Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1393 page)

   Limitations

   If the assay is used with cord blood as the specimen source, positive results should be interpreted with caution. The presence of IgG antibodies to measles in cord blood may be the result of passive transfer of maternal antibody to the fetus. A negative result, however, may be helpful in ruling out infection.

METHICILLIN-RESISTANT
STAPHYLOCOCCUS AUREUS
CULTURE (RULE OUT)
*

   Definition and Use

   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.

   Interpretation

   
Expected results:
Negative.

   Any growth of
S
.
aureus
likely represents MRSA; confirmation of isolate identification and oxacillin resistance by standardized susceptibility testing is recommended.

   Limitations

   
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.