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DURHAM, N.C. In one of the largest studies to date of surgical site infections (SSI) in adults, Duke University Medical Center researchers found SSI risk increases with each year in age until patients are 65 years old. Then, SSI drops in a steady decrease with each additional year. No infections were reported in patients more than 95 years old.
"I was surprised by the results," said lead author Keith Kaye, MD, assistant professor of infectious diseases at Duke University Medical Center. "Generally, risk for infection increases with age and I expected surgical site infection risk to also increase with age."
Surgical site infections occur in more than 325,000 patients each year in the U.S. and cost more than $1 billion per year in care, primarily due to increased hospital stay and increased morbidity and mortality in patients, Kaye said. Of the nearly 145,000 patients who underwent surgery during the Duke study, the risk of surgical site infection increased by 1.1 percent per year between ages 17 and 65. The SSI risk decreased by 1.2 percent per year for patients 65 years and older, he said.
The results raise important questions about why elderly patients have a decreased risk of SSI, Kaye said. "If elders are at a decreased risk due to a surgical selection bias -- in which only healthier elders are selected for surgery -- this is important to know and to address. Maybe age shouldn't be weighted heavily in surgical selection issues. Alternatively, if the decreased risk is associated with an innate immune phenomenon, it would be important to understand this process and its implications for other types of infections," Kaye said.
The study appears in the April 1, 2005, issue of The Journal of Infectious Diseases. Funding was provided by the Infectious Diseases Society of America; the Association of Subspecialty Professors; the John A. Hartford Foundation; Elan Pharmaceuticals; a Mid-Career Award in Patient-Oriented Research from the National Institute of Allergy and Infectious Diseases, National Institutes of Health; the Center for the Study of Aging and Human Development, Duke University Medical Center; and the Geriatric Research, Education and Clinical Center, Durham Veterans Affairs Medical Center.
According to Kaye, earlier studies examining the relationship between age and risk of SSI produced conflicting results. Some showed increasing age was linked to a greater risk of all types of postoperative infections, while others concluded that increasing age was not an independent risk factor for SSI. Most studies involved small sample sizes, were performed at a single hospital or examined only one category of surgery, Kaye said.
The Duke study included 144,485 patients who underwent surgery at 11 hospitals participating in the Duke Infection Control Outreach Network. All surgical site infections were prospectively identified by trained infection-control practitioners using standardized Centers for Disease Control and Prevention criteria for hospital-acquired (nosocomial) infections. The surgeries took place between February 1991 and July 2002.
The patients were divided into two groups -- one to explore the relationship between age and SSI risk and one to validate this relationship. The large sample size allowed researchers to statistically control for the effects of length of surgery and type of wound, the type of procedure and for hospital.
The Duke researchers identified 1,684 surgical site infections in their patient sample, a rate of 1.2 percent. This is comparable with the average SSI rate in the U.S., which varies between 1 percent and 3 percent, Kaye said. The risk of SSI increased linearly until age 65, then decreased linearly with additional age. There were no SSIs among the oldest patients, between 95 and 108 years old.
There are several potential explanations for the finding that patients older than 65 do not have an increased risk of SSI, Kaye said. "It's possible that older patients who have an increased risk of complications, such as frail elderly patients with comorbid conditions, have surgery less often than their healthy peers because doctors and/or patients judge their risk of adverse outcome to be too high," he said. Another possibility is that older patients had operations with a lower risk of SSI, while younger patients underwent more complicated surgeries with a greater SSI risk. However, the distribution of types of surgical procedures was similar for patients younger than and older than 65 years of age.
The decreased risk for very old patients, more than 80 years old, may be due to a "hardy survivor" effect - people who survive to much older ages may have a genetic makeup that enables them to better withstand health threats, Kaye said.
Study co-authors include Kristine Schmit, Richard Sloane, Kathleen F. Caughlan, Daniel J. Sexton and Kenneth E. Schmader, all of Duke.
Source: Duke University Medical Center