Staphylococcus Aureus (SA) and Methicillin-Resistant Staphylococcus Aureus (MRSA)

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Subject: Staphylococcus Aureus (SA) and Methicillin-Resistant Staphylococcus Aureus (MRSA)

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Definition

  • The MRSA/SA molecular assay is a qualitative diagnostic test for the rapid detection of Staphylococcus aureus (SA) and methicillin-resistant Staphylococcus aureus (MRSA). Staphylococcal resistance to oxacillin/methicillin occurs when an isolate carries an altered penicillin-binding protein, PBP2a, which is encoded by the mecA gene. Most MRSA infections occur in people who have been in hospitals or other health care settings. These infections are known as health care–associated MRSA (HA-MRSA). HA- MRSA isolates often have multiply resistant to commonly used antimicrobial agents, including all β-lactam agents, erythromycin, clindamycin, and tetracycline. Community-associated MRSA (CA-MRSA) infection increases. CA-MRSA isolates are often resistant only to β-lactam agents and erythromycin. If clinically indicated susceptibility testing should be ordered.

Use

  • Diagnostic test—skin and soft tissue, surgical wound infections, many people who have a staph skin infection often mistake it for a spider bite; less commonly MRSA can cause the urinary tract infection, pneumonia, or infection of the bloodstream.

  • Preoperative testing—screening of patients for MRSA is associated with a significant reduction in subsequent MRSA surgical site infections.

  • Target screening—elective admissions, ICU admissions, neonatal units, trauma and burn units, patients with previous positive MRSA cultures, transfers from residential care facilities, and patients from specific high-risk wards (e.g. cardiothoracic, neurosurgery, orthopedic, renal).

  • Universal screening—at hospital admission; identified MRSA-colonized individuals are managed to prevent transmission and reduce MRSA prevalence in the patient population.

  • Nucleic acid amplification tests, such as the polymerase chain reaction (PCR), can be used to detect the mecA gene, which mediates oxacillin resistance in staphylococci.

Limitations

  • A negative MRSA or Staphylococcus aureus result should not be used as the sole basis for diagnosis, treatment or management decisions. Negative test results may occur from improper specimen collection, handling or storage, or presence of inhibitors, or because the number of bacteria in the specimen is below the analytical sensitivity of the test.

Considerations

  • Specimens should be kept between 2°C and 25°C during transport and protected against freezing or exposure to excessive heat.

  • Specimens can be stored up to 48 hours at 15–25°C or 5 days at 2–8°C before testing.

  • PCR test can be affected by genetic rearrangements or the presence of rare bacterial DNA variants.

  • Mutations or polymorphisms in primer- or probe-binding regions may affect detection of new or unknown MRSA/SA variants resulting in a false-negative result with the PCR assay.

 
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