If you’re covered by traditional Medicare, you currently need only a doctor’s prescription to get expensive imaging tests such as MRIs, CAT scans, and nuclear imaging. But that will soon change.

Next year, instead of relying solely on your doctor's judgment on when you need imaging, the doctor will need to go through an electronic portal to determine whether the test meets “appropriate use” guidelines for your condition. This requirement will be voluntary in 2018 but mandatory in 2019.

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Appropriate use criteria are intended to help a physician determine the best treatment decisions for a specific clinical condition.

This is different from the prior authorization process—which may also be called preauthorization or precertification—you’ve probably experienced when getting certain medical treatments through a Medicare Advantage or commercial insurance plan.

Under prior authorization, or PA, your insurer, not your doctor, may decide whether certain therapies, procedures, prescription drugs, or equipment (such as an electronic wheelchair) are medically necessary and thus covered.

Getting prior authorization for a medical treatment isn’t always straightforward. Here’s what to know:

Rulers Vary From Insurer to Insurer
Every insurance company has its own rules about which medical treatments and services need prior authorization.

For example, Aetna, my carrier, requires PA for inpatient stays (except hospice care), the use of an air ambulance, outpatient physical therapy, chiropractic treatment, pain management, and many other items, including a long list of medications.

They Can Take Time or Go Astray
If prior authorization is needed, your doctor or office staff should let you know and request it from your insurer. But if you don’t hear from your healthcare provider about the status of the request within five days, check back with the office.

Sometimes insurers simply take their time arriving at a decision. A December 2016 survey of 1,000 doctors by the American Medical Association found that on average, 20 percent reported waiting three to five days for PA decisions from health plans, and 6 percent reported that it took more than five business days.

Your doctor’s office staff may also get backed up, forget to request a PA, or neglect to notify you that it has been attained.

When it’s approved, be sure to ask for and write down the authorization number, because treatment facilities usually require it.

Prior Authorizations Could Slow Your Care
Few doctors have the staff to handle the demands that PAs entail, and the new Medicare regulations will probably add to the burden.

Indeed, 80 percent of the nearly 1,100 physicians who participated in a 2017 Medical Economics “scorecard” reported that the process represented a significant challenge.

And the 2016 AMA survey found that an average of 16.4 hours a week of physician and staff time was spent completing prior authorization requests. Ninety percent of surveyed physicians reported that the process delayed patient access to necessary care.

Prior Authorization Isn’t Always Enough
Getting prior authorization isn’t an absolute guarantee that your insurer will cover a service. That’s because a given treatment must also meet all of the conditions in the Summary Plan Description of your health plan’s policy.

If your doctor is unsure whether a treatment will satisfy these conditions, call the insurer yourself.

Prior Authorization Numbers Are Time-Limited
The prior authorization number you receive when a PA is approved won’t last forever. For example, the authorization number for commercial United Healthcare plans is valid only for 45 days.

Ask your doctor what length of time the service has been authorized for—and jot it down—so you can avoid problems.

You Can Fight a Denial
If you receive a denial, ask your doctor why the medical treatment or service failed to meet appropriate-use guidelines. (Sometimes it may be that it’s not medically necessary.)

But be aware that your insurer’s decision can be appealed. Your doctor can usually even ask for a peer-to-peer review so that he or she can explain the rationale for requesting the service to an insurance-company physician.

Clarification: This article has been updated to reflect the differences between prior authorization and appropriate use criteria.