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Rapid antimicrobial susceptibility testing: Technical and clinical considerations
Clinical Microbiology Newsletter October 15, 1993
Vol. 15, No. 20
Rapid Antimicrobial Susceptibility Testing: Technical and Clinical Considerations John A. Washington, M.D. Section of M &robiology The Cleveland Clinic Foundation Cleveland, OH 44195 Rapid antimicrobial susceptibility testing has engendered broad interest among clinical microbiologists, primarily based on the assumption that the earlier availability of susceptibility results would provide an opportunity for earlier adjustment of antimicrobial therapy than might the case with conventional testing. Whether this assumption is correct and whether existing commercially available technology provides rapid susceptibility test results accurately are the subjects of this article.
Commercial Systents---Early Beginnings and Problems The first commercially available rapid susceptibility testing device was the Autobac, inlroduced by Pfizer Diagnostics in the 1970s and now no longer available. This system was followed by the Abbott MS-2/Avantage system. Both systems were based on disk elution technology and provide results in 5 h; however, neither system has been critically evaluat_edfor its accuracy in detecting more recently described gramnegative resistance, such as that due to Bush group 1 beta-lactamase or the expanded spectrum plasmid-mediated TEM- ~ SHV-derived cephalosporinases. Moreover, neither has been critically evaluated with respect to accuracy in detecting expression class 1 or 2 heterogeneously methicillin-resistant staphylococci. One important and often
overlooked observation was, however, made by Lampe et al. (1), who found that the detection of resistance of Enterobacter cloacae to ampicillin was not made by the MS-2 within the fhst 6 h of incubation. In other words, although there was a small increase in optical density initially, logarithmic growth of E. cloacae in the presence of ampicillin was not observed for at least 6 h, reflecting then the growth of mutants that inactivated ampiciUin. Such mutants are now variously known to conlain inducible beta-lactamase,to be derepressed for Bush group 1 beta-lactamase, or to have constitutive Bush group 1 beta-lactamaseand are most frequenlly associated with Citrobacterfreundii, Enterobacter spp., Morganella morganff, Providencia stuartii, Serratia marcescens, and Pseudomonas aerugi-
Vitek System (2). The corresponding rates of very major errors for the derepressed mutants were both 2.7%. Although this percent of very major errors is higher than the 1% maximum that is commonly regarded as acceptable, it should be emphasized that the test population was highly selective. At any rate, it seems likely that the Vitek System detected beta-lactam resistance in the majority of instances with this particular group of problem organisms because of the somewhat longer incubation time (mean, 7.6 h) than those of the Autobac and the MS-2 (5 h). As already pointed out, even the detection of ampicillin resistance in E. cloacae requires at least 6 h. The AutoSCAN Walk-Away (W/A) system (Baxter Diagnostics, West Sac-
Ilosa.
The Vitek System (bioMerieux, Hazelwood, MO) provides susceptibility results for this group of gram-negative bacilli in an average of nearly 8 h. In a comparison of this system with agar dilution, broth macrodilution, and microdilution in tests of beta-lactams against isogenic pairs of wild type and stably derepressed mutants for Bush group 1 beta-lactamase and of stable mutants without wild type pairs of gram-negative bacilli, the overall very major error rate in 352 organism-betalactam combinations resulting in 840 tests was 3% between the reference agar or macrodilution and microdilution methods and 2% between the reference agar or macrodilution method and the
In This Issue i
Rapid Antimicrobial Susceptibility Testing: Technical and Clinical Considerations . . . . . . . . . . . . . 153 A review of the problems associated with rapid ASTs with specific reference to selected organism~antibiotic combinations and the utility of the data
In Search of the Golden Kilo-The Development of Reference Standards for Mueiler.Hinton Agar . . . . . . . . . . . . . . . . . . . . . . . .
