Mayo Clinic will start charging patients for some online messaging exchanges with their doctors, closing an increasingly popular backdoor route to free medical advice.

Patients will be warned, starting Friday, that their messages could result in charges of up to $50 if Mayo doctors respond with diagnostic information. The Rochester-based health care system announced the switch Monday in response to a recent 132% increase in patient messages, which sometimes averted the need for billable clinic visits.

Mayo is the latest large U.S. health system to start billing for online messaging, after the federal Medicare program outlined the conditions under which it would pay for the service.

Mayo practitioners received more than 6.4 million messages from patients in 2022, and were running out of time to respond while seeing other patients in clinic and virtual appointments, said Dr. Conor Loftus, chairman of Mayo's outpatient practice subcommittee.

"The volume and type and complexity of these messages has been increasing really exponentially," he said. "Within those messages, the type of care being delivered ... often involved care coordination and complex decision-making."

Mayo leaders believe a small fraction of exchanges will qualify for billing — only those initiated by patients that require doctors to make clinical decisions. There will be no cost when patients send messages about appointments or prescription refills or have follow-up questions about recent surgeries or exams.

Only 1.4% of messages qualified for billing after a similar switch at the University of California-San Francisco, according to recent research. That rate might have been suppressed by doctors declining to bill patients for fear of alienating them. The volume of patient messages declined slightly, but the change did not appear to increase clinic or virtual appointments, the study found.

Loftus said billing will slow the growth of messages flooding Mayo doctors' inboxes, but the health system doesn't want cost to be a barrier to care. Patients will be connected to financial services to cover costs of any exchanges they can't afford.

Sherri Fike of Boulder, Colo., said she used online messaging to gain a second opinion from Loftus about new medications and valued the exchange so much that she asked him to bill her. The 62-year-old retired aerospace engineer said online messaging to doctors allows her to explain her symptoms clearly and to keep a written record of the conversation.

"It really takes up their time, which is so valuable," she said. "I would hate to see them take telehealth or patient portal messaging away because they couldn't charge for it. That would be moving backward."

Medicare defines a billable exchange as a series of messages that requires at least five minutes of a clinician's time over seven days. Exchanges must be with established patients seeking help with new problems and not follow-up messages about recent treatments.

Online messaging has been more of an afterthought for doctors, who at Mayo were expected to reply within four days. Loftus, a gastroenterologist, said he often replied between office visits, during lunch hours or from bed at night. There were 80 messages waiting for him when he checked Sunday morning.

"You try to keep up," he said. "You hate to feel like there is a patient out there waiting for your answer."

Billing for online messaging will raise expectations and make the service mainstream — like video telehealth visits and virtual visits by which common medical problems are diagnosed through digital questionnaires. Doctors may need to carve out more time in their regular shifts to respond, Loftus said.

Mayo is researching the use of artificial intelligence to help doctors triage messages and identify the most urgent ones. Patients with new and complex symptoms that doctors need to see to diagnose will be referred to appointments or video visits without charge for the online exchange.

Among other Minnesota providers, Essentia Health and Children's Minnesota said in statements that they do not bill for online messaging. HealthPartners reported that it bills in rare circumstances when messages or e-visits don't involve existing diagnoses or recent appointments.

Allina Health said it doesn't bill for messaging and is looking at other strategies to reduce the volume of messages and the burden on doctors.