Clinical laboratory evaluation of the Abbott MS-2 automated antimicrobial susceptibility testing system: report of a collaborative study.

The MS-2 system (Abbott Diagnostics, Division of Abbott Laboratories, Dallas, Tex.) was evaluated for its efficacy in determining the susceptibilities of both clinical and selected challenge (nonfastidious, facultative, and aerobic) isolates. The MS-2 results were compared with standard Kirby-Bauer disk diffusion and microdilution results by using fresh clinical isolates. For gram-positive isolates other than enterococci, overall agreement between MS-2 and reference results was 93 to 98%. With enterococci, MS-2 agreement with disk diffusion was 68% but with microdilution was 86% (agreement between disk diffusion and microdilution was 73%). The main discrepancies with enterococci were with cephalothin, penicillin, gentamicin, and kanamycin. With clinical gram-negative isolates, the overall agreement was 91 to 93%, with most discrepancies occurring with Enterobacter spp. and beta-lactam antibiotics (MS-2 versus disk diffusion, 84%; MS-2 versus microdilution, 84%; disk diffusion versus microdilution, 87%) and with Serratia spp. and colistin (false-susceptible results). The agreement of MS-2 results with established reference antibiograms of a special collection of challenge strains was 91 to 97% for the gram-positive cocci and 86 to 98% for the gram-negative strains. (With Enterobacter spp., agreement was 86%, but was greater than 90% for all other organism groups.) Of the 98 finite MS-2 minimum inhibitory concentrations (MICs) that could be directly compared with microdilution MICs, 77 (79%) were within +/- 1 well of the geometric mean microdilution MIC. MS-2 analysis time ranged from 2.8 to 6.5 h (mean, 4.2 h). On the basis of these results, we conclude that the MS-2 can be expected to yield rapid and accurate results with most nonfastidious, facultative, and aerobic pathogens.     

Evaluation of the Cobasbact automated antimicrobial susceptibility testing system

The Cobasbact is an automated system for use in the microbiological laboratory which is capable of performing antimicrobial susceptibility tests within five hours. The performance of the Cobasbact system was evaluated using 2000 non-fastidious, facultative and aerobic clinical bacterial isolates. The standard Kirby-Bauer disk diffusion test served as reference method. For gram-positive isolates, the rate of overall full and essential agreement between Cobasbact and reference antibiograms was 90 % and 95.5 % respectively, forEnterobacteriaceae 91 % and 95.5 % and forPseudomonas spp. 90% and 96 %. Reproducibility studies yielded an essential agreement rate of 98 %. On the basis of this preliminary evaluation it would seem that the Cobasbact system can rapidly produce reasonably accurate and reproducible results of antimicrobial susceptibility tests for gram-positive cocci,Enterobacteriaceae andPseudomonas spp.     

Evaluation of a novel automated chemiluminescent assay system for antimicrobial susceptibility testing

The newly developed Rapid Lumi Eiken/IS60 (RL/IS60) system automatically determines MICs by detecting chemiluminescence produced in the reaction of a chemiluminescent probe and oxygen metabolites from living microorganisms. The present study evaluated this system for accuracy in antimicrobial susceptibility testing. Chemiluminescence intensities after 4 h of cultivation of clinically important strains were plotted against various concentrations of antimicrobial agents, which resulted in curves reflecting the levels of susceptibility. Sixty-percent inhibitory concentrations based on the susceptibility curves agreed with MICs determined by the reference microdilution method. When the MICs of antimicrobial agents for four quality control (QC) strains (Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, and Pseudomonas aeruginosa) were determined by the RL/IS60 system, most (91.1%) of them were within the QC limits proposed by the National Committee for Clinical Laboratory Standards. The system was further assessed for a total of 162 clinical isolates, including E. coli, Citrobacter freundii, Enterobacter cloacae, Klebsiella pneumoniae, Serratia marcescens, Proteus mirabilis, Morganella morganii, P. aeruginosa, Haemophilus influenzae, S. aureus, coagulase-negative staphylococci, Enterococcus faecalis, Enterococcus faecium, and Streptococcus pneumoniae. Overall, there was 90.6% agreement between the RL/IS60 system and the reference microdilution method. Our results suggest that the RL/IS60 system provides rapid and reliable MICs of a variety of antimicrobial agents for clinical isolates as well as QC strains.     

