Antimicrobial susceptibility testing (AST): a review of changing trends, quality control guidelines, test accuracy, and recommendation for the testing of beta-lactam drugs

Multicenter Evaluation of Ceftazidime-Avibactam Susceptibility Testing of Enterobacterales and Pseudomonas aeruginosa on the Vitek 2 System

In this multisite study, Vitek 2 AST-Gram-Negative Ceftazidime/Avibactam test results for 1,073 isolates (866 Enterobacterales and 207 Pseudomonas aeruginosa) were compared to the Clinical and Laboratory Standards Institute (CLSI) broth microdilution (BMD) reference method. The results were analyzed for essential agreement (EA), category agreement (CA), major error rates, and very major error rates following FDA/ISO performance criteria using the FDA-recognized CLSI/EUCAST breakpoints (sensitive [S], ≤8/4 μg/ml; resistant [R], ≥16/4 μg/ml). The overall EA was 94.5% (1,014/1,073) and CA was 98.7% (1,059/1,073). No very major errors were reported. The major error rate was 1.4% (14/998). Out of 14 major errors, 9 were within EA. Based on the EA and lack of an intermediate category for ceftazidime-avibactam (CZA), the adjusted major error rate for FDA criteria was 0.5% (5/998). The performance for ISO criteria after error resolutions included EA of 94.5% (1,014/1,073), CA of 98.9% (1,061/1,073), major error of 1.2% (12/998), and no very major error. Vitek 2 met the ISO and FDA criteria of ≥95% reproducibility and ≥95% quality control (QC) results within acceptable ranges for QC organisms. Vitek 2 overall performance for Enterobacterales and P. aeruginosa met or exceeded the FDA and ISO performance criteria; thus, it is a reliable alternative to the BMD reference method for routine CZA susceptibility testing.

Educational antimicrobial susceptibility testing as a critical component of microbiology laboratory proficiency programs: American Proficiency Institute results for 2007-2011

External laboratory proficiency programs are an important requirement for test quality assurance (EQA) and compliance to regulatory guidelines (Clinical Laboratory Improvement Amendments and inspections). The American Proficiency Institute (API) regularly distributes EQA sample challenges (test events) including an Educational Sample (ES) for antimicrobial susceptibility testing. Beginning in 2007, API has sent 3 ES samples annually, each a well-characterized (molecular/phenotypic methods) strain having an interesting/emerging mechanism of resistance. Hundreds of USA laboratories, usually serving small- to medium-size hospitals and clinics, participate in the API ungraded ES test event. Analysis of responses is made and reported electronically as ES critiques addressing contemporary susceptibility testing issues that affect patient therapy. Seven Gram-positive (+) and 8 Gram-negative (-) ES strains were tested over the 5 years (2007-2011) with organism identification (graded) accuracy of 95.3% (range, 91.0-99.2%) for Gram (-) and 97.0% (range, 94.2-100.0%) for Gram (+) challenges. Susceptibility testing categorical accuracy was generally greatest for the disk diffusion test (91.0/97.0%) compared to the MIC methods (commercial automated or manual) combined (89.9/96.1%, for Gram [-]/Gram [+], respectively). The most worrisome observations of these ES samples were as follows: 1) poor recognition of ESBL- and serine carbapenemase-producing strains (various types including Klebsiella pneumoniae carbapanemase) due to delayed application of Clinical and Laboratory Standards Institute [CLSI] guidelines; 2) overcalling of ESBL in organisms having wild-type non-ESBL enzymes (OXA series; OXA, 1/30) due to commercial system or participant interpretive error; and 3) occasional drug-bug discords noted in nonfermentative Gram (-) bacilli. In conclusion, the API ES series of ungraded susceptibility testing challenges (accuracy was >90%) has been well received by subscribers and has provided detailed educational opportunities to improve laboratory testing performance. ES samples have delivered guidance to enable laboratories to rapidly comply with CLSI document changes of interpretive breakpoints such as those for β-lactams when testing Enterobacteriaceae and Pseudomonas aeruginosa; the program was sustained into 2012 and beyond to document quality of susceptibility tests in USA laboratories.

Performance accuracy of antibacterial and antifungal susceptibility test methods: report from the College of American Pathologists Microbiology Surveys Program (2001-2003)

CONTEXT

The College of American Pathologists Microbiology Surveys Program provides external proficiency samples that monitor the performance of nearly 3000 laboratories that perform and report antimicrobial susceptibility tests.

