After officials in the last holdout state enacted a new rule, all 50 states and Washington, D.C. have mandates that require some level of insurance coverage for the treatment of autism.

The Tennessee Department of Commerce and Insurance now requires all individual, small and large group insurance plans issued in the state to cover treatment for autism that is “medically necessary and appropriate and is not experimental,” including applied behavior analysis, or ABA, at the same level as any other medical condition.

Before the new rule was issued in August, insurance companies in the state could define their own mental health benefits, but now they must cover the treatment of autism and any other condition as listed in the Diagnostic and Statistical Manual of Mental Disorders.

Stuart Spielman, senior vice president of advocacy at Autism Speaks, said the new rule should also stop insurance companies in Tennessee from limiting the number of treatment hours that are covered under their plans.

The parent-led movement to extend insurance coverage for autism throughout the country started in the early 2000s.

“When I started down this advocacy path a dozen years ago, the only families who could access ABA for their children with autism were wealthy families and those lucky enough to live in the few states (that funded therapy programs),” said Lorri Unumb, CEO of the Council of Autism Service Providers, a Massachusetts-based nonprofit that advocates for and supports autism care agencies. “Now, most families in the United States have coverage for ABA.”

State mandates on insurance coverage are effective at increasing access to autism treatment, particularly for younger children, according to researchers who have studied claims data in states with and without the required coverage.

“The mandates result in slightly increased numbers of people getting treatment and more treatment for those already getting treatment,” said David Mandell, director of the Penn Center for Mental Health at the University of Pennsylvania.

While Tennessee’s move is a victory for advocates for people with autism, many gaps in insurance coverage remain. Most states only mandate coverage for children with autism, with cutoffs at age 18 or 21. And self-funded plans, which cover about 90 million Americans in the private sector, are not regulated by states.

“The country is a patchwork of autism laws,” said Unumb, who before joining the Council of Autism Service Providers earlier this year helped lead the push for autism insurance coverage in her more than 10 years as vice president for state government affairs at Autism Speaks. “The mandates are really strong in some states and somewhat weak in others.”

Unumb said advocates continue to educate families about their rights under federal and state laws, and lobby state lawmakers to improve their rules.

“It’s what keeps me awake at night,” said Unumb, whose adult son has autism. “After all of the progress we have made in all 50 states, still some child will get diagnosed tomorrow and will be unable to afford the treatment recommended by the doctor, notwithstanding being covered by insurance. That’s wrong.”