Clinical evaluation of automated antibiotic susceptibility testing with the MS-2 system.

The MS-2 (Abbott Laboratories) system for automated antimicrobial susceptibility testing was evaluated for both accuracy and general utility in our clinical laboratory. A total of 984 fresh clinical bacterial isolates (745 gram-negative, 239 gram-positive) were tested with the MS-2 system, and results were compared directly with those from a conventional agar disk diffusion method. Discrepancies between the two methods were categorized as very major, major, and minor. For gram-positive isolates, full accord (all discrepancies considered) was 91.6%, and essential accord (minor discrepancies not included) was 96.2%. With gram-negative isolates, full accord was found to be 93.9%, with essential accord of 97.9%. Aggrement as a function both of organism group and of antimicrobial agent was determined. Full accord of 90% or more was found for all major organism groups tested, with the exception of enterococci, where discrepant results between the two methods were observed. Mean test time for all isolates tested was 4.3 h. The MS-2 was found to be an accurate and highly automated instrument which required minimal technician time and was readily adaptable to work flow in our clinical laboratory.     

Evaluation of the BD Phoenix automated microbiology system for identification and antimicrobial susceptibility testing of Enterobacteriaceae

We evaluated the accuracy of the BD Phoenix system for the identification (ID) and antimicrobial susceptibility testing (AST) of 251 isolates of the family Enterobacteriaceae representing 31 species. Organisms were inoculated onto the Phoenix panel according to the manufacturer's instructions. The results from conventional biochemical tests were used for the reference method for ID. Agar dilution, performed according to the CLSI guidelines, was the reference AST method. Essential and categorical agreements were determined. The overall levels of agreement for the genus- and species-level identifications were 95.6% and 94.4%, respectively. Fourteen isolates were incorrectly identified by the Phoenix system; 10 of these were incorrectly identified to the species level. Three of these were Enterobacter (Pantoea) species and four of these were Shigella spp. misidentified as Escherichia coli. For AST results, the essential and categorical agreements were 98.7% and 97.9%, respectively. The very major error, major error, and minor error rates were 0.38%, 0.33%, and 1.8%, respectively. Six isolates (three E. coli isolates and three Klebsiella isolates) were extended-spectrum beta-lactamase producers. All six were flagged by the Phoenix system expert rules. The Phoenix system compares favorably to traditional methods for ID and AST of Enterobacteriaceae.     

Evaluation of the Sceptor microdilution antibiotic susceptibility testing system: a collaborative investigation.

A multiwell, dried antimicrobial agent susceptibility test system, Sceptor (BBL Microbiology Systems, Cockeysville, Md.), was tested. The system was compared directly with a reference microdilution method by using two collections of stock cultures and 305 fresh clinical isolates. Sceptor was found to be in agreement (+/- log2 dilution) with the reference microdilution method in 96.9 to 98.3% of 9,840 minimal inhibitory concentration determinations performed on stock strains and 95.0% of 7,308 minimal inhibitory concentrations obtained from the clinical isolates. The intralaboratory and interlaboratory reproducibility on stock strains was 97.6 and 97.2%, respectively. The intralaboratory reproducibility for the clinical isolates was 96.9%. Sceptor accurately categorized representative challenge strains of methicillin-resistant staphylococci, beta-lactamase-producing bacteria, organisms producing other antimicrobial agent-inactivating enzymes, and permeability mutants as resistant. Only 0.2% very major errors (false-sensitive minimal inhibitory concentrations by Sceptor) were identified among the clinical isolate test results, the majority being clinically insignificant. The product is accurate and reliable, has a long shelf life, and seems applicable for routine use in clinical laboratories.     