156
The history behind the establishment of the Mueller-Hinton agar medium refer-
ence standard and the continuing efforts to maintain that standard i
CMNEEJ 15(20)153-160,1993
Elsevier
0196-4399/93/$0.00 + 06.00
ramento, CA) provides susceptibility test results in 5 to 7 h. This system has recently undergone critical evaluation with respect to its accuracy in detecting resistance among gram-negative fermentative bacilli (3). In this study, tests of common clinical isolates were supplemented with isolates likely to be highly antibiotic resistant. The standard microdilution test served as the reference method. The overall essential agreement between the W/A system and microdilution was 92.7%; however, less than 90% agreement occurred for piperacillin, cefazolin, cefotetan, and ceftriaxone, although the results for cefazolin were suppressed for certain organism groups. Without exclusion of certain organism-antimicrobial combinations that were suppressed by the W/A system, there were 906 determinations of resistance, of which 658 were classified as resistant by the W/A system and 133 as moderately susceptible, leaving 115 very major discrepancies (12.7% of the 906 resistant organismantimicrobial combinations or 2.8% of the total tests). After exclusion of organism-antimicrobial combinations suppressed by the W/A system, there were 708 resistant determinations, of which 517 were called resistant by the W/A system and 93 were called moderately susceptible, leaving 98 very major discrepancies (13.8% of the 708 resistant determinations or 2.6% of the total determinations). The W/A had difficulty detecting narrow-spectrum cephalosporin resistance in Klebsiella spp. and broad-spectrum cephalosporin resistance in C. freundii, Enterobacter spp., M. morganii, and Serratia spp. Only 7 3 % of the resistant organism -cephalosporin combinations in this latter group of the Enterobacteriaceae were called resistant by the W]A system. Whether the small difference in incubation time between the W/A and Vitek systems is sufficient to explain the difference in accuracy in detection of resis-
tance to beta-lactams by species typically associated with Bush group 1 betalactamase is unclear. These two studies emphasize the need for evaluation of susceptibility testing systems, rapid or otherwise, with not only common clinical isolates but also isolates characterized by resistance to various classes of antimicrobial agents.
Detection of Methicillin Resistance The accurate detection of methiciUin or oxacillin resistance in staphylococci, particularly those heterogeneously resistant in expression class 1 or 2 (4), remains a concern. A recently published report of a provincial quality control program in Canada found that laboratories experienced particular difficulty in detecting resistance in an expression class 1 strain (5), Skulnick et al. (6) evaluated the MicroScan Positive MIC panel and the Vitek GPS-SA card, along with the oxacillin agar screen, in comparison with the standard broth microdilution method and a DNA probe encoding the mec gene. They reported 14.2% very major errors (i.e., false susceptible results) with Vitek and 6.7% very major errors with MicroScan. It was not clear, however, what Vitek software program was used in their study. In a comparable preliminarily reported (7) and recently completed study m our laboratory of the Vitek GPS-SA card with the 6.1 software upgrade we found no very major errors when results were compared with the standard broth microdilution test, the oxacillin agar screen, population analysis profiles, and DNA probe studies for the mec gene. Our studies did include isolates belonging to expression classes 1 arid 2, as well as 3 and 4. Reanalysis of our data with the 7.1 software upgrade had no effect on either very major or major errors. Whether the difference between our results and those of Skulnick et al. (6) can be ascribed to their use of the older 4.2 GPS-SA program is unknown since they did not specify which soft-
ware program they used. It is also not clear what expression class the resistant isolates studied by them belonged to. To summarize the discussion up to this point, it would appear that false susceptibility is a problem when testing gram-negative bacilli associated with Bush group 1 beta-lactamase against expanded spectrum cephalosporins and penicillins with rapid methods providing results within 5 or 6 h of incubation. Whether detection of this type of resistance can be made more accurate by incorporating a low concentration of an inducing agent (e.g., imipenem, 0.1 mg/l, as suggested by Reeves et al. (8) for agar dilution) in the medium for a rapid test would improve accuracy is uncertain but may be worth ex ploring Clinical C o n s i d e r a t i o n s Technical considerations aside, the big question is whether faster is better. From a theoretical standpoint, the more rapid availability of susceptibility results should, in turn, lead to the more rapid administration of appropriate antimicrobial therapy, whether that is defined by effective therapy based on an organism's susceptibility in vitro and/or the administration of an active but less costly antibiotic regimen. In most instances patients with suspected infections are started on antimicrobial therapy empirically, and in most instances today the antimicrobial agent started is active against the organism involved. Thus, inappropriate therapy involving the administration of an antimicrobial agent to which the organism is resistant is not a frequent occurrence except perhaps in instances of suspected polymicrobial infections, in which case two or three antimicrobial agents may have already been started. The patient's clinical course on empirical therapy, rather than the arrival of the susceptibility report, may then be the ultimate determinant of whether to modify therapy.
