Health insurance deductibles were designed, in part, to influence patient behavior. And new data suggests they have, in at least one way: creating an uptick in visits in the final months in the year, when patients are likely to pay less out of pocket.

An analysis of 13.8 million patient visits on the athenahealth network in 2015 found that commercially insured patients paid 27 percent less out of pocket for visits to primary care physicians at the end of the year than the start.

Meanwhile, the volume of patient visits rose 17.3 percent between the first and fourth quarter — and there were 11 percent more visits in the fourth quarter than in any other quarter.

Patient obligations also fell — and volumes rose — for visits to pediatricians and orthopedists.

Those numbers match what many physicians have noticed anecdotally. Mark D. Moncino, M.D., a practitioner at Kaiser Permanente Medical Center near Atlanta, says he frequently sees more patient visits at the end of the year — along with more requests for costly services.

In just the past month, Moncino says, he has seen a patient request for a sleep study that was denied earlier this year; a patient request that a genetic evaluation be declared an emergency so it could be performed before December 31; and many more requests for expedited referrals to specialists.

“This will continue, as it always does, and the calls just keep on coming," Moncino says.

And patients who wait to visit their primary care doctors often try to cram multiple issues into a well visit, since it's free, said Budd Shenkin, M.D., who practices preventive medicine in Oakland, California.

“It gets very hard to control the visits in your office," he says. “As the doctor you want to take care of everything, but you can't do all the work that goes into both a well visit and sick visit for free. The system shifts the cost of medical care more on the patient."

Physicians can also speak to cramming in visits later in the year, themselves. Katherine Frederick, M.D., who practices pediatric medicine in Scarsdale New York, said last year her deductible was $4,000.

“Speaking as a patient, we saw every specialist in December, [from getting a mammogram] at the gynecologist to [going to the] endocrinologist," she says.

And the data shows a rise in visits to orthopedists late in the year, as out of pocket payments go down. An analysis of 3.5 million visits in 2015 found that the average out-of-pocket obligation for an orthopedic visit dropped from $63.23 in January to $39.60 in December — as visit volumes rose 13 percent.

David Jevsevar, M.D., chair of the American Academy of Orthopaedic Surgeons, says his New Hampshire practice is typically 10 to 15 percent busier in December. That's when patients' deductibles are paid off, he says, and many have time off from work or school, which they want to use to recuperate from procedures.

“We get lots of complaints because people wait 'till the last second and are mad when it gets full," he says.

Jevsevar said most of his patients aren't risking their health by putting off visits — but the danger is greater for patients with other health issues.

“With orthopedic surgery, some of it is elective or semi-elective, so there's not much detriment in waiting until December for a procedure," he says. “In other specialties, like diabetic care, there's a huge detriment to waiting."

And if patients aren't informed about how their deductibles work, doctors say, they can make choices that neither improve their health nor contain costs systemwide.

“When you put off something until you've met your deductible, it just means you spend the money elsewhere first," says David A. Horowitz, M.D., an adjunct professor in pediatrics at the University of North Carolina. “Whether you meet your deductible in my office, the specialist office, or the hospital, it costs you exactly the same amount of money."

At the same time, physicians worry about patients who never meet their deductibles, and might forego care altogether, says Glenn Schlundt, M.D., a pediatrician in California and former instructor at Harvard Medical School.

“Their deductibles are in effect never met, because their deductible resets after a year and is never reached," Schlundt says. “This is especially concerning when the high deductibles apply to high-cost medications."

Data analysis by Stewart Richardson. Illustration by Michelle Kondrich.