Emergence of SCCmec variants causing false-negative MRSA results by Xpert SA Nasal Complete: a need for culture back-up?

BACKGROUND: Staphylococcus aureus (SA) is a major human pathogen and an important cause of healthcare-associated infections. Hospital-acquired methicillin-resistant S. aureus (MRSA) is associated with increased morbidity and mortality. Screening for nasal carriage of SA followed by decolonization has been shown to reduce healthcare-associated MRSA transmission and infection. Nucleic acid amplification assays (NAATs) are widely used for MRSA screening and are associated with shorter turnaround times, fewer isolation days, reduced MRSA-related infections, and improved clinical outcomes and cost savings. CASE SUMMARY: Two patients underwent preoperative nasal screening using Xpert SA Nasal Complete, and corresponding culture results were discordant with the molecular results. In both cases, the Xpert SA assay reported "SA detected and MRSA not detected," whereas culture and antimicrobial susceptibility testing (AST) demonstrated the presence of MRSA. Additional testing supported the culture-based identification of MRSA. Whole-genome sequencing and molecular typing revealed that both MRSA isolates harbored SCCmec variants that were not detected by the Xpert assay, leading to false-negative (FN) MRSA results. CONCLUSION: While NAATs remain highly sensitive and reliable tools for MRSA screening and the overall risk of FN MRSA detection may be low, our cases highlight the importance of ongoing surveillance of local MRSA epidemiology and understanding the genetic inclusivity of the molecular assays used for detection. In high-risk or targeted patients, consideration of reflex culture may be warranted.