OBJECTIVE

To summarize results obtained with bacterial and yeast challenge samples (2001 through 2003).

DESIGN

One organism every 4 months was tested by surveys participants against antibacterials/antifungals by routinely used methods. Reports were graded by interpretive category (susceptible, intermediate, resistant) based on an 80% consensus of referees/participants.

RESULTS

The most common antibacterial test methods/systems were Vitek (38%-43%), MicroScan (39%-43%), and the disk diffusion test (14%-15%), although Etest was most used for fastidious species. YeastOne was the dominant antifungal test (50%-55%). Antifungal results demonstrated continuous, improved accuracy (83%-88%), highest for YeastOne (96%) and broth microdilution (95%) methods. Antibacterial test accuracy was consistently greater than 97% against gram-positive organism challenges and greater than 98% against gram-negative challenges. For gram-negative strains with well-characterized resistance mechanisms, the accuracy by method was disk diffusion greater than broth microdilution greater than automated systems. Major problems identified were (1) Haemophilus influenzae control ranges require re-evaluation, (2) overuse of beta-lactamase tests, (3) errors among Enterococcus faecium against penicillins (Vitek 2, MicroScan), (4) false-susceptible results with trimethoprim/sulfamethoxazole against coagulase-negative staphylococci (MicroScan), (5) macrolide false-susceptibility for beta-hemolytic streptococcus (MicroScan), (6) flawed reporting for antimicrobials not active at the infection site, (7) use of outdated interpretive criteria, and (8) failure to follow Clinical and Laboratory Standards Institute testing/reporting recommendations.

CONCLUSIONS

Susceptibility tests were generally performing satisfactorily as measured by the surveys, but serious errors were identified with some drug/organism combinations that may require action by the Clinical and Laboratory Standards Institute and/or the Food and Drug Administration.

Method preferences and test accuracy of antimicrobial susceptibility testing: updates from the College of Amercian Pathologists Microbiology Surveys Program

OBJECTIVE

To summarize the antimicrobial susceptibility testing results from the College of American Pathologists (CAP) Microbiology Surveys Program for 2000. Specifically, the frequency of tests used and the quantitative and qualitative (susceptibility category) accuracy were assessed.

DESIGN

The CAP Microbiology Surveys challenged subscribers in 2000 with 3 well-characterized organisms for antimicrobial susceptibility testing in pure culture. Each laboratory was to use the test method and reporting procedures routinely applied to patient samples. The strains were National Committee for Clinical Laboratory Standards (NCCLS) quality control organisms with precisely defined antimicrobial susceptibility patterns and reproducibility. Results reported by participants (2685-2979/sample) were graded for categorical accuracy and quantitative performance by comparing reported minimal inhibitory concentrations (microg/mL) or zone diameters (mm) against quality control ranges published by the NCCLS. The appropriateness of reported drugs was determined in the context of the type and anatomic location of the infection.

RESULTS

The tests most often used varied by the species of the organism and growth characteristics of the isolated strains. Nonfastidious, rapid-growing Surveys unknowns (Escherichia coli ATCC 25922, Pseudomonas aeruginosa ATCC 27853) were most often tested with commercial systems (MicroScan, 42.0%-42.4%; Vitek, 41.5%-43.0%) or with the standardized disk diffusion method (12.8%-13.9%). In contrast, fastidious species, such as Streptococcus pneumoniae (ATCC 49619), were predominantly tested by Etest (40.3%), followed by disk diffusion (27.6%) and MicroScan (23.2%). Categorical accuracy was essentially equal between dilution (98.9%) and diffusion (99.0%) methods. Among the minimal inhibitory concentration methods used to test penicillin against S pneumoniae, Etest method quantitative accuracy (96.3%) was greater than that of MicroScan (92.4%). Quantitative accuracy was greatest for dilution minimal inhibitory concentration methods, with more than 90% of results within NCCLS quality control ranges for nearly all reported antimicrobials. Reevaluations of quality control ranges may be needed for 4 to 7 agents, depending on method. Reporting errors were also detected in 2 areas: (1) reporting results for drugs not active at the site of infection and (2) reporting results for drugs tested with suboptimal methods without published NCCLS interpretive criteria.

CONCLUSIONS

Antimicrobial susceptibility testing methods used in US laboratories were dominated by commercial products with relatively high accuracy (qualitative and quantitative). As available methods have become better suited to both fastidious and rapid-growing species, reporting errors have assumed a higher level of concern to the CAP Surveys in an effort to minimize prescription errors.

