Staying in a hospital raises the risk of contracting a multidrug-resistant infection, according to a recent study.

Researchers from the Medical University of South Carolina found that if a patient contracts an infection while in the hospital, each day of hospitalization increases by 1 percent the likelihood that the infection will be multidrug-resistant.

Hospital-acquired infections represent a large and possibly preventable segment of hospital-related deaths and have been rising in recent years. A European study suggested that Gram-negative infections account for two thirds of the 25,000 hospital-acquired infection deaths each year.

"Our findings emphasize one of the risks of being in the hospital, acquiring a multidrug-resistant infection" John Bosso, an author of the paper, said in a statement. "At the very least, this observation argues against both unnecessary hospitalization and unnecessarily long hospitalization."

For the study, researchers gathered and analyzed historical data from 949 documented cases of Gram-negative infection at their academic medical center. In the first few days of hospitalization the percentage of infections associated with Gram-negative bacteria classified as multidrug-resistant was about 20 percent and rose fairly steadily until four or five days, then jumped dramatically, peaking at over 35 percent at 10 days. Statistical analysis suggested an additional 1 percent risk per day of hospitalization.

The data revealed several other surprising findings. The chances that a patient would become infected with a multidrug-resistant pathogen varied from one organism to another. This could have implications for clinicians and others hoping to reduce dangerous hospital-acquired infections, Bosso added.

On any given day, about 1 in 25 hospital patients has at least one healthcare-associated infection and over a third of these infections are caused by Gram-negative bacteria, many of which are resistant to one or more classes of antibiotics. This study is the first to quantify the risks for patients over time, researchers said.

The findings were recently presented at the 54th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) an infectious disease meeting of the American Society for Microbiology.