Utility of antimicrobial susceptibility-based algorithms for the presumptive identification of genotypically-defined community-associated methicillin-resistant Staphylococcus aureus at a London teaching hospital.
OBJECTIVE
Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA) strains are classically characterised by susceptibility to most non-β-lactam antimicrobial agents. We sought to determine whether antimicrobial susceptibility (AMS)-based algorithms could be used to presumptively identify CA-MRSA in a hospital MRSA collection.


METHODS
Over a three-month period, all MRSA were tested for AMS, staphylococcal cassette chromosome mec (SCCmec) type, presence of the Panton-Valentine leukocidin (PVL) genes and spa type. CA-MRSA isolates were defined genotypically using a combination of spa and SCCmec type. AMS based algorithms were developed and tested for their sensitivity, specificity, positive predictive value (PPV) and negative predictive value (NPV).


RESULTS
Ciprofloxacin susceptibility (p < 0.001) and fusidic acid resistance (p  = 0.044) were independent predictors of CA-MRSA in a multivariate model. Although 98.5% of HA-MRSA were ciprofloxacin resistant, so too were 36.6% of CA-MRSA. Algorithms based on ciprofloxacin-susceptibility and fusidic acid resistance performed best, with specificity and NPV >90% and sensitivity and PPV >70%.


CONCLUSIONS
Our data indicate that while ciprofloxacin-susceptible isolates are likely to be CA-MRSA, the use of ciprofloxacin-susceptibility as a marker of CA-MRSA would miss approximately one third of CA-MRSA isolates. Therefore, AMS patterns have limited utility for the identification of genetically-defined CA-MRSA in our setting.
Department of Infection, St. Thomas' Hospital and King's College London, 5th Floor, North Wing, Lambeth Palace Road, London, SE1 7EH, Canada. jonathan.otter@kcl.ac.uk
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Otter
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French
D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000465:Algorithms; D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D000942:Antigens, Bacterial / Q000235:genetics; D002648:Child; D002675:Child, Preschool; D002939:Ciprofloxacin / Q000494:pharmacology; D017714:Community-Acquired Infections / Q000382:microbiology*; D024881:Drug Resistance, Bacterial; D004739:England; D005672:Fusidic Acid / Q000494:pharmacology; D005838:Genotype; D006784:Hospitals, Teaching; D006801:Humans; D007223:Infant; D008297:Male; D055624:Methicillin-Resistant Staphylococcus aureus / Q000145:classification* / Q000187:drug effects / Q000235:genetics / Q000302:isolation & purification; D008826:Microbial Sensitivity Tests; D008875:Middle Aged; D010641:Phenotype; D013203:Staphylococcal Infections / Q000382:microbiology*
D000900:Anti-Bacterial Agents; D000942:Antigens, Bacterial; C119204:streptococcal protective antigen; D005672:Fusidic Acid; D002939:Ciprofloxacin