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

Prize-Winning Case Report

From Dr. Anne Dubouix, Director of Laboratoire de Microbiologie, Clinique de L'Union, Toulouse, France. (Figure 1)

A 44-year-old male presented to the Emergency Department of our institution on November 29, 2009 with inability to ambulate and pain in the right knee. On examination, the patient had a large knee effusion with significant erythema.

The patient reported that his problems with his knee had started nearly two weeks before he finally presented to the hospital. He also stated that because he had been previously diagnosed with sinusitis, he had been taking pristinamycin (similar to quinupristin/dalfopristin, trade name Synercid®) for the previous 8 days as prescribed by his general practitioner.

Laboratory tests revealed a slightly above-normal leukocyte count (11,300/mm3) mainly due to an increase in polymorphonuclear leukocytes (9,800/mm3) and an elevated C-reactive protein (230 mg/l).

Ultrasonography and magnetic resonance imaging confirmed a patellar bursitis and an infection was suspected, therefore the patient was taken to surgery on an emergency basis. Three peri-operative swabs were obtained from the open wound site during surgery and sent to the clinical laboratory. A Gram stain was performed on one of the samples and did not reveal any organisms. For two of the swabs, standard culture media including Columbia agar + 5% sheep blood, Columbia CNA agar + 5% sheep blood, Chocolate agar + Polyvitex® and Brain Heart Infusion broth (BioMérieux®, France) were inoculated and incubated aerobically while an Anaerobes agar + 5% sheep blood plate was incubated anaerobically.

The third swab was used to perform the Xpert® MRSA/SA SSTI assay. Following the product insert, the swab was broken off into the elution reagent vial, the reagent vial was vortexed for 10 seconds, and a pipette was used to inoculate the entire contents of the vial into the cartridge (Figure 2).

Results were available within an hour of receipt of the sample indicating that a MRSA was present in the sample. The physician was advised and dual therapy combining vancomycin (50 mg/kg/day) and rifampicin (20 mg/kg/day) was immediately started.

Of note, the culture results confirmed the molecular results but only 5 days after the samples were received in the laboratory, probably because of the patient's previous treatment with pristinamycin for his sinusitis (Figure 3). By that time, the patient's outcome was already looking very positive with a reduction of all symptoms and a decrease of inflammatory markers.

Using the GeneXpert® MRSA/SA SSTI assay definitively improved the clinical management of this patient, as the infecting MRSA was detected in less than one hour after sampling and the appropriate and effective antibiotic therapy was given to the patient less than 2 hours after sampling.

This would not have been the case if the Gram stain (negative) and cultures (delayed) were the only techniques used for microbiological documentation of the etiology of his infection. Furthermore, this patient did not present any risk factors for MRSA carriage and would probably not have had such a positive outcome if the antibiotic treatment was only empiric (oxacillin, in this case) in the absence of microbiological documentation.