title: Rapid detection of methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-susceptible Staphylococcus aureus (MSSA) using the KeyPath MRSA/MSSA blood culture test and the BacT/ALERT system in a pediatric population.
abstract: BACKGROUND
  Timely initiation of directed antimicrobial therapy for Staphylococcus aureus bacteremia is dependent on rapid identification of S. aureus to ascertain methicillin-susceptibility status.
  
  
  OBJECTIVE
  To investigate the performance of the rapid KeyPath (MicroPhage, Inc, Longmont, Colorado) methicillin-resistant S. aureus (MRSA) and methicillin-susceptible S. aureus (MSSA) blood culture test (MMBT).
  
  
  METHODS
  Positive BacT/ALERT Pediatric FAN (fastidious antibiotic neutralization) blood culture bottles (bioMérieux, Inc, Durham, North Carolina) were tested prospectively using MMBT and routine bacterial identification and antibiotic susceptibility testing procedures as the gold standard. The MMBT uses an S. aureus-specific bacteriophage cocktail that infects bacterial cells and replicates them, resulting in cellular lysis. Bacteriophage-specific antibodies detect the increase in bacteriophage concentration in an immunoassay device. Phage amplification, in both the presence and absence of cefoxitin, indicates the presence of MRSA. The sensitivity, specificity, positive predictive value, and negative predictive value of MMBT in detecting S. aureus, MSSA, and MRSA were calculated.
  
  
  RESULTS
  Of 188 positive blood cultures tested, 199 (63%) had Gram-positive cocci in clusters, 46 (24%) grew S. aureus (26 MSSA [57%], 20 MRSA [43%]) with the MMBT detecting 40 of 46 (87%). The sensitivity, specificity, positive predictive value, and negative predictive value among blood cultures with Gram-positive cocci in clusters were 87%, 100%, 100%, and 92% for S. aureus; 81%, 100%, 100%, and 95% for MSSA; and 95%, 100%, 100%, and 99% for MRSA. All blood cultures without growth of S. aureus tested negative by MMBT.
  
  
  CONCLUSIONS
  The MMBT detected MSSA and MRSA directly from positive BacT/ALERT PF bottles with positive predictive values of 100%, suggesting that positive results could be reported immediately, but the sensitivity of this assay limited immediate reporting of negative results.
authors.affiliation: Department of Pathology and Laboratory Medicine, The Children's Hospital of Philadelphia, Philadelphia, Pennsylvania 19104, USA. otak@email.chop.edu
authors.forename: Kaede V
authors.initials: KV
authors.lastname: Sullivan
authors.forename: Nicole N
authors.initials: NN
authors.lastname: Turner
authors.forename: Sylvester S
authors.initials: SS
authors.lastname: Roundtree
authors.forename: Karin L
authors.initials: KL
authors.lastname: McGowan
mesh_terms: D016470:Bacteremia / Q000175:diagnosis / Q000188:drug therapy / Q000382:microbiology; D001431:Bacteriological Techniques / Q000379:methods* / Q000706:statistics & numerical data; D002648:Child; D006801:Humans; D016106:Methicillin Resistance; D055624:Methicillin-Resistant Staphylococcus aureus / Q000187:drug effects / Q000302:isolation & purification*; D008826:Microbial Sensitivity Tests / Q000379:methods / Q000706:statistics & numerical data; D011237:Predictive Value of Tests; D011446:Prospective Studies; D012680:Sensitivity and Specificity; D013203:Staphylococcal Infections / Q000175:diagnosis / Q000188:drug therapy / Q000382:microbiology; D013204:Staphylococcus Phages; D013211:Staphylococcus aureus / Q000187:drug effects / Q000302:isolation & purification* / Q000821:virology