By DENISE GRADY

string of mysterious infections at a hospital from 1998 to 1999 were traced to a most unexpected source, doctors are reporting today: a respiratory therapist who is suspected of having used a needle and syringe to steal a narcotic from the bags of intravenous medicine that were hanging by the bedsides of critically ill patients. The therapist is believed to have contaminated the medications while tapping into them.

The case, being described today in The New England Journal of Medicine, was uncommon, said Dr. Belinda E. Ostrowsky, who led an investigation by the Centers for Disease Control and Prevention. But it sheds light on the wider problem of drug abuse among health care workers and its potential to harm patients.

In an article accompanying Dr. Ostrowsky's report, Dr. Abraham Verghese of the Texas Tech Health Sciences Center wrote: "What is unusual is that the health care worker in question was a respiratory therapist and not a physician. When narcotics are misappropriated, the person implicated is usually a physician."

About 4 to 6 percent of doctors become addicted to drugs, and 10 to 15 percent abuse alcohol — proportions similar to those among the rest of the population. There are 676,000 practicing doctors in the United States.

In the outbreak described in the medical journal today, 26 patients developed a bloodstream infection with a bacterium called Serratia marcescens while in the surgical intensive care unit of a 455-bed medical center. In keeping with C.D.C. practice intended to promote investigatory cooperation and candor, Dr. Ostrowsky declined to name the institution. But she said it was not the Medical College of Virginia, in Richmond, where she now works.

The infected patients stayed longer in intensive care than other patients, possibly because of the infection, Dr. Ostrowsky said, adding that studies had shown that catching an infection in a hospital can add $10,000 to $40,000 to the cost of a person's care.

She said she and her colleagues had compared the records of people who had become infected with those of people who had not. They found that the infected patients were 30 times as likely as uninfected ones to have received the drug fentanyl, a narcotic that is 80 times as potent as morphine and that has been used by drug abusers.

The researchers found no reports of infections linked to fentanyl at other hospitals, and tests showed that unopened vials of the drug were sterile. But bags containing the drug that were taken from the bedsides of two infected patients contained bacteria identical to those found in the patients' blood.

In the meantime, Dr. Ostrowsky said, two nurses had told hospital officials that they had seen a respiratory therapist tampering with a patient's intravenous lines and crouching by the patient's bed with a needle. Respiratory therapists do not usually give injections or intravenous drugs. The patient in this incident subsequently became infected.

Dr. Ostrowsky knew that someone was under suspicion, she said, but asked the hospital not to tell her who it was right away, so as not to bias her investigation. She and her colleagues studied the records to find out which health care workers had been taking care of the patients.

They found that infected patients were 13 times as likely as uninfected ones to have been treated by a worker whom they identified as Respiratory Therapist 18. He turned out to be the same man the nurses had seen.

The therapist agreed to let Dr. Ostrowsky take bacterial cultures from his hands to look for Serratia, and the cultures came back negative. But the therapist also agreed to let the hospital test a sample of his hair, and that test came back positive for fentanyl, meaning that he had recently used the drug. Dr. Ostrowsky said she thought he might have infected the patients by injecting some nonsterile liquid back into their intravenous bags to prevent nurses from noticing that the level had dropped too quickly.

"To the end, he denied that he was involved," Dr. Ostrowsky said.

The therapist was fired, and the outbreak stopped. The hospital reported the case to the district attorney, who, Dr. Ostrowsky said, declined to pursue it because the evidence was considered insufficient.

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