The university hospital system did not address patients' fears that the mistake led to an increased risk of autoimmune or other noninfectious disorders.

In a letter sent this week to nearly 4,000 patients, the university cited a risk study done by an outside researcher it hired.

"We regret when any patient suffers," Duke said, adding that there is "always some risk of an undesirable outcome in any procedure."

Hydraulic Fluid Instead of Soap

For two months late last year, surgeons at Duke Health Raleigh Hospital and Durham Regional Hospital unknowingly used instruments that had been washed with hydraulic fluid instead of soap.

The error happened after elevator workers drained hydraulic fluid into empty soap containers without changing the labels.

Among the 4,000 patients was Shelley Bassett, who underwent a lumpectomy at the Raleigh hospital. Her husband, Kelley, said his wife soon developed gastrointestinal symptoms and lost 38 pounds.

"She was exposed to used hydraulic oil with cancer-causing agents and poisonous materials," he said, adding that infection was not a top concern.

'Immediate Jeopardy'

The federal Centers for Medicare and Medicaid Services in a report earlier this year said the hospitals had put patients in "immediate jeopardy" by not detecting the problem, despite complaints from medical staff about slick tools.

Duke said its tests were conducted by scientists led by William Rutala, director of the Statewide Program in Infection Control and Epidemiology at the University of North Carolina School of Medicine.

"We found that replacing cleaning detergent with hydraulic fluid did not alter the effectiveness of the sterilization process as high numbers of clinically relevant bacteria and standard test spores ... were completely inactivated," the letter quoted Rutala as concluding.