III
N O T E : No rc~pousibility is assmned by the Publisher for any injury and/or damage to persons or property as a m a t e r of products liability, negligence or otherwise, or from any use or operation of any methods, products, instmutiotts or i d s ~ contained in the nmmrial ber¢in. No suggested test o r p ~ t ~ should be carried out unlms, in the r~d©r's judgment, its risk is justified. Because of rapid advances in medical sciences, w e m m e n d that the indqpeudmt vor~e~tinn of diagnoses and drug dosages should be made. Disc,tmsitus, views, and
reo~mmctadations as to medical procedures, choice of drags, and drug dosages are the responsibility of the authors.
Clinical Microbiology Newsletter (ISSN 0196-4399) is issued twice monthly in one indexed vohaneper year by Elsevier Science Publishing Co., Inc.. 655 Avenue of the Americas. N e w York. NY 10010. S/lbscription price per year. $145.00 including pottage and handling in the United States. Canada. and Mexico. Add $56.00 for postaSe in the rest of the world. Second*class postage paid at New York, NY and at additional mailing offices. Postmaster: Send address changes to Climcal Microbiology Newsletter. Elsevier Science Publishing Co.. Inc., 655 Avctlue of the Americas, New York. NY 10010.
154
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© 1993 Elsevier Science Publishing Co,. inc.
Clinical Microbiology Newsletter ! 5:20.199]
Another problem in defining appropriate therapy is the tendency of laboratory personnel to focus on the organism isolated from a particular site and the organism's antimicrobial susceptibility without knowledge or consideration of other potential sites of infection. In other words, while an Escherichia coli or Proteus mirabilis from the urine of a patient receiving imipenem is fully susceptible to ampicillin or trimethoprim/sulf:~methoxazole, the same patient may have what appears to be a nosocomial pneumonia with diverse gram-negative flora on Gram stain alad culture of respiratory secretions. Under these circumstances, the choice of therapy may have been a prudent one, despite the susceptibility of the E. coli or P. mirabilis to less expensive antimicrobial agents. In another example, a patient may be receiving vancomycin for an infection due to a methicillin-susceptible Staphylococcus spp.; however, this regimen may be completely appropriate ff the patient is on hemodialysis. The point of presenting these two examples is that a rapid result in these instances would probably have no major impact on the antibiotic regimens being administered.
Acceptance of Same-Day Reporting There are very few hard data documenting the clinical value of rapid testing, due at least in part to the fact that studies on the use of susceptibility resuits have repeatedly demonstrated physician ignorance of the availability or existence of susceptibility results in up to 50% of cases. In an early study of rapid testing, Matsen (9) found that only 51.7 % of physicians at the University of Utah were aware of the susceptibility results. Similar results were reported by Pestotnik et al. (10). These investigators found that intervention constituted the only effective way to modify antimicrobial therapy. Matsen (9) reported a study in which isolates from patients' specimens were tested by both a 3- to 5-h semiautomated disk elution method and the disk diffusion method. Results from the rapid method were reported the same afternoon, while those from the disk method were reported the next day. AfClinical Microbiology Newsletter 15:20,1993
ter receiving both results, physicians were questioned as to whether receiving the earlier result had influenced their choice of antimicrobial therapy. In 31.7% of cases physicians responded that receipt of the preliminary culture result with the rapid susceptibility report had led them to start therapy other than that which they might have started. In another 17% of cases physicians indicated that they "probably" would have made a different antibiotic selection on the basis of the earlier report. The only problems with this study were the biases introduced when the investigators called attention to susceptibility reports that had previously been ignored in about half the cases and then further called attention to the existence of two reports--one rapid and one conventional. By contrast, Vincent et at. (11) conducted a prospective, randomized study in a surgical unit in which half the patients' positive cultures were tested by a rapid disk elution method and the other half by a conventional method. They reported that therapy was modified in 14.5% of patients whose susceptibility results were reported rapidly in contrast with 8.8% of those whose results were reported conventionally (P< 0.001); however, there was no statistically significant difference between the two groups as regards length of stay. While both of these studies demonstrated that therapeutic changes were made, apparently in response to notation of a rapid susceptibility test result, it is unclear what reporting systems were in place in either institution at the time. The current availability of interfaces between automated instruments and laboratory information systems and the availability of these results to the medical staff via remote inquiry terminals has further facilitated the opportunity for rapid communication of results and potential modification of therapy. Moreover, interfaces between the microbiology laboratory and the pharmacy permit intervention by an infectious disease pharmacist, when appropriate (10). Evangelista (12) audited the impact of rapid computerized results generated by the Vitek system on antimicrobial therapy in patients with positive blood cultures. In contrast with the previously cited studies, this study did not corn-
Summary In the final analysis, the effectiveness of microbiology reports reflects a complex web of factors including proper specimen collection and transport, process control by the laboratory as regards the establishment of appropriate culturing and antimicrobial susceptibility testing protocols, reporting procedures, interaction with clinicians, and understanding of infectious diseases. Rapid turnaround time of tests may be helpful in certain critical cases
O 1993 Elsevier Science Publishing Co., Inc.
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pare the impact of rapid testing with that of conventional testing but instead assessed adequacy of therapy and the impact of computerized reporting on modification of therapy. The study confirmed the fact that the majority of patients (56%) were receiving adequate therapy based on in vitro susceptibility results and demonstrated that modifications in therapy were made in 22% of cases following the Gram stain report and in 28% of cases following the preliminary identification/susceptibility report. Of interest was the observation that therapy was modified only after a printed report was charted on 11% of patients. In examining respiratory, wound, and urine reports, Evangelista found that empirical therapy was adequate in only 25% of cases and that modifications in therapy were made in 8% of cases on the basis of Gram stain and in only I7% of cases following preliminary identification/susceptibility reports. In this portion of the study modifications were made in an additional 25% of cases following charting of the reports. Inappropriate or no change in therapy was noted in 21% of cases. It was also noted that fewer patients in this audit had been placed on empirical antimicrobial therapy than in the blood culture audit, which was ascribed to differences in disease severity between the two groups. One other explanation is that a positive wound or respiratory culture, which is often mixed, is less likely to equate with disease than a positive blood culture. At any rate, the results of this study send a mixed message but may provide greater support for the value of rapid computerized reporting of results, especially those of blood cultures.
155
but may also be helpful only when linked with a prospective intervention program. While early studies in this area focused on the effects of rapid testing on length of stay, current DRG pressures to minimize length of stay may have superseded the impact of rapid testing since home parenteral and oral antimicrobial therapy have become commonplace. Nonetheless, one cannot argue that appropriate therapy is good medicine and that its administration should be sought by whatever means possible. Microbiologists, however, need to recognize that having the technical capability to produce rapid results is not sufficient in of itself to optimize therapy. References 1. Lampe, M.F., B.H. Minshew, and J.C. Sherris. 1979. In vitro response of Enterobacter to ampicillin. Antimicrob. Agents Chemother. 16:458--462. 2. Washington, J. A. 1I, C. C. Knapp, and C. C. Sanders. 1938. Accuracy ofmicrodilution and the AutoMicrobie System in detection of beta-lactam
resistance in gram-negative bacterial mutants with derepressed beta-lactamase. Rev. Infect. Dis. 10:.824--829. 3. York, M. K., G. F. Brooks, and E. H. Fiss. 1992. Evaluation of the autoSCAN-W/A rapid system for identification and susceptibility testing of gram-negative fermentative bacilli. J. Clin. Microbiol. 30:2903--2910. 4.
Tomasz, A., S. Naclunan, and H. l.#~f. 1991 Stable classes of phenotypic expression in methicillin-resistant clinical isolates of staphylococci. Antimicrob. Agents Chemother. 35:124--129.
