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Subject: Methicillin-Resistant Staphylococcus Aureus Culture (Rule out)
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This test is usually ordered to detect methicillin-resistant S. aureus (MRSA) carriage in asymptomatic patients for infection control purposes. It is indicated to screen patients at risk for MRSA self-infection or transmitting MRSA to close contacts, such as other hospitalized patients. The test may also be requested to document clearance of MRSA carriage.
Patient specimens are plated onto selective agar, typically containing 4–6 μg/mL of oxacillin. A base agar selective for gram-positive organisms (like PEA) or staphylococci (mannitol–salt agar) is often used to improve sensitivity of detection of MRSA. Selective chromogenic agar is commercially available to screen for MRSA carriage. These agars provide increased sensitivity and decreased turnaround time for detection of MRSA carriage.
Special collection and transport instructions: Swab specimens of the anterior nares, throat, axilla, perineum, and/or umbilicus (neonates) are usually submitted for MRSA screening cultures.
Turnaround time: 48–72 hours.
Expected results: Negative.
Any growth of S. aureus likely represents MRSA; confirmation of isolate identification and oxacillin resistance by standardized susceptibility testing is recommended.
Common pitfall: The MRSA screening culture is not recommended for evaluation of potentially infected material. Because only selective media are used, other potential pathogens would be missed if MRSA screening culture only is performed. MRSA isolates grow well in routine bacterial cultures submitted for evaluation of patient specimens.
Commercially available molecular diagnostic methods have been developed for detection of MRSA carriage. These assays have been shown to be more sensitive for detection of MRSA carriage, but the clinical implication for detection of very low level MRSA carriage has not been clearly defined.