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Highlights from ICAAC/IDSAFocusing on Healthcare-Associated Infections
The American Society for Microbiology’s Interscience Congress on Antimicrobial Agents and Chemotherapy and the Infectious Diseases Society of America’s annual meeting were combined and held in Washington, DC this October. More than 14,000 attendees acquired the latest information on new diagnostics, new therapies, microbial pathogenesis, and more. Here is a summary of a few abstracts whose findings focus on healthcare-associated infections. The majority of recent studies are proving that infection control based on surveillance is cost-effective and beneficial, even when surveillance is restricted to limited inpatient units.C2-106. Detection of Oxacillin/Methicillin Resistant and Susceptible Staphylococcus aureus by Culture and RT-PCR in Patients Scheduled for Orthopedic Surgery Nasal swabs from 275 patients scheduled for orthopedic surgery at Baptist Hospital (Jacksonville, FL) were tested by CHROMagar and BD GeneOhm MRSA in Phase I and another 137 patients by the Xpert™ MRSA test (using a frozen swab obtained at the same time) during Phase II. Rates of positivity more than doubled with the Xpert MRSA test. Both molecular systems usually agreed. The authors concluded that molecular methods were more sensitive than culture and that the GeneXpert® System gave more rapid results with less hands-on time than did the SmartCycler® System. C2-1066. Real-Time PCR (RT-PCR) for Methicillin-Resistant Staphylococcus aureus (MRSA) Detection: Implications in a Neonatal Intensive Care (NIC) Setting In 2005, National Childrens’ Hospital in Washington DC tested for MRSA at admission and weekly thereafter using cultures. After they switched to rt-PCR (GeneXpert, Cepheid) in 2007 the incidence of MRSA increased from 6.9% to 10.8%. This enhanced detection led to a statistically significant reduction in hospital acquisitions of colonizing MRSA. During the time of culture surveillance 30.4% of MRSA-colonized patients developed an MRSA infection, whereas during PCR-based testing, only 9.1% of MRSA-positive patients developed MRSA infection. They concluded that PCR-based testing enhanced infection control measures for MRSA. K-3367. A Pilot Study of Active Surveillance for MRSA by PCR on Admission to ICUs at a Tertiary Care Center Mem. Hermann TX Medical Ctr., Houston, performed the BD GeneOhm assay once daily on all patients admitted to the adult ICU. Patients were initially all placed on modified contact precautions and positive patients were placed on full contact precautions and flagged for future infection control actions. During the 3-month intervention period, 11.6% of patients tested were MRSA positive. There was a statistically significant decrease in healthcare-associated ventilator-associated pneumonias and bloodstream infections overall in the ICU, from 59 to 38 per 1000 admissions. MRSA infections dropped from 9% to 5%. Importantly, the authors extrapolated costs to one year. Even considering the cost of testing and isolation, the cost avoidance associated with preventing healthcare-associated infections by PCR screening and subsequent isolation was estimated at $567,000. What’s good and what’s disappointing about the various surveillance methods?D-1137. Comparison of Direct Plating and Broth Enhanced Culture for Recovery of MRSA from PCR Positive Nasal Swabs To resolve discrepancies between positive GeneXpert (Cepheid) MRSA results and negative culture results, Loyola scientists evaluated enhanced culture methods for recovery of MRSA. During Phase I, the second swab from patients whose first swab yielded a positive or inconclusive PCR was plated directly onto blood agar, two types of MRSA selective CHROMagars and then incubated in 1 mL of 6.5% NaCl in trypticase-soy broth (TSB) overnight. Broths from negative plate cultures were plated to the same 3 agars. In Phase II, the second swab was vortexed in sterile saline, and inoculated onto the 3 agars and into 6.5% and 3.25%NaCl TSBs. Broths were subcultured if direct plating did not yield MRSA. In Phase I, direct plating recovered 68% of those with positive results in the Xpert MRSA test. Seven more MRSA were recovered from TSB subcultures (total 76% recovery). However, 13 of 21 MRSA culture negative samples grew MSSA. In Phase II, cultures overall yielded 87% on blood agar and 92-93% on CHROMagars. Of 20 MRSA-negative cultures , 13 grew MSSA. The MSSA isolates were further characterized; 2/3 were found to have a deletion in the mecA region. The isolation of MRSA from PCR-positive patients was