A contract dispute between Blue Cross and Blue Shield of Minnesota and Fairview Health Services means up to 170,000 patients could lose access to Fairview services next year if an agreement isn't reached.

Eagan-based Blue Cross started sending letters about the dispute to subscribers on Monday. Fairview says it will start sending letters to patients, too.

Without a resolution by year's end, Minneapolis-based Fairview would become an out-of-network option for Blue Cross subscribers.

Patients could still get care from Fairview health care providers, but they would pay much higher rates that could be prohibitively expensive in some cases.

"Discussions are ongoing and an agreement can be reached at any time," Blue Cross said in a statement. "While it is very rare for Blue Cross not to reach an agreement before a current contract expires, we want to make sure our members stay informed."

Blue Cross is one of the state's largest health insurers. Fairview is one of the state's largest operators of hospitals and clinics, running both the University of Minnesota Medical Center in Minneapolis and Fairview Southdale Hospital in Edina.

Fairview officials said in a statement that they hope to resolve the dispute before patients are affected. The health system said it is negotiating for "the necessary reimbursement level to fund our mission and support exceptional patient care."

"Fairview is communicating directly with its patients who have Blue Cross coverage — about 170,000 patients in the previous 12 months," the health system said Tuesday in a statement. "They will receive a letter from Fairview in the mail, and we also have information posted on our website for patients."

In its letter this week to subscribers, Blue Cross says if a contract agreement isn't reached, Fairview would become a "nonparticipating provider" effective Jan. 1.

"There is no change in your coverage or access before this date, but after this date, if you choose to continue receiving services from Fairview Health Services, you will likely incur additional out-of-pocket costs and claims may be paid differently," Blue Cross said in the letter, a copy of which was obtained by the Star Tribune.

Contract disputes between insurers and health care providers occur periodically.

A contract dispute between Minnetonka-based Medica and Minneapolis-based Children's Hospitals and Clinics of Minnesota went public in November 2014, and was settled within a few weeks. There was no patient impact.

That was also true of a dispute between Blue Cross and Regions Hospital in St. Paul that settled in December 2013.

But not all disputes between health insurers and health care providers come to a resolution.

Last year, Medica and the mental health care provider PrairieCare could not reach an agreement on financial terms before a contract expired.

Twitter: @chrissnowbeck