Ann Biol Clin (Paris). 2021 Oct 12. doi: 10.1684/abc.2021.1672. Online ahead of print.
In hospitalized patients, staphylococcal blood infection is common and mortality is high. Rapid diagnosis using molecular assay aims to identify the presence of Staphylococcus aureus and its resistance to methicillin as soon as the blood culture is positive. We evaluate performance of GeneXpert MRSA/SA Blood Culture assay (Cepheid®) before and after interpretation of the positivity levels of the various probes estimated by the Cycle threshold (Ct), as well as its contribution to the characterization of coagulase-negative staphylococci blood cultures not offered by the supplier. The study involved 145 samples with gram-positive cocci bacteremias. Ct analysis of the different probes revealed a few positive results with very high Ct values distants from the mean. The reclassification of these results as negative improves the specificity of the probes (spa: 100% vs. 96,8% and mec 100% vs. 91,9%) without degrading the sensitivity (spa: 98,1% vs. 100% and mec 98,6% vs. 98,6%). Then, based on an algorithm integrating the amplification results of each target, we extrapolated the results to the coagulase-negative staphylococci. In the end, reclassifying probes with extreme Ct values as negative and using the algorithm for coagulase-negative staphylococci resulted in a 97,9% (142/145) agreement between the molecular assay conclusion and conventional culture.