CHICAGO – The rate of hospitalizations for bacterial infective endocarditis has risen sharply in the United States, mainly driven by cases caused by Staphylococcus aureus, investigators reported at the Interscience Conference on Antimicrobial Agents and Chemotherapy.

In a nationwide population-based study, a group led by Jerome J. Federspiel, an MD-PhD student at the University of North Carolina at Chapel Hill, assessed trends in and characteristics of such hospitalizations during a recent 10-year period (1999-2008).

Study results, based on nearly 84,000 hospitalizations for bacterial endocarditis, showed that the annual rate increased by 46%, although most of that increase occurred before 2006.

Rates of endocarditis hospitalizations for most causative pathogens remained fairly stable during the study period, but rates of hospitalizations for S. aureus endocarditis roughly doubled. Moreover, patients with S. aureus endocarditis had poorer clinical outcomes.

"Infective endocarditis–related hospitalization increased between 1999 and 2008, and this increase appears driven by Staph aureus–related hospitalizations," Mr. Federspiel commented. "Staph aureus–related admissions are associated with substantially higher mortality as well as greater service use, whether measured by length of stay or inpatient charges."

For the study, the investigators analyzed data from the Nationwide Inpatient Sample, using diagnostic codes to identify hospitalizations for bacterial infective endocarditis. Analyses were based on 83,700 hospitalizations for which patient disposition was known, representing an estimated 409,665 such hospitalizations nationwide.

Study results, based on nearly 84,000 hospitalizations for bacterial endocarditis, showed that the annual rate increased by 46%, although most of that increase occurred before 2006.

The hospitalized patients had a mean age of 62 years, the majority were male (59%), and 57% had Medicare insurance. On average, their length of stay was 15 days and their total inpatient charges were $97,630. A total of 13% died during hospitalization and 31% were discharged to long- or intermediate-term care facilities.

Trends showed an increase in the overall rate of hospitalizations from bacterial endocarditis over time, from 11.4 to 16.6 discharges/100,000 population-years between 1999 and 2008 (P less than .0001). Most of the increase occurred in the first 7 years.

The investigators were able to determine the pathogen for 56% of the hospitalizations, said Mr. Federspiel. Within this subset, the rates of hospitalization for endocarditis remained essentially stable during the study period when the disease was caused by coagulase-negative staphylococci; streptococci/enterococci; and multiple or other pathogens. In contrast, the hospitalization rate for S. aureus endocarditis roughly doubled, with most of the increase again occurring in the first 7 years.

"The number of unidentified cases was roughly parallel to the number of total cases," he noted. "Based on this evidence, we do not believe that the observed trends are the result of improved organism coding."

Analysis of the specific roles of methicillin-resistant S. aureus (MRSA) and methicillin-sensitive S. aureus (MSSA) was not possible, as coding for these pathogens was a fairly recent development in the study period. And origin of infection could not be determined.

"I think the issue of community-acquired vs. nosocomial infective endocarditis is a really important one," but getting a better handle on this issue will require additional information from other types of studies, said Mr. Federspiel.

In analyses limited to hospitalizations in 2002-2008 (when comorbidity data were available), patients with endocarditis caused by S. aureus were younger and more likely to abuse drugs than were patients with endocarditis due to coagulase-negative staphylococci or streptococci/enterococci.

But whether the rates of S. aureus endocarditis reflected increased intravenous drug use is unknown, according to Mr. Federspiel. "We didn’t specifically look at the rate of IV [intravenous] drug use over time, and what we show is simply drug use; there is no specific code for IV drug use in the United States," he explained.

The patients with S. aureus endocarditis also had longer lengths of stay and higher inpatient charges. And they were more likely to die: After adjustment, their rate of in-hospital mortality was 16.1%, compared with 10.5% for their counterparts with streptococcus/enterococcus endocarditis and 10.2% for their counterparts with coagulase-negative staphylococcus endocarditis (P less than .0001).

The study was limited by its reliance on administrative data, Mr. Federspiel acknowledged. "We don’t have the kind of rich clinical data you would need" to assess etiology in greater detail. "This is simply a complementary approach, in concert with our existing work in multicenter and geographically defined cohort studies, to understand infectious endocarditis epidemiology."

"We believe our results underscore the need to continue efforts to prevent endocarditis and to improve its treatment," he concluded at the conference, which was sponsored by the American Society for Microbiology.

Mr. Federspiel reported that he had no relevant conflicts of interest.

S. aureus Driving Upswing in Bacterial Endocarditis