A head-to-head comparison of 2 automated, polymerase chain reaction–based systems for the detection of methicillin-resistant Staphylococcus aureus (MRSA) in nasal swabs and culture samples from 119 high-risk patients has demonstrated the superiority in terms of detection accuracy and speed of the Xpert MRSA Gen 3 system over the BD-MAX MRSA system.

Martin Rottman, MD, PhD, Hôpital Raymond Poincaré AP-HP, Garches, France, and colleagues presented findings that the ongoing evaluation of the diagnostic performance of MRSA molecular assays is necessary, given the emergence of new strains with mec gene variations or altered staphylococcal cassette chromosomal junctions. This study focused on the performance of Xpert and BD-MAX automated molecular assays in comparison to 2 culture-based methods on nasal swab samples acquired from patients in the intensive care unit, orthopedic surgery unit, and rehabilitation medicine unit of Hôpital Raymond Poincaré. A hospital policy had designated these units as being at high risk for MRSA outbreaks in the tertiary-care center.

Consecutive nasal swab samples were collected from both anterior nares of 119 patients over a 45-day period. These and 36 nasal swab samples collected from patients who were culture-positive for MRSA were used for the 2 automated molecular assays as well as 2 culture-based methods. The culture-based methods were the Tryptic Soy Broth enriched culture method (gold standard) followed by MRSA select agar (culture time, 24 to 72 hours), and direct culture on MRSA select agar (culture time, 18 hours). A bank of 12 control strains was also tested, including a bovine strain that contained mecC. Each sample was dispensed in 500 µL distilled deionized water, with aliquots of 100 µL used for each of these four methods. A total of 119 consecutive samples were tested, followed by testing of 36 direct positive samples. In the subsequent statistical analyses, the agreement of each method was compared with the Tryptic Soy Broth enriched culture (gold standard).

Based on the results of the gold standard method, 12 of 119 patients (10%) harbored MRSA in their nasal passages. Both automated assays detected the mecC strain. The Xpert system correctly identified MRSA in 45 of 47 samples (95.7%) known to be MRSA-positive, with 98.7% agreement with the gold standard method. The BD-MAX assay correctly identified 42 of 48 samples (87.5%), with a 94.0% agreement with the gold standard method. The difference in accuracies significantly favored the Xpert system (P = .023, McNemar nonparametric test). When MRSA culture-negative samples were tested, the Xpert assay correctly identified all 107 samples tested. The BD-MAX assay correctly identified 100 of 103 samples. The 3 false-positive results in the BD-MAX assay were traced to problems with the interpretive software of the instrument. Positive predictive values were 100% and 93.3% with Xpert and BD-MAX, respectively, whereas negative predictive values were 99.0% and 96.1%, respectively.

The hands-on time needed to run 4 samples in the Xpert system was 12 minutes, with a turn-around time of 91 minutes. The comparable times for the BD-MAX system were 15 and 117 minutes.

The data demonstrate the detection superiority and lower hands-on time of the Xpert automated assay compared to the BD-MAX automated assay in detecting MRSA in nasal samples obtained from patients in units regarded to be at high risk for MRSA.