Tests for bactericidal effects of antimicrobial agents: technical performance and clinical relevance

Bactericidal testing has been used for several decades as a guide for antimicrobial therapy of serious infections. Such testing is most frequently performed when bactericidal antimicrobial agent therapy is considered necessary (such as when treating infectious endocarditis or infection in an immunocompromised host). It has also been used to ensure that the infecting organism is killed by (not tolerant to) usually bactericidal compounds. However, few data are available to support the role of such tests in direct patient care. Several important variables affect the reproducibility of the test results; however, proposed reference methods are now available for performing the MBC test. With minor modifications, these can provide a standardized approach for laboratories that need to perform them. Currently, little evidence is available to support the routine use of such testing for the care of individual patients. However, testing of new (investigational) antimicrobial agents can be beneficial in determining their potential to provide bactericidal antimicrobial activity during clinical use. New methods to assess bactericidal activity are being developed, but as yet none have been rigorously tested in patient care settings; further, for most of these methods, little information is available as to which technical parameters affect their results. In clinical laboratories, all bactericidal tests must be performed with rigorously standardized techniques and adequate controls, bearing in mind the limitations of the currently available test procedures.

Growth of Bacteroides fragilis group in agar and broth media

The rate of bacterial growth of four Bacteroides fragilis group organisms was determined in agar and broth media. Exponential bacterial growth occurred in agar media within 4 to 8 h, while such growth was delayed in broth media and occurred within 12-24 h after inoculation. This phenomenon may explain why antimicrobials which manifest an 'inoculum effect' may show increased resistance to antimicrobials when tested in agar media.

Comparison of BIOGRAM and commercial microdilution antimicrobial susceptibility test systems

The BIOGRAM (Difco Laboratories, Detroit, Mich.) system, which is designed to calculate MICs from disk diffusion zone diameters, was compared with two commercial microdilution antimicrobial susceptibility systems. A total of 111 clinical isolates were evaluated with each test system. Six additional isolates were tested in a comparison between BIOGRAM and Sceptor (Johnston Laboratories, Inc. Towson, Md.) systems. BIOGRAM demonstrated an overall correlation with the Sceptor microdilution method of 95.7% for 1,287 organism-antimicrobial susceptibility combinations. The BIOGRAM and UniScept (Analytab Products, Inc., Plainview, N.Y.) systems were in agreement in 90.3% of 1,048 organism-antimicrobial susceptibility combinations tested. All methicillin-resistant staphylococci were detected by the standard disk agar diffusion method used with the BIOGRAM system. The BIOGRAM system provides an acceptable alternative to these commercial systems for the determination of quantitative susceptibility.

Evaluation of the BIOGRAM antimicrobial susceptibility test system

BIOGRAM is an antimicrobial susceptibility test system for the determination of MICs from the standard disk diffusion test zone diameters. The system was challenged with 511 recent clinical isolates of members of the family Enterobacteriaceae, nonfermentative gram-negative bacteria, staphylococci, and enterococci. Results were compared with those obtained with the broth microdilution method. Appropriate control organisms were included with each test series. A total of 10,085 organism-drug combinations were evaluated. BIOGRAM demonstrated an overall correlation of 95.9% with the reference broth microdilution method.

Accuracy and reproducibility of the AutoMicrobic System Gram-Negative General Susceptibility-Plus Card for testing selected challenge organisms

A total of 180 selected strains of gram-negative bacilli were tested for susceptibility to nine antibiotics by the prototype General Susceptibility Card and reference broth microdilution MIC method, and 112 of 180 were tested by the modified Gram-Negative General Susceptibility-Plus Card (Vitek Systems, Inc., Hazelwood, Mo.). When category calls--susceptible, intermediate, and resistant for the prototype card and very susceptible, moderately susceptible, and resistant for the modified card--were identical or compatible (very susceptible by one method and moderately susceptible by the other), the methods were considered in qualitative or categorical agreement. The overall categorical agreement improved from 83% for the prototype card to 91.5% for the modified card, and the frequency of major and very major disagreements was reduced from 7.5 to 1%, respectively. Overall agreement between the modified card and reference method for both category calls and MICs (+/- 2 dilutions) was 90%. Of the results, 67% were identical, 22% were more susceptible with the modified card, and 11% were more resistant. Reproducibility of identical or compatible category calls and MICs obtained from three trials was 94.5%. The poorest accuracy and reproducibility were observed when testing Pseudomonas aeruginosa. The study suggests that the modified card can rapidly and efficiently perform susceptibility tests with an acceptable level of accuracy and reproducibility provided that the system is appropriately used for testing strains of the family Enterobacteriaceae with preset criteria for excluding organism-antibiotic combinations. The evaluation also indicates dramatic improvement in the technical performance of the modified card compared with its earlier prototype.

