Surgery patients end up back in the hospital most often because of incision infections that don't show up until after they're sent home, according to a study that found unexpected readmission rates vary widely.

Among six common surgeries, the lowest readmission rate was just under 4 percent for hysterectomy patients. The highest rate - almost 15 percent - was for artery disease patients who had surgery to reroute blood flow in the legs.

"Most of these things are clearly related to the surgery, well-known accepted complications that we all try to reduce," said Dr. Karl Bilimoria, the study's senior author and director of the surgical quality improvement center at Northwestern University's Feinberg School of Medicine.

Bilimoria said surgery patients need clear instructions with information on warning signs and who to contact day or night if symptoms occur after they go home. Patients should seek treatment at the earliest sign of an infection, which may include redness or swelling, he said.

The researchers analyzed 2012 data from 346 hospitals involved in an American College of Surgeons quality improvement program. Results were published Tuesday in the Journal of the American Medical Association.

The study notes that hospital readmissions are a focus of nationwide efforts to control hospital costs and improve quality of patient care.

Infections at the surgery site typically are caused by bacteria entering the incision area, sometimes due to bacteria from the patient's own body, or from inadequately sterilized surgical instruments. In recent years, hospitals have beefed up efforts to fight them, with staff hand-washing campaigns and other methods.

But these infections can be difficult to prevent and in most patients studied, symptoms didn't show up until several days after they went home.

Some patients are more vulnerable, including those with artery disease affecting the legs, who have poor blood flow and often other illnesses.

Incision infection was the reason for more than 1 in 3 unplanned readmissions for these patients, and for at least 1 in 4 patients who had either hysterectomies, abdominal hernia repair, or colorectal surgery. Among hip or knee replacement patients, it was the reason for almost 1 in 5 readmissions, and for obesity surgery patients, just over 11 percent of readmissions.

Dr. Lucien Leape, a prominent patient safety advocate and Harvard School of Public Health professor, said incision infections are very rarely the patient's fault.

"The real focus has to be on what's happening in the operating room," said Leape, who wrote a JAMA editorial.

He noted that some surgeries seldom result in infections and said the solution might be to have other surgeons watch what goes on in operating rooms where patients are rarely infected.

"Go out there and copy the best," he said. "There's pretty good evidence that that works."