Performance of the Cepheid Methicillin-Resistant Staphylococcus aureus/S. aureus Skin and Soft Tissue Infection PCR Assay on Respiratory Samples from Mechanically Ventilated Patients for S. aureus Screening during the Phase 2 Double-Blind SAATELLITE Study

We investigated the performance of the Xpert methicillin-resistant Staphylococcus aureus (MRSA)/S. aureus skin and soft tissue (SSTI) quantitative PCR (qPCR) assay in SAATELLITE, a multicenter, double-blind, phase 2 study of suvratoxumab, a monoclonal antibody (MAb) targeting S. aureus alpha-toxin, for reducing the incidence of S. aureus pneumonia. The assay was used to detect methicillin-susceptible S. aureus (MSSA) and MRSA in lower respiratory tract (LRT) samples from mechanically ventilated patients. LRT culture results were compared with S. aureus protein A (spa) gene cycle threshold (C-T) values. Receiver operating characteristic (ROC) and Youden index were used to determine the C-T cutoff for best separation of culture-S. aureus-negative and S. aureus-positive patients. Of 720 screened subjects, 299 (415%) were S. aureus positive by qPCR, of whom 209 had culture data: 162 (77.5%) were S. aureus positive and 47 (22.5%) were S. aureus negative. Culture results were negatively affected by antibiotic use and cross-laboratory variability. An inverse linear correlation was observed between C-T values and quantitative S. aureus culture results. A spa Cr value of 29 (approximate to 2 x 10(3) CFU/mL) served as the best cutoff for separation between culture-negative and culture-positive samples. The associated area under the ROC curve was 83.8% (95% confidence interval (CI), 78 to 90%). Suvratoxumab provided greater reduction in S. aureus pneumonia or death than placebo in subjects with low S. aureus load (C-T >= 29; relative risk reduction [RRR], 50.0%; 90% CI, 2.7 to 74.4%) versus the total study population (RRR, 252%; 90% CI, -43 to 46.4%). The qPCR assay was easy to perform, sensitive, and standardized and provided better sensitivity than conventional culture for S. aureus detection. Quantitative PCR C-T output correlated with suvratoxumab efficacy in reducing S. aureus pneumonia incidence or death in S. aureus-colonized, mechanically ventilated patients.

Automated summary

We investigated the performance of Xpert MRSA/S. aureus SSTI qPCR assay for reducing the incidence of S. aureus pneumonia. The study found that the qPCR assay was more sensitive than conventional culture method and its output correlated with the efficacy of suvratoxumab in reducing pneumonia incidence or death in mechanically ventilated patients colonized with S. aureus.