American Board of Internal Medicine (ABIM) Certification Practice Exam

What laboratory finding helps confirm a diagnosis of methicillin-resistant Staphylococcus aureus in a patient with suspected bacteremia?

1. Positive blood culture for MRSA

2. Negative blood culture for MRSA

3. Positive urinalysis for bacteria

4. Chest X-ray confirming pneumonia

The correct answer is: Positive blood culture for MRSA

The confirmation of a diagnosis of methicillin-resistant Staphylococcus aureus (MRSA) in the context of suspected bacteremia is primarily established through the identification of the organism in blood cultures. A positive blood culture specifically for MRSA means that the laboratory has successfully isolated this strain of Staphylococcus aureus resistant to methicillin, confirming the presence of MRSA in the patient's bloodstream. Blood cultures are essential in diagnosing bacteremia because they allow for the detection of bacteria present in the circulatory system. In the case of MRSA, the laboratory utilizes specific techniques to test for resistance to methicillin, which distinguishes it from other strains of Staphylococcus aureus that may be sensitive to this antibiotic. Therefore, the presence of MRSA in blood cultures not only confirms the diagnosis but also guides appropriate antibiotic treatment, which is critical given the antibiotic resistance associated with MRSA. Other options, such as negative blood cultures or findings from a urinalysis or chest X-ray, do not provide specific confirmation of MRSA bacteremia and instead may indicate other conditions or organisms. Thus, the positive blood culture for MRSA is the definitive laboratory finding to support the diagnosis.