DURHAM, N.C. — Duke University Health System's chief executive officer is defending the health care giant's actions in the wake of a medical mix-up.

Last fall, doctors used tainted surgical tools to perform close to 4,000 operations at two Duke-affiliated hospitals. When the mistake was discovered, patients received a letter from the health care system informing them of what had happened.

Duke University Health CEO Dr. Victor Dzau said he had wanted to wait until Duke got the lab analysis back from the hydraulic fluid before talking with the media, but with all the attention surrounding the medical mix-up, he felt it was best to speak out now.

"We went through the investigation; we found the problem; we informed the doctors and patients," Dzau said.

Patients, however, said they are frustrated and upset because six months after they were informed, Duke still has not released the components in the hydraulic fluid.

"I fully empathize with patients saying, 'I haven't heard from you, what's going on?'" Dzau said.

Seven months after her surgery where doctors used tainted instruments, Carol Svec said it is frustrating for her to think that her slow recovery may be because of the hydraulic fluid. Since her surgery, she has had frequent headaches and some swelling, she said.

"There's been complete silence from Duke and we need answers," Svec said. "I'm worried because I don't know the effects on my future health."

When asked if he could be sure the patients would not have any ill long-term effects, Dzau said he thought no one can ever be sure.

"We need these people followed and tested," he said.

Dzau insists, however, that the infection rate involving the 4,000 patients is no greater than the average of 1 percent.

He said the hydraulic fluid used is currently being analyzed by an independent lab and expects to release the fluid ingredients in the next couple of weeks.

"Every patient that feels they were not well-served, we have to say we'll do better," Dzau said. "But that's not the equivalent of 'We did something wrong.'"

Duke University Health System is sending out new letters this week to physicians to update them and their patients.