Determination of interpretive breakpoints for ceftazidime disc-diffusion susceptibility testing using single-strain regression analysis

Interpretive breakpoints for ceftazidime disc-diffusion susceptibility testing were determined using single-strain regression analysis (SRA). Regression lines were determined for a total of 58 strains representing 15 species, from inhibition zone diameters obtained for discs containing six different ceftazidime concentrations. Statistical analysis for excluding non-linearity of test-results was performed. A minimum of five tests on consecutive days was required for maximal precision of regression analysis according to the SRA-method. Calculated regression lines showed similarities within individual and groups of bacterial species. A minimum of five strains could be used to represent these groups. Interpretive breakpoints according to recommended MIC-limits were determined for each species taking into consideration confidence limits for zone correlates of MIC-values. Single-strain regression analysis for the determination of interpretive breakpoints for ceftazidime disc-diffusion susceptibility testing in individual laboratories.

The clinical microbiology laboratory. Utilization and cost-effectiveness

Utilization of the microbiology laboratory is subject to limitations posed by the diversity of microbial agents causing infection, multiple and often poorly accessible sites of infection, contamination of specimens with indigenous flora, and failure of communication between clinician and microbiologist. Cost constraints increasingly limit the extent of microbiologic services available on-site and lead to decentralization of laboratory services and possible loss of quality control. Increased attention is being paid to process control of cultures, rapid screening and diagnostic tests as culture substitutes, and limitations on antibiotic susceptibility testing.

Comparison of a beta-lactamase induction test with a test that detects low-frequency resistance to cefotaxime

A paper strip test that detects cefotaxime-resistant variants of gram-negative bacilli was described and compared with a beta-lactamase induction test. Both tests demonstrated a potential for resistance that is not indicated by standard agar dilution and agar diffusion tests.

Effect of the source of Mueller-Hinton agar and resistance frequency on the detection of methicillin-resistant Staphylococcus aureus

Inconsistencies in the results of disk diffusion tests of oxacillin against Staphylococcus aureus that occurred when using commercially prepared Mueller-Hinton agar from different sources led us to evaluate the ability of media from different sources to detect resistance to oxacillin, methicillin, and nafcillin in S. aureus. Mueller-Hinton agar from five manufacturers was prepared in our laboratory and used for standard disk diffusion and agar dilution tests. Ten oxacillin-resistant S. aureus isolates, of which three were definitive-resistant and seven were occult-resistant, were examined. All definitive-resistant strains were resistant to all three antimicrobial agents on four out of five agars. The occult-resistant strains were consistently detected as resistant on only one of the agars. With only slight differences, oxacillin, methicillin, and nafcillin resistance was more readily detected by disk diffusion and agar dilution when initially incubated at 30 degrees C, and extended incubation improved the detection. The frequency of resistance within a population of occult-resistant cells was low compared with the frequency within a population of definitively resistant cells. The heterogeneity of colony morphology and apparent growth rates within a population of occult-resistant cells contributed to the problem of detecting some resistant isolates. Definitive-resistant isolates were characterized by a very high and stable frequency of resistance. Occult-resistant strains were characterized by a lower frequency of resistance, although the true frequency of resistance may be difficult to ascertain because of heterogeneity in growth rates.

Determination of species- and laboratory-related interpretive breakpoints for doxycycline susceptibility testing using single-strain regression analysis

Single-strain Regression Analysis (SRA) was performed for doxycycline on a total of 68 bacterial strains representing 16 different species. Species- and laboratory-related zone diameter breakpoints were determined and compared with histograms of zone diameter values obtained from 942 routine susceptibility tests. Calculated breakpoints were similar within individual species. When considering the homogeneity of susceptibility groups within bacterial species, the calculated breakpoints gave rise to relatively few interpretive errors. In contrast, general breakpoints for doxycycline as recommended by the Swedish Reference Group (SRG) (R less than = 20 mm and S greater than = 26 mm) would give rise to a high proportion of false interpretations in the present laboratory. Fifty-three per cent of H.influenzae strains would have been assigned to the wrong susceptibility group. For E.coli and K.pneumoniae, 22 and 35 per cent, respectively, would have been erroneously categorized using SRG breakpoints. E.cloacae and E.aerogenes would have been assigned another category in 39 and 50 per cent, respectively. The procedure for setting species-specific and laboratory-related interpretive breakpoints is described. Determination of species- and laboratory-related interpretive breakpoints using SRA provides a new approach towards improved accuracy of disc-diffusion susceptibility testing.

