Methicillin-resistant Staphylococcus aureus (MRSA) infections doubled at U.S. academic medical centers between 2003 and 2008, according to a new report published in Infection Control and Hospital Epidemiology.

MRSA infections, which cannot be treated with antibiotics related to penicillin, have become common since the late 1990s. These infections can affect any part of the body, including the skin, blood stream, joints, bones, and lungs.  The findings run counter to a recent CDC study that found MRSA cases in hospitals were declining. The CDC study looked only at cases of invasive MRSA—infections found in the blood, spinal fluid, or deep tissue. It excluded infections of the skin, which the new study includes.

The researchers estimate hospitalizations increased from about 21 out of every 1,000 patients hospitalized in 2003 to about 42 out of every 1,000 in 2008, or almost 1 in 20 inpatients.

The researchers attribute much of the overall increase they detected to community-associated infections—those that were contracted outside the healthcare setting. When MRSA first emerged it was primarily contracted in hospitals or nursing homes. 

The study utilized the University HealthSystem Consortium (UHC) database, which includes data from 90 percent of all not-for-profit academic medical centers in the U.S. However, like many such databases, the UHC data are based on billing codes hospitals submit to insurance companies, which often underestimate MRSA cases. For example, hospitals might not report MRSA cases that do not affect insurance reimbursement for that particular patient. In other cases, hospitals might be limited in the number of billing codes they can submit for each patient, which can result in a MRSA code being left off the billing report if it was not among the primary diagnoses.

They corrected for these errors by using detailed patient observations from the University of Chicago Medical Center and three other hospitals. They looked at patient records to find the actual number of MRSA cases in each hospital over a three-year period. The team then checked the insurance billing data to see how many of those cases were actually recorded. They found that the billing data missed one-third to one-half of actual MRSA cases at the four hospitals. They used that rate of error as a proxy to correct the billing data from other 420 hospitals in the UHC database and arrive at the final estimates. 

"The rapid increase means that the number of people hospitalized with recorded MRSA infections exceeded the number hospitalized with AIDS and influenza combined in each of the last three years of the survey: 2006, 2007, and 2008," said Michael David, MD, PhD, an assistant professor of medicine at the University of Chicago and one of the study's authors. "I think this is still an underestimate of actual cases. But we can say with some assurance that this correction gives us a more accurate lower bound for how many cases [of MRSA] there actually are. What's clear from our data is that cases were on the rise in academic hospitals in 2003 to 2008."