trained medical record reviewer?"

The letter asks UnitedHealth Group for information on billing practices raised by both the Medicare Office of Inspector General (OIG) and the Wall Street Journal.

Last year, the company rejected the OIG's findings, saying the watchdog agency's report presented a misleading and incomplete view of how UnitedHealth Group uses risk adjustment data as well as a program for providing in-home assessments to vulnerable seniors in Medicare.

The company said last week the Wall Street Journal "has engaged in a year-long campaign to defend a legacy [Medicare] system that rewards volume over keeping patients healthy and addressing their underlying conditions. Any suggestion that our practices are fraudulent is outrageous and false."

Separately, UnitedHealth Group is defending itself against a lawsuit in U.S. District Court in Minnesota where plaintiffs allege the company has used a faulty artificial intelligence program to deny coverage for post-acute care needed by Medicare patients.

UnitedHealth Group says the lawsuit is based on unfounded allegations, mischaracterizes the work of its clinicians and obscures how medical directors — not artificial intelligence — make coverage decisions in accordance with Medicare criteria.

UnitedHealth Group is Minnesota's largest company by revenue. It runs UnitedHealthcare, which is the nation's largest health insurer, plus a fast-growing division for health care services called Optum.