Evaluation of a fully automated system (RAISUS) for rapid identification and antimicrobial susceptibility testing of Staphylococci

RAISUS is a system for rapid bacterial identification and antimicrobial susceptibility testing. RAISUS and VITEK showed 97.8% and 75.9% agreement in identification of 45 Staphylococcus aureus strains and 58 coagulase-negative staphylococci (CoNS), respectively, and RAISUS and CLSI (formerly NCCLS) methods showed 87.2% and 87.9% agreement in the MICs for S. aureus and CoNS, respectively. RAISUS provided these data within 3.75 h, suggesting its utility for clinical bacteriological laboratories.     

Two-center collaborative evaluation of performance of the BD phoenix automated microbiology system for identification and antimicrobial susceptibility testing of gram-negative bacteria

The performance of the BD Phoenix Automated Microbiology System (BD Diagnostic Systems, Sparks, MD) was assessed for identification (ID) and antimicrobial susceptibility testing (AST) of the majority of clinically encountered bacterial isolates in a European collaborative two-center trial. A total of 494 bacterial isolates including various species of the Enterobacteriaceae and 110 nonfermentative gram-negative bacteria were investigated: of these, 385 were single patient isolates, and 109 were challenge strains tested at one center. The performance of the Phoenix extended-spectrum beta-lactamase (ESBL) test was also evaluated for 203 strains of Escherichia coli, Klebsiella pneumoniae, and Klebsiella oxytoca included in the study. Forty-two antimicrobial drugs were tested, including members of the following drug classes: aminoglycosides, beta-lactam antibiotics, beta-lactam/beta-lactamase inhibitors, carbapenems, cephems, monobactams, folate antagonists, quinolones, and others. Phoenix system ID results were compared to those of the laboratories' routine ID systems (API 20E and API CHE, ATB ID32E, ID32GN, and VITEK 2 [bioMérieux, Marcy l'Etoile, France]); Phoenix AST results were compared to those of frozen standard broth microdilution (SBM) panels according to NCCLS (now CLSI) guidelines (NCCLS document M100-S9, approved standard M7-A4). Discrepant results were repeated in duplicate. Concordant IDs of 98.4 and 99.1% were observed for the Enterobacteriaceae and the nonfermentative group, respectively. For AST results, the overall essential agreement was 94.2%; the category agreement was 97.3%; and the very major error rate, major error rate, and minor error rate were 1.6, 0.6, and 1.9%, respectively. In terms of ESBL detection, Phoenix results were 98.5% concordant with those of the reference system, with 98.0% sensitivity and 98.7% specificity. In conclusion, the Phoenix ID results showed high agreement with results of the systems to which they were being compared: the AST performance was highly equivalent to that of the SBM reference method, and the system proved to be very accurate for the detection of ESBL producers.     

Evaluation of the BD Phoenix automated microbiology system for identification and antimicrobial susceptibility testing of staphylococci and enterococci

We evaluated the Phoenix automated microbiology system (BD Diagnostic Systems, Sparks, MD) for the identification (ID) and antimicrobial susceptibility testing (AST) of challenge and clinical staphylococci and enterococci recovered from patients in a tertiary-care medical center. In total, 424 isolates were tested: 90 enterococci; 232 Staphylococcus aureus isolates, including 14 vancomycin-intermediate S. aureus isolates; and 102 staphylococci other than S. aureus (non-S. aureus). The Phoenix panels were inoculated according to the manufacturer's instructions. The reference methods for ID comparisons were conventional biochemicals and cell wall fatty acid analysis with the Sherlock microbial identification system (v 3.1; MIDI, Inc. Newark, DE). Agar dilution was the reference AST method. The overall rates of agreement for identification to the genus and the species levels were 99.7% and 99.3%, respectively. All S. aureus isolates and enterococci were correctly identified by the Phoenix panels. For the non-S. aureus staphylococci, there was 98.0% agreement for the ID of 16 different species. The AST results were stratified by organism group. For S. aureus, the categorical agreement (CA) and essential agreement (EA) were 98.2% and 98.8%, respectively. Three of three very major errors (VMEs; 1.7%) were with oxacillin. For non-S. aureus staphylococci, the CA, EA, VME, major errors, and minor error rates were 95.7%, 96.8%, 0.7%, 1.7%, and 2.9%, respectively. The two VMEs were with oxacillin. For the enterococci, there was 100% CA and 99.3% EA. All 36 vancomycin-resistant enterococci were detected by the Phoenix system. The Phoenix system compares favorably to traditional methods for the ID and AST of staphylococci and enterococci.     

