trialregister.nlIdentifier: NTR1780.

A newly marketed rapid agglutination kit for the identification ofStaphylococcus aureus is one of the most frequently isolated pathogens in clinical specimens (8). In fact, S. aureus is currently the most common cause of infections in hospitalized patients (1). Misidentification of S. aureus as coagulase-negative staphylococcus (CoNS) can result in a costly search for other pathogens or unwarranted broad-spectrum empiric antimicrobial coverage (11). Although the tube coagulase test for the detection of free coagulase is generally considered the standard test for differentiating S. aureus from CoNS, the timeconsuming nature of the test (incubation for 4 to 24 h is required) often forces clinical microbiology laboratories to use more rapid alternatives. The slide coagulase test, which detects bound coagulase (clumping factor) is rapid (Ͻ1 min), but 10 to 15% of S. aureus strains may yield a false-negative result (7). Through the years many manufacturers have developed commercial agglutination kits for the rapid identification of S. aureus. The earliest of these tests consisted of erythrocytes sensitized with fibrinogen for the detection of clumping factor. Thereafter, a second generation of products was marketed, which employed coated latex particles and/or sensitized sheep erythrocytes to identify S. aureus by the simultaneous detection of clumping factor and protein A (12). Initially, these tests were reported to be very accurate, but later reports documented false-negative results among methicillin-resistant S. aureus (MRSA) strains (4,8,9,13,14). It was hypothesized that these false-negative MRSA strains do not expose clumping factor or protein A on their surface, which might be explained by the presence of a large amount of capsular polysaccharides masking other cell wall structures. The observation that capsular polysaccharide serotype 5 predominated among MRSA isolates which were not identified by rapid agglutination methods offered a target for improvement of the available tests (4). Third-generation tests were developed, which incorporated antibodies against capsular polysaccharides or antibodies against group-specific antigens on the S. aureus cell surface (4,5,12).In this international multicenter study, a newly marketed rapid latex agglutination test, Slidex Staph Plus (bioMérieux, Marcy-l'Etoile, France), was compared to two other latex agglutination tests of the same generation, Staphaurex Plus (Murex Diagnostics Ltd., Dartford, England) and Pastorex Staph-Plus (Sanofi Diagnostics Pasteur, SA, Marnes-La-Coquette, France). All three tests detect clumping factor and staphylococcal protein A, in addition, Slidex Staph Plus and Staphaurex Plus detect group-specific antigens on the S. aureus cell surface, and Pastorex Staph-Plus detects capsular polysaccharides.(This study was presented at the 10th European Congress of Clinical Microbiology and Infectious Diseases, Stockholm, Sweden, 2000.) MATERIALS AND METHODSThe study was conducted at three clinical microbiology laboratories in three European countries...

The mecA gene was lost in 36 (14.4%) of 250 methicillin-resistant Staphylococcus aureus isolates after 2 years of storage at ؊80°C with the Microbank system (Pro-lab Diagnostics, Austin, Tex.). Further analysis of 35 of these isolates confirmed loss of the mecA gene in 32 isolates. This finding has important implications for the management of strain collections.

Two new selective media, oxacillin resistance screening agar base (ORSAB) and CHROMagar Staph aureus (CSA), were evaluated for identification of Staphylococcus aureus and for screening of methicillin resistance by addition of antimicrobial agents to these media. A well-defined collection consisting of 1,140 staphylococci was used. A total of 624 were S. aureus, of which 358 were methicillin susceptible and 266 were methicillin resistant, and 516 were coagulase-negative staphylococci. The methicillin-resistant S. aureus (MRSA) strains were selected based on the results of phage typing; 247 different types were included in the analysis. For identification of S. aureus, both media performed better after 24 h than after 48 h. The sensitivities at 24 h were comparable (CSA, 98.6%; ORSAB, 97.1%), but the specificity of CSA was significantly higher (CSA, 97.1%; ORSAB, 92.1%). For screening of methicillin resistance, antibiotic supplements were added to both media. The sensitivity was lower after 24 h (CSA, 58.6%; ORSAB, 84.2%) and increased significantly after 48 h (CSA, 77.5%; ORSAB, 91.4%). At both time intervals ORSAB was significantly more sensitive than CSA. However, the specificities of both media were high after 24 h (CSA, 99.1%; ORSAB, 98.3%) and decreased significantly after 48 h of incubation (CSA, 94.7%; ORSAB, 95.5%). In conclusion, for identification of S. aureus, CSA is more accurate than ORSAB because of a significantly higher specificity. For screening of MRSA, ORSAB performs better than CSA, but the usefulness in clinical practice is limited because a significant number of strains are not detected.

The MRSA screen test (Denka Seiken Co., Ltd.), a commercially available, rapid (20-min) slide latex agglutination test for the determination of methicillin resistance by detection of PBP 2a inStaphylococcus aureus, was compared with the oxacillin agar screen test and PCR detection of the mecA gene. A total of 563 S. aureus isolates were tested. Two hundred ninety-six of the isolates were methicillin-susceptible isolates from cultures of blood from consecutive patients. Also, 267 methicillin-resistant isolates that comprised 248 different phage types were tested. Methicillin resistance was defined as the presence of themecA gene. Of the 267 mecA gene-positive isolates, 263 were positive by the MRSA screen test (sensitivity, 98.5%), and all the mecA-gene negative strains were negative by the MRSA screen test (specificity, 100%). The oxacillin agar screen test detected methicillin resistance in 250 of themecA gene-positive isolates (sensitivity, 93.6%). The sensitivity of the MRSA screen test was statistically significantly higher than the sensitivity of the oxacillin agar screen test (P < 0.05). The MRSA screen test is a highly sensitive and specific test for the detection of methicillin resistance. Also, it offers results within half an hour and is easy to perform, which makes this test a valuable tool in the ongoing battle against methicillin-resistant S. aureus.