1027. The KeyPath MRSA/MSSA Blood Culture Test BT Accurately Identifies Staphylococcal aureus Isolates Carrying Incomplete Staphylococcal Cassette Chromosome (SCC) Element (mecA dropouts) as MSSA
Session: Poster Abstract Session: Diagnosis of Bloodstream / Cardiovascular Infections
Saturday, October 22, 2011
Room: Poster Hall B1
Background: Timely, accurate determination of methicillin resistance and susceptibility in S. aureus is crucial for appropriate therapy and disease management. PCR methods that target the staphylococcal cassette chromosome mec (SCCmec) for MRSA identification have gained popularity. However, genotypic methods do not test directly for susceptibility and can diverge from phenotypic test results.  False positive MRSA calls for isolates carrying partial SCCmec elements (mecA dropouts) that are phenotypically MSSA have been reported. The prevalence of such strains can be as high as 13% among MSSA strains. The KeyPath MRSA/ MSSA Blood Culture Test - BT is a phenotypic test for positive blood cultures that returns a call of MRSA, MSSA or NSA (non-S. aureus) in about 5 hours. Here we examine the ability of KeyPath Test to return accurate ID/AST results from a set of well characterized mecA dropout clinical isolates.

Methods: Each test strain was spiked into BACTECTM Plus Anaerobic blood culture bottles, charged with blood drawn from volunteers and grown to alarm. Culture positive samples were first diluted 10-fold to achieve cell concentrations similar to limit of detection and then inoculated into the KeyPath MRSA/MSSA Blood Culture Test – BT. Tests were run according to manufacturer’s instructions. Test outcome was visually interpreted as MRSA, MSSA or NSA, by a panel of readers blinded to strain identity.

Results: A total of 28 S. aureus strains that were classified as MRSA by commercial PCR tests but determined to be MSSA by culture methods were tested. All 28 strains were correctly identified as S. aureus and also as MSSA by the KeyPath test.

Conclusion: Rapid commercial PCR based tests carry a high risk of misidentifying MSSA isolates as MRSA. This misidentification could result in inappropriate antibiotic therapy, and unnecessary implementation of contact precautions.  The KeyPath Test, which is a phenotypic test of cefoxitin susceptibility and resistance, is not subject to artifacts arising from gene mutations and rearrangements.  It is thus better able to identify MSSA strains irrespective of SCC element status and provide significant clinical utility by reducing false positive MRSA calls.

Subject Category: D. Diagnostic microbiology

Breanna Dreiling, BS, Kirsten Reed, BS, Tiffany Steinmark, BS, Dipankar Manna, PhD and Drew Smith, PhD, MicroPhage Inc, Longmont, CO


B. Dreiling, MicroPhage: Employee, Salary

K. Reed, MicroPhage: Employee, Salary

T. Steinmark, MicroPhage: Employee, Salary

D. Manna, MicroPhage: Employee, Salary

D. Smith, MicroPhage: Employee, Salary

Findings in the abstracts are embargoed until 12:01 a.m. EST Thursday, Oct. 20 with the exception of research findings presented at IDSA press conferences.