Even as the inspector general’s report was issued, on Sept. 27, doctors and patients and members of Congress were expressing concern about some practices of Medicare Advantage plans.

“Patients may be encountering barriers to timely access to care that are caused by onerous and often unnecessary prior authorization requirements,” said a letter sent to the Trump administration this past week by a bipartisan group of more than 100 lawmakers.

Insurers defend the requirements. They “protect patients from unnecessary and inappropriate care” and help reduce costs, said Matt Eyles, the president and chief executive of America’s Health Insurance Plans.

The growth of Medicare Advantage is driven not only by consumer choice, but also by policies set in Washington. Several factors have contributed to a favorable environment for Medicare Advantage plans, allowing them to reduce premiums or add benefits:

■ The Trump administration approved a big increase in payments to private plans for 2019, saying it was “committed to unleashing and strengthening the Medicare Advantage program.”

■ In addition to the general business tax cuts enacted last year, Congress provided additional tax relief to health insurers for 2019, suspending a fee imposed on them by the Affordable Care Act.

■ Medicare Advantage plans will be able to offer extra benefits, which could include transportation to the doctor’s office, home delivery of hot meals and safety features in the home like wheelchair ramps and bathroom grab bars. Such benefits, which are generally not available in the traditional Medicare program, may be attractive to older Americans who want to combine social and medical services.

“One in two American seniors will be in Medicare Advantage by 2021, given the industry-friendly rate and tax policies of the Trump administration,” said Ana Gupte, a health care analyst at Leerink Partners in New York.