increased 10% by vortexing swabs in saline before plating and 7% by enriching overnight in TSB. These results further support the superior sensitivity of molecular methods for detection of MRSA colonized patients. D-1136. Surveillance Swabs for Detection of Methicillin-Resistant Staphylococcus aureus (MRSA): Diagnostic Yield of Different Anatomic Sites and Comparison of Provider and Patient-Collected Samples Using cultures, workers at the Univ. of Pennsylvania (Philadelphia) compared swabs from different sites collected by healthcare workers and by the patients themselves. Axillae, nares, throat, perineum, and groin swabs were evaluated. Patient-collected swabs compared favorably with professionally collected samples. The best agreement between patient- and provider-collected samples was 95% for the groin. To achieve >90% detection of colonized persons, >1 site had to be sampled. In this population, the best single sites were nares (84% sensitive) and throat (65% sensitive). Groin and perineum cultures were more likely to be positive in community-onset patients (81%) than hospital-onset patients (41%). Nares, throat, and groin together achieved 100% sensitivity and either nares and throat or nares and groin reached 91% sensitivity for recovery of MRSA. K-3355. A Targeted Approach to MRSA Colonization Screening Using a PCR Assay and Multi-Site Sampling Strategy Colleagues at Montefiore Medical Center in the Bronx increased the sensitivity of surveillance cultures almost 24% by collecting swabs from axilla, nares, and groin vs. nares alone. 268 patients, of whom 21.3% were positive from at least one site: 17.2% nares, 10.1% axilla, and 13.4% groin, were evaluated. Targeting screening to those patients who had either long-term care residence, a previous MRSA isolate, or antibacterial use within the last 6 months, yielded a sensitivity of 80.7% and the need to test 43.7% of admissions. By adding the risk factor of hospitalization within the last 6 months, sensitivity increased to 91.2% with the need to screen 57.8% of admissions. They concluded that a targeted approach to surveillance and a two-swab (nares and skin) sample could result in near-complete capture of MRSA-colonized patients. Use of PCR as a point-of-care test: at this time, there is only one moderate complexity PCR assay FDA-cleared for use in the United States.D-1139. Potential Use of the Cepheid Xpert™ MRSA Test for Near-Patient Testing Computer-interfaced Cepheid GeneXpert® Systems were installed in the Critical Care and Admissions wards of two hospitals in Birmingham, UK. Double swabs were collected from patients in the wards and one swab was used for near-patient testing, the other was sent to the central laboratory. Valid paired results were available for 735 swabs, 62 of which were MRSA positive. There was 97.1% agreement. Sampling errors were thought to account for all discrepancies. The results of tests performed at the wards were available in ≤ 2 hours for 97.7% of the swabs collected. The authors said that the use of the GeneXpert System at the patient point of care dramatically reduced the time to result compared with testing in the laboratory. Late Breaker: First look at the new Xpert MRSA/SA test for SSTI and blood culture broths.D-2250a. Automated PCR to Detect Methicillin-Resistant (MR) or -Susceptible (MS) Staphylococcus aureus (SA) in Blood Cultures (BC) and Wound Swabs (WS) 132 blood cultures yielding gram-positive cocci in clusters were tested using the Xpert MRSA/SA test. All 25 MRSA and 7 MSSA were correctly identified by the Xpert test in 50 minutes. One blood culture (negative for S. aureus) was identified as MRSA by the Xpert test. The sensitivities of the Xpert test for blood cultures were 100% for MRSA and MSSA and 99% for no S. aureus. Specificities were 99% for MRSA and 100% for MSSA. Sensitivity of the Xpert test for wounds culture-positive for MRSA was 99%. Sensitivities for wounds yielding MSSA and no S. aureus on culture were 91% and 79%; and specificities were all >91%. Of 80 MRSA cultured from 235 wound swabs, the Xpert test correctly identified 79. The Xpert MRSA/SA test correctly identified 30 of 33 MSSA isolates from wounds; 2 were incorrectly called MRSA, and one was not detected. From 122 wound cultures where no S. aureus were isolated, the Xpert test detected MRSA in 12 samples and MSSA in 14 samples. Because resolving tests were not performed, the true nature of the discrepant results is not known yet. Dr. Musher suggested that previously treated patients may account for some of the inconsistencies.
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