Northwestern got a “C” on the latest Hospital Safety Score report. So did all four North Shore suburban hospitals affiliated with the University of Chicago.

The report, a national survey by the non-profit group Leapfrog , combines data on infections, problems with surgery, and other issues to compile letter grades for more than 2,500 hospitals.

Northwestern pulled “A” and “B” grades in previous editions of the report, and for the most part so did the hospitals that are part of the Northshore University Health System.

In Northwestern’s case, the hospital’s grade fell because of new data added to the reporting system, on two serious kinds of infection: MRSA, a drug-resistant staph infection and Clostridium difficile, another drug-resistant bacteria.

“These are really prevalent and difficult infections, and with Northwestern Memorial, they scored really poorly on that,” says Tim Vogus, a Vanderbilt University professor who advised Leapfrog on the construction of its rating system.

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Another Leapfrog adviser says the measurements aren’t perfect, but they’re meaningful.

“If all else was absolutely equal, I think you’d be better off going to a hospital that consistently got As than hospitals that got Bs and Cs,” says Robert Wachter, a professor at the University of California San Francisco medical school and author of the book “Understanding Patient Safety.”

He hopes that hospitals will pay at least as much attention to the results as consumers— and focus on ways to improve.

“Hospitals like Northwestern, or University of Chicago or my own—you know, we’re used to getting As on tests,” he says. “So when you see yourself getting a B or a C, you say, ‘Boy, what are we doing here? Can we do better?’”

The data behind the ratings is posted to the Hospital Safety Score site, as are some of the technical guidelines that went into compiling the letter grades . “If people really want to nerd out on this, they can go find the root data,” Vogus says.

Twenty-seven local hospitals got “A” grades, including Rush and the University of Chicago Medical Center in Hyde Park.

Northwestern Memorial and Northshore University Health Systems had no comment on the report.

Dan Weissmann is a reporter for WBEZ. Follow him @danweissmann.