High Vancomycin MICs in MRSA Patients Signal Endocarditis

Fran Lowry

February 15, 2012

February 15, 2012 (Houston, Texas) — When the vancomycin minimum inhibitory concentration (MIC) is 2 µcg/mL or more in patients with methicillin-resistant Staphylococcus aureus (MRSA) bacteremia and severe sepsis, it could mean that they are at risk for endocarditis, according to a study presented here at the Society of Critical Care Medicine 41st Critical Care Congress.

"When this is the case, we should start thinking about other antibiotics, rather than just sticking to vancomycin," lead author Anindita Chowdhury, MD, from Washington Hospital Center, Washington, DC, told Medscape Medical News after her poster presentation.

Dr. Anindita Chowdhury

Factors associated with the development of endocarditis have been unclear. In this study, Dr. Chowdhury and her team conducted a retrospective review of 225 patients with MRSA bacteremia and severe sepsis at their hospital from January 2009 to December 2010 to see how many of them went on to develop endocarditis.

They also noted potential confounders, including comorbidities, severity of illness, and demographic and infection characteristics.

The investigators used logistic regression to determine independent factors associated with endocarditis to establish a risk score to help determine the likelihood of the disease.

Of the 225 subjects, 12.4% developed endocarditis, and a vancomycin MIC of 2 µcg/mL occurred in 5.3%.

The researchers found that endocarditis was seen in 11.3% of patients when the MIC was less than 2 µcg/mL, and in 33.3% when the MIC was 2 µcg/mL (P = .047).


In addition, subjects diagnosed with bacteremia in the intensive care unit (ICU) were 4.1 times more likely to suffer endocarditis (95% confidence interval, 1.7 to 10.1).

Three factors other than MIC were independently linked to endocarditis: care in the ICU, underlying congestive heart failure (CHF), and primary bacteremia or infection related to an intravascular device.

The researchers assigned points to each risk factor, allotting 2 points for ICU care, and 1 point each for underlying CHF, primary bacteremia, or a MIC of 2 µcg/mL.

The more of these risk factors a patient had, the greater the risk for endocarditis.

When none of these factors were present, the prevalence of endocarditis was 0.0%; when all were present, it was 66.7% (P < .001).

"This is an interesting idea, especially since we treat MRSA endocarditis with a longer course of antibiotics," said Todd W. Rice, MD, from Vanderbilt University School of Medicine, Nashville, Tennessee.

"If we knew who was at increased risk of getting endocarditis, maybe we could be more aggressive with antibiotic coverage initially," explained Dr. Rice, who was not part of the study.

However, this is difficult to determine, he said.

"Knowing...when the endocarditis actually happens, when the heart valve actually becomes seeded with the bacteria, is tough. Many of these characteristics may be associated with endocarditis already being present, rather than with its development," Dr. Rice said. "The valve may have already been seeded before the patient went to the ICU, or the 'primary bacteremia' may just be bacteria in the blood stream from the infected heart valve that hasn't been identified yet."

Dr. Rice pointed out that most hospitals and clinicians do not have access to vancomycin MIC values. Even where these values are obtainable, they are not done routinely but by special order, and so not available on every patient.

It will be important to see if this score will enable us to "provide different antibiotic coverage, with a longer course or more antibiotics, and actually effectively prevent or lower the incidence of endocarditis from developing," he said.

Dr. Chowdhury and Dr. Rice have disclosed no relevant financial relationships.

Society of Critical Care Medicine (SCCM) 41st Critical Care Congress: Abstract 289. Presented February 6, 2012.