Changing patterns of resistance to new beta-lactam antibiotics. In vitro efficacy of cefoperazone against bacterial pathogens

No evidence for significant increase in resistance to cefoperazone was detected in susceptibility test surveillance programs or in the literature through 1983, but rare endemic resistance to cefoperazone and some other newer beta-lactams was found. Medical centers contemplating the use of a third-generation drug for cost-containment should be aware of the susceptibility of isolates in their hospitals, and the drug's ability to withstand beta-lactamase hydrolysis by local pathogens. The possible dangers of inducible cephalosporins in certain Enterobacteriaceae and strains of Pseudomonas aeruginosa are discussed in the light of the physical and chemical characteristics of the newer antimicrobial agents. Criteria for judging the values of investigational beta-lactams are presented. These focus on the interaction of antimicrobial agents with host defense mechanisms and circulating blood elements.

Single-strain regression analysis for quality control of cephalothin-susceptibility testing and determination of interpretive breakpoints

Histogram analysis of inhibition zone diameters around the 30 micrograms cephalothin disk for E. coli, P. mirabilis, and K. pneumoniae in samples from 1975 to 1982 showed a marked reproducibility of the disk-diffusion antibiotic-susceptibility test in the routine laboratory. A comparison of interpretive breakpoints with histograms for E. coli, P. mirabilis, K. pneumoniae, S. aureus, coagulase-negative staphylococci, and S. faecalis showed a higher proportion of possible misinterpretations using the breakpoints of the Swedish Reference Group, SRG, as compared to international (NCCLS) breakpoints. Further analysis using single-strain regression analysis revealed two major causes of interpretive errors. Firstly, the laboratory-related regression line for a bacterial species can be different from the general regression line of the reference laboratory. This difference has to be corrected by using species-related breakpoints. For E. coli, a species-specific breakpoint was determined to R = greater than 13 mm. Secondly, MIC limits recommended for the susceptibility categories of cephalothin by SRG are lower than the international limits and close to the true MIC values of many bacterial isolates, leading to misinterpretations due to the methodological variation. These studies suggest an adoption of international MIC limits for the susceptibility categories of cephalothin in Scandinavia. The "I" category should denote an indeterminate zone. A multi-laboratory quality control assessment using histogram analysis is recommended with optional single-strain regression analysis to determine breakpoints for problem combinations of bacterial species and antibiotics.

Comparison of two automated instrument systems for rapid susceptibility testing of gram-negative bacilli

The Vitek AutoMicrobic System with GSC-plus cards and the Abbott MS-2 system were tested in parallel and the results were compared directly with those of a reference microdilution minimal inhibitory concentration (MIC) procedure on a group of 262 clinical isolates of the family Enterobacteriaceae and of Pseudomonas aeruginosa. Results of both systems were compared with the reference MIC for category agreement, and in addition, the Vitek MICs were compared with those obtained by the reference procedure. The Vitek system provided an essential category correlation of 89.4% for enteric bacteria and 97.0% for P. aeruginosa. Vitek MICs agreed within 1 twofold dilutional increment for 86.3% of the enteric bacteria tested and for 96.2% of the P. aeruginosa isolates. The Abbott MS-2 essential categoric agreement was 92.0% for enteric bacteria and 92.4% for P. aeruginosa. If only aminoglycosides or carbenicillin were considered for P. aeruginosa isolates, the essential category agreement was 92.5% for the Vitek and 93.3% for the MS-2. The majority of MS-2 category errors (13 of 19) with P. aeruginosa involved gentamicin results on isolates whose reference MICs were 8 micrograms/ml and whose MS-2 results were susceptible (MIC less than or equal to 4 micrograms/ml). Retesting of the P. aeruginosa isolates in calcium-supplemented MS-2 broth increased the essential agreement for the aminoglycosides to 97.5%.