Controlled multicenter evaluation of a bacteriophage-based method for rapid detection of Staphylococcus aureus in positive blood cultures

Abstract

Staphylococci are a frequent cause of bloodstream infections (BSIs). Appropriate antibiotic treatment for BSIs may be delayed because conventional laboratory testing methods take 48 to 72 h to identify and characterize isolates from positive blood cultures. We evaluated a novel assay based on bacteriophage amplification that identifies Staphylococcus aureus and differentiates between methicillin-susceptible and methicillin-resistant S. aureus (MSSA and MRSA, respectively) in samples taken directly from signal-positive Bactec blood culture bottles within 24 h of positive signal, with results available within 5 h. The performance of the MicroPhage KeyPath MRSA/MSSA blood culture test was compared to conventional identification and susceptibility testing methods. At four sites, we collectively tested a total of 1,165 specimens, of which 1,116 were included in our analysis. Compared to standard methods, the KeyPath MRSA/MSSA blood culture test demonstrated a sensitivity, specificity, positive predictive value, and negative predictive value of 91.8%, 98.3%, 96.3%, and 96.1%, respectively, for correctly identifying S. aureus. Of those correctly identified as S. aureus (n = 334), 99.1% were correctly categorized as either MSSA or MRSA. Analysis of a subset of the data revealed that the KeyPath MRSA/MSSA blood culture test delivered results a median of 30 h sooner than conventional methods (a median of 46.9 h versus a median of 16.9 h). Although the sensitivity of the test in detecting S. aureus-positive samples is not high, its accuracy in determining methicillin resistance and susceptibility among positives is very high. These characteristics may enable earlier implementation of appropriate antibiotic treatment for many S. aureus BSI patients.

Fig 1

Performance characteristics of the KeyPath MRSA/MSSA blood culture test for identification of S. aureus. Of 366 isolates identified as S. aureus by the standard methods (catalase, tube coagulase, and Staphaurex), the KeyPath MRSA/MSSA blood culture test correctly identified 336 directly from the positive Bactec blood culture bottle. Thirteen non-S. aureus blood culture isolates were misidentified by the KeyPath MRSA/MSSA blood culture test as S. aureus, and 30 S. aureus isolates were misidentified as non-S. aureus. SA, S. aureus; NSA, not S. aureus; SN, sensitivity; SP, specificity; PPV, positive predictive value; NPV, negative predictive value.

Fig 2

Categorical agreement between the KeyPath MRSA/MSSA blood culture test and cefoxitin disk test for prediction of oxacillin susceptibility. Two S. aureus isolates judged to be cefoxitin resistant using the disk test were misclassified as susceptible by the KeyPath MRSA/MSSA blood culture test (very major error). One cefoxitin susceptible isolate was misclassified as susceptible by KeyPath MRSA/MSSA blood culture test (major error). Repeat testing on two of the isolates (one incorrectly classified as susceptible and one incorrectly classified as resistant) yielded results concordant with those of cefoxitin disk testing.