Prenatal testing for Huntington's disease: a European collaborative study

This European study involving seven genetic centres from six countries - Aberdeen, Cardiff (UK), Leiden (Netherlands), Leuven (Belgium), Paris (France), Rome (Italy), Athens (Greece) has gathered information on prenatal testing by direct mutation analysis and exclusion testing for Huntington's disease (HD) from the six European countries during the period 1993-1998. Data describing the parent belonging to the HD family was collected; this included their sex and age as well as their risk of developing HD. Information about previous pregnancies, the rank of the pregnancy being tested and its outcome was also gathered. In addition the number of previous prenatal tests for HD was recorded. Three hundred and five results were recorded by the participating countries between 1993 and 1998. The largest groups came from the UK (157) and the Netherlands (90). The mean age for the parent from the HD family was 30.8 years. In half of the tests the prospective parent was an asymptomatic gene carrier, 42% remained at risk, and 6% of the prospective parents were already showing clinical features of HD. 65% of tests performed used mutation analysis.     

Validation of the automated reading and incubation system with Sensititre plates for antimicrobial susceptibility testing

The present study compared the antimicrobial susceptibility testing (AST) results generated by the Automated Incubation and Reading System (ARIS) with custom Sensititre plates (TREK Diagnostic Systems, Cleveland, Ohio) and MicroScan PC10 GP and NUMIC10 GN plates interpreted with the WalkAway-96 system (Dade Behring, West Sacramento, Calif.) for gram-positive (GP) and gram-negative (GN) organisms as part of an in-house validation. A total of 326 isolates (3,689 antimicrobial agent-organism combinations) were evaluated. Sensititre plates were inoculated according to the instructions of the manufacturer with a suspension adjusted to a 0.5 McFarland standard, while the Prompt Inoculation System was used for the MicroScan plates. ARIS and the WalkAway system were used for automated reading of the Sensititre and MicroScan plates, respectively, at 18 to 24 h. The results were analyzed for essential (+/-1 twofold dilution) and categorical (sensitive, intermediate, or resistant) agreements. Plates that resulted in ARIS interpretations with major (falsely resistant) or very major (falsely susceptible) errors compared to the results obtained with the WalkAway system were read manually to corroborate instrument readings. Isolates for which very major or major errors were obtained and for which the results were not resolved by manual reading were retested in parallel. Isolates for which very major or major errors were obtained and for which the results were not resolved upon repeat testing were tested by the National Committee for Clinical Laboratory Standards M7-A5 frozen reference microdilution method. Essential agreement was 95.8% for 246 GN isolates. The following categorical error rates were obtained for the GN isolates: 1.3% minor errors, 0% major errors, and 0.4% very major errors. For 95 GP isolates, there was 93.5% essential agreement. Categorical error rates for GP isolates were 0.9% minor errors, 0.6% major errors, and 0.4% very major errors. ARIS-Sensititre is a diagnostic system feasible for use for automated AST in a clinical laboratory.     

Evaluation of the MicroScan antimicrobial susceptibility system with the autoSCAN-4 automated reader.

The American MicroScan (American MicroScan, Mahwah, N.J.) identification and antimicrobial susceptibility system consists in part of an automated reading system (autoSCAN-4) with data management capabilities. We evaluated the system with 404 gram-negative and 170 gram-positive facultative anaerobic and aerobic bacteria. We compared MicroScan results read automatically and visually with each other and with the results obtained by the reference method (read visually). The overall agreement within +/- 1 log2 dilution was 94.3% when the MicroScan system (read automatically) was compared with the reference method (read visually), 96.4% when MicroScan panels (read visually) were compared with reference panels, and 97.4% when the autoSCAN-4 automated reading was compared with the visual reading of the MicroScan panels. Total discrepancies (susceptibility interpretation category changes) for the MicroScan system compared with the reference method were 7%, with 6.2% considered a minor discrepancy. The autoSCAN-4 and the complete MicroScan system yielded accurate results compared with the reference method.     