5.
Mackenzie,A.M.R. et al. 1993. Accuracy of reporting of meliaicillin-resistant Staphylococcus aureus in provincial quality control program: A 9-year study. J. Clim Microbiol. 31:1275-1279.
6.
Skuinick, M. et al. 1992. Evaluation of commercial and standard methodology for determination of oxacillin susceptibility in Staphylococcus aureus. J. Clin. Microbiol. 30:1985--1988.
7.
Knapp, C. C., M. D. Ludwig, and J. A. Washington. 1992. Evaluation of screening disk diffusion method and Vitek GPS-SA card for detection of ox-
acillin-resistant staphylococci. Abstract C274. Abstracts. 92rid General Meeting of the American Society for Microbiology. 8.
Reeves, D. S., M. J. Bywater, and H. A. Holt. 1993. Activity of eefpirome and ten other antimicrobial agents against 2,858 clinical isolates collected from twenty centres. J. Antimicrob. Chemother. 31:345-362.
9,
Matsen, J. M. 1985. Means to facilitate physician acceptance and use of rapid test results. Diagn. Microbiol. Infect. Dis. 3:773S-778S.
10. Pestotnik, S. L. et aL 1990. Therapuetic antibiotic monitoring: Surveillance using computerized expert system. Am. J. Med. 88:43-48, 11. Vincent. P. et al. 1985. Int6r~t cliniques des resuitats rapides de bacteriologic au de l'infection nosocomiales: Comparaison avec les methods traditionelles. Presse M6d. 14:1697-1700. 12. Evangelista, A.T. 1989. The clinical impact of automated susceptibility rctx~ing using a computer interface. In D. Klegcr (cal.), Rapid Methods in Clinical Microbiology. New York, Plenum Press.
ii
Editorial
In Search of the Golden Kile Mueller.Hinton Agar
The Development of.Reference Standards for
George L. Evans, Ph.D. Research and Development Becton Dickinson Microbiology Systems Cockeysville, MD 21030
Mueller-Hinton agar since it was a Class II in vitro diagnostic product and a written NCCLS standard was available for the single disk diffusion (BauerKirby) procedure. A draft outfine of a written standard for Mueller-Hinton agar and a study proposal were prepared in 1979. The new sroup became a subcommittee of the NCCLS Area Committee on Microbiology and Harry Frankel and Helen Pollock were appointed co-
The National Committee for Clinical Laboratory Standards (NCCLS) appointed a Working Group on Microbiology Culture Media in 1977, with Harry Frankel as Chairholder. The original p r e l a t e was to prepare written standards for culture media, Which were be-
ing reviewed at that time by the F D A Advisory Subcommittee for Microbiology Products. It was decided that those culture media to be put in Class II,and therefore that require written standards, would be addressed f'wst. Performance standards were the original goal of the group, with emphasis on testing methodology. It was decided by late 1977 to concentrate on 156
0196-4399/93/$0.00 + 06.0o
chairholders. The concept of areagcnt
standard that would be stored at the Centers for Disease Control was decided upon in 1980. The project was announced at the ASM meeting that year and a budget of $63,000 was estimated. Initially, funding was expected from the FDA, but later, the media manufacturers were solicited for support. The pro© 1993 Elsevier Science Publishing Co., Inc.
tocol for the study was drafted in 1981. but due to delays in funding, the project did not begin until 1982. At the heart of the Mueller-Hinton project, as it was known in the early days, was the concept of two standards, a primary and a secondary reference standard. The secondary standard was for media manufacturers to use in conjunction with a protocol for the testing o f production lots of dehydrated Muelter-Hinton agar. The primary reference standard was to be used only for the selection of the secondary standard and for the selection of new reference media. In 1982, a multicenter study was conducted to select the first primary
and secondary standards. Seven manufacturers of dcl~ydrated media each provided one lot ofMueUer-Hinton agar for the project.The medium (25 kg) Clinical MJc~biology Newsletter 15:20.1993
Vol. 15, No. 20
Rapid Antimicrobial Susceptibility Testing: Technical and Clinical Considerations John A. Washington, M.D. Section of M &robiology The Cleveland Clinic Foundation Cleveland, OH 44195 Rapid antimicrobial susceptibility testing has engendered broad interest among clinical microbiologists, primarily based on the assumption that the earlier availability of susceptibility results would provide an opportunity for earlier adjustment of antimicrobial therapy than might the case with conventional testing. Whether this assumption is correct and whether existing commercially available technology provides rapid susceptibility test results accurately are the subjects of this article.
