Friday, December 23, 2016 Written by Ellen Jo Baron, Ph.D., D(ABMM), Prof. Emerita, Stanford University Director of Medical Affairs, Cepheid

Summaries of selected abstracts presented in Europe recently on the use of GeneXpert® MRSA/SA SSTI

From the European Federation of National Associations of Orthopaedics and Traumatology, June 2011:

Contribution of GeneXpert MRSA-SA SSTI® assay to fast diagnosis of MRSA and MSSA positive prosthesis joint infection

Laurent F.1,2, Bouaziz A.1, Blanc-Pattin V.1, Rasigade JP.1,2, Ferry T.3, Chanard E.4, Neyret P.5, Tigaud S.1, Lustig S.5
1Laboratoire de Bactériologie, Groupement Hospitalier Nord, Hospices Civils de Lyon; 2Centre National de Référence des Staphylocoques, Inserm U851, Lyon; 3Service des Maladies Infectieuses, Groupement Hospitalier Nord, Hospices Civils de Lyon; 4Laboratoire UNILAB (membre du réseau NOVESCIA), Lyon; 5Service d'Orthopédie, Groupement Hospitalier Nord, Hospices Civils de Lyon, Lyon, France.

This retrospective study evaluated 91 frozen samples including 24 joint aspirates, 42 bone biopsies, and 25 tissue biopsies. Of the 63 samples positive for methicillin-susceptible Staphylococcus aureus (MSSA) by culture, 59 were positive and designated as SA by the Xpert® MRSA/SA SSTI assay. The other 4 were considered to be false negative Xpert results. All of the 9 culture-positive methicillin-resistant Staphylococcus aureus (MRSA) infections were detected by Xpert. Among the 19 culture-negative samples, there were 16 Xpert results concordant with cultures from the same sample and 3 GeneXpert® positive results in patients with previously documented S. aureus infections, suggesting that PCR was more sensitive than culture in these cases. GeneXpert results were available within 58 minutes of cartridge inoculation. The overall sensitivity of Xpert MRSA/SA SSTI assay for the frozen samples compared to true positive results (positive culture from same or additional sample) was 94.4%. and specificity was 100%. The authors suggest further studies to evaluate clinical and pharmaco-economic impact of use of this assay in the setting of prosthetic joint infections.


From the French Congress of Infectious Diseases (Journées Nationales d'Infectiologie), also in June, 2011:

Rapid detection of methicillin-resistance in chronic osteo-articular infections in patients with prostheses.

M. Titécat2, C. Loïez1,2, H. Dezèque1,3, H. Migaud1,3, L. Legout1,4, R.J. Courcol1,2, E. Senneville1,4
1Centre de Référence des Infections Ostéo-articulaires Complexes Nord-Ouest, CHU Lille -CH Tourcoing; 2Service de bactériologie, Centre de Biologie Pathologie, CHRU Lille; 3Service de Chirurgie Orthopédique, Hôpital Salengro, CHRU Lille; 4Service de Maladies Infectieuses, Hôpital Dron, CH Tourcoing

The Xpert® MRSA/SA SSTI was used to test for the presence of the mecA gene in 39 culture-negative fluids and 69 culture-positive samples including joint fluids, tissue biopsies, and bone biopsies. The purpose was to determine if patients who did not have methicillin-resistant staphylococcal infections by culture (including either S. aureus or coagulase-negative staphylococci) could be taken off vancomycin therapy earlier than they would be if caregivers had to wait for culture results. Note that this is not an indication supported in the product insert for this product. The comparator systems of phenotypic characterization and MALDI-TOF were used to identify the organism, and Vitek2 was used for antibiotic susceptibility testing. There were 3 samples containing methicillin-resistant coagulase-negative staphylococci for which the Xpert® assay did not detect mecA and 5 samples in which mecA was detected but the cultures failed to reveal a methicillin-resistant strain. Another 30 samples had inconsistent results with culture. After resolution, the sensitivity of Xpert for detecting mecA was 92.3% and the predictive value of a negative result was 94.4%. Using this technique, the authors showed a savings of 102 days of vancomycin therapy (average of 6 days per patient for whom methicillin-resistant staphylococci were not detected). Costs savings included the antibiotic administration, avoidance of antibiotic monitoring tests (required for vancomycin to avoid toxicity), and nursing labor. A publication by these authors (Dr. Senneville first author) has been accepted by Clinical Infectious Diseases.