Evaluation of a new system, VITEK 2, for identification and antimicrobial susceptibility testing of enterococci

We evaluated the new automated VITEK 2 system (bioMérieux) for the identification and antimicrobial susceptibility testing of enterococci. The results obtained with the VITEK 2 system were compared to those obtained by reference methods: standard identification by the scheme of Facklam and Sahm [R. R. Facklam and D. F. Sahm, p. 308-314, in P. R. Murray et al., ed., Manual of Clinical Microbiology, 6th ed., 1995] and with the API 20 STREP system and, for antimicrobial susceptibility testing, broth microdilution and agar dilution methods by the procedures of the National Committee for Clinical Laboratory Standards. The presence of vanA and vanB genes was determined by PCR. A total of 150 clinical isolates were studied, corresponding to 60 Enterococcus faecalis, 55 Enterococcus faecium, 26 Enterococcus gallinarum, 5 Enterococcus avium, 2 Enterococcus durans, and 2 Enterococcus raffinosus isolates. Among those isolates, 131 (87%) were correctly identified to the species level with the VITEK 2 system. Approximately half of the misidentifications were for E. faecium with low-level resistance to vancomycin, identified as E. gallinarum or E. casseliflavus; however, a motility test solved the discrepancies and increased the agreement to 94%. Among the strains studied, 66% were vancomycin resistant (57 VanA, 16 VanB, and 26 VanC strains), 23% were ampicillin resistant (MICs, >/=16 microgram/ml), 31% were high-level gentamicin resistant, and 45% were high-level streptomycin resistant. Percentages of agreement for susceptibility and resistance to ampicillin, vancomycin, and teicoplanin and for high-level gentamicin resistance and high-level streptomycin resistance were 93, 95, 97, 97, and 96%, respectively. The accuracy of identification and antimicrobial susceptibility testing of enterococci with the VITEK 2 system, together with the significant reduction in handling time, will have a positive impact on the work flow of the clinical microbiology laboratory.     

Laboratory evaluation of an automated antimicrobial susceptibility system

A newly introduced automated method for antibiotic susceptibility testing, AUTOBAC 1, has been evaluated by comparison with the disk agar diffusion method (Bauer-Kirby). A total of 2,518 strains of gram-positive (540) and gram-negative (1,978) organisms isolated from clinical specimens was examined by both methods with eight or ten antibiotics, including Tobramycin. An overall agreement of 97.4% was obtained when results were compared by individual antibiotic. However, many discrepancies were observed when individual genera or species were analyzed. Of 2,518 strains examined, 651 (26%) showed discrepancies in response to one or more antibiotics. Strains, showing discrepancies were re-examined by the broth dilution susceptibility method. The results obtained favored the disk agar diffusion method. Reproducibility experiments revealed a greater inconsistency in the AUTOBAC 1 system than in the agar diffusion test. It is concluded that although a rapid automated system for antibiotic sensitivity testing is desirable, the conventional disk agar diffusion method is easier to perform, more reliable, and a less expensive procedure for antibiotic sensitivity determination.     

Rapid automated antimicrobial susceptibility testing of Streptococcus pneumoniae by use of the bioMerieux VITEK 2

The VITEK 2 is a new automated instrument for rapid organism identification and susceptibility testing. It has the capability of performing rapid susceptibility testing of Streptococcus pneumoniae with specially configured cards that contain enriched growth medium and antimicrobial agents relevant for this organism. The present study compared the results of testing of a group of 53 challenge strains of pneumococci with known resistance properties and a collection of clinical isolates examined in two study phases with a total of 402 and 416 isolates, respectively, with a prototype of the VITEK 2. Testing was conducted in three geographically separate laboratories; the challenge collection was tested by all three laboratories, and the unique clinical isolates were tested separately by the individual laboratories. The VITEK 2 results of tests with 10 antimicrobial agents were compared to the results generated by the National Committee for Clinical Laboratory Standards reference broth microdilution MIC test method. Excellent interlaboratory agreement was observed with the challenge strains. The overall agreement within a single twofold dilution of MICs defined by the VITEK 2 and reference method with the clinical isolates was 96.3%, although there were a number of off-scale MICs that could not be compared. The best agreement with the clinical isolates was achieved with ofloxacin and chloramphenicol (100%), and the lowest level of agreement among those drugs with sufficient on-scale MICs occurred with trimethoprim-sulfamethoxazole (89.7%). Overall there were 1.3% very major, 6.6% minor, and no major interpretive category errors encountered with the clinical isolates, although >80% of the minor interpretive errors involved only a single log(2) dilution difference. The mean time for generation of susceptibility results with the clinical isolates was 8.1 h. The VITEK 2 provided rapid, reliable susceptibility category determinations with both the challenge and clinical isolates examined in this study.     

Evaluation of the VITEK 2 system for identification and antimicrobial susceptibility testing of medically relevant gram-positive cocci

A study was conducted to evaluate the new VITEK 2 system (bioMérieux) for identification and antibiotic susceptibility testing of gram-positive cocci. Clinical isolates of Staphylococcus aureus (n = 100), coagulase-negative staphylococci (CNS) (n = 100), Enterococcus spp. (n = 89), Streptococcus agalactiae (n = 29), and Streptococcus pneumoniae (n = 66) were examined with the ID-GPC identification card and with the AST-P515 (for staphylococci), AST-P516 (for enterococci and S. agalactiae) and AST-P506 (for pneumococci) susceptibility cards. The identification comparison methods were the API Staph for staphylococci and the API 20 Strep for streptococci and enterococci; for antimicrobial susceptibility testing, the agar dilution method according to the procedure of the National Committee for Clinical Laboratory Standards (NCCLS) was used. The VITEK 2 system correctly identified to the species level (only one choice or after simple supplementary tests) 99% of S. aureus, 96.5% of S. agalactiae, 96.9% of S. pneumoniae, 92.7% of Enterococcus faecalis, 91.3% of Staphylococcus haemolyticus, and 88% of Staphylococcus epidermidis but was least able to identify Enterococcus faecium (71.4% correct). More than 90% of gram-positive cocci were identified within 3 h. According to the NCCLS breakpoints, antimicrobial susceptibility testing with the VITEK 2 system gave 96% correct category agreement, 0.82% very major errors, 0.17% major errors, and 2.7% minor errors. Antimicrobial susceptibility testing showed category agreement from 94 to 100% for S. aureus, from 90 to 100% for CNS, from 91 to 100% for enterococci, from 96 to 100% for S. agalactiae, and from 91 to 100% for S. pneumoniae. Microorganism-antibiotic combinations that gave very major errors were CNS-erythromycin, CNS-oxacillin, enterococci-teicoplanin, and enterococci-high-concentration gentamicin. Major errors were observed for CNS-oxacillin and S. agalactiae-tetracycline combinations. In conclusion the results of this study indicate that the VITEK 2 system represents an accurate and acceptable means for performing identification and antibiotic susceptibility tests with medically relevant gram-positive cocci.     

Evaluation of the Vitek 2 system for identification and antimicrobial susceptibility testing of Staphylococcus spp.

The Vitek 2 system was assessed against reference methods with 197 methicillin-resistant Staphylococcus aureus from Belgian hospitals and 121 clinically significant blood culture isolates of Staphylococcus spp. Vitek 2 identified 95% of staphylococcal isolates correctly, detected oxacillin resistance with a sensitivity/specificity of 99/96%, and showed acceptable accuracy for susceptibility testing of five of eight other evaluable antibiotics. The median time for reporting results was 2 h 45 min for identification and 7 h for susceptibility tests.