Commercial Systents---Early Beginnings and Problems The first commercially available rapid susceptibility testing device was the Autobac, inlroduced by Pfizer Diagnostics in the 1970s and now no longer available. This system was followed by the Abbott MS-2/Avantage system. Both systems were based on disk elution technology and provide results in 5 h; however, neither system has been critically evaluat_edfor its accuracy in detecting more recently described gramnegative resistance, such as that due to Bush group 1 beta-lactamase or the expanded spectrum plasmid-mediated TEM- ~ SHV-derived cephalosporinases. Moreover, neither has been critically evaluated with respect to accuracy in detecting expression class 1 or 2 heterogeneously methicillin-resistant staphylococci. One important and often
overlooked observation was, however, made by Lampe et al. (1), who found that the detection of resistance of Enterobacter cloacae to ampicillin was not made by the MS-2 within the fhst 6 h of incubation. In other words, although there was a small increase in optical density initially, logarithmic growth of E. cloacae in the presence of ampicillin was not observed for at least 6 h, reflecting then the growth of mutants that inactivated ampiciUin. Such mutants are now variously known to conlain inducible beta-lactamase,to be derepressed for Bush group 1 beta-lactamase, or to have constitutive Bush group 1 beta-lactamaseand are most frequenlly associated with Citrobacterfreundii, Enterobacter spp., Morganella morganff, Providencia stuartii, Serratia marcescens, and Pseudomonas aerugi-
Vitek System (2). The corresponding rates of very major errors for the derepressed mutants were both 2.7%. Although this percent of very major errors is higher than the 1% maximum that is commonly regarded as acceptable, it should be emphasized that the test population was highly selective. At any rate, it seems likely that the Vitek System detected beta-lactam resistance in the majority of instances with this particular group of problem organisms because of the somewhat longer incubation time (mean, 7.6 h) than those of the Autobac and the MS-2 (5 h). As already pointed out, even the detection of ampicillin resistance in E. cloacae requires at least 6 h. The AutoSCAN Walk-Away (W/A) system (Baxter Diagnostics, West Sac-
Ilosa.
The Vitek System (bioMerieux, Hazelwood, MO) provides susceptibility results for this group of gram-negative bacilli in an average of nearly 8 h. In a comparison of this system with agar dilution, broth macrodilution, and microdilution in tests of beta-lactams against isogenic pairs of wild type and stably derepressed mutants for Bush group 1 beta-lactamase and of stable mutants without wild type pairs of gram-negative bacilli, the overall very major error rate in 352 organism-betalactam combinations resulting in 840 tests was 3% between the reference agar or macrodilution and microdilution methods and 2% between the reference agar or macrodilution method and the
In This Issue i
Rapid Antimicrobial Susceptibility Testing: Technical and Clinical Considerations . . . . . . . . . . . . . 153 A review of the problems associated with rapid ASTs with specific reference to selected organism~antibiotic combinations and the utility of the data
In Search of the Golden Kilo-The Development of Reference Standards for Mueiler.Hinton Agar . . . . . . . . . . . . . . . . . . . . . . . .
156
The history behind the establishment of the Mueller-Hinton agar medium refer-
ence standard and the continuing efforts to maintain that standard i
CMNEEJ 15(20)153-160,1993
Elsevier
0196-4399/93/$0.00 + 06.00
ramento, CA) provides susceptibility test results in 5 to 7 h. This system has recently undergone critical evaluation with respect to its accuracy in detecting resistance among gram-negative fermentative bacilli (3). In this study, tests of common clinical isolates were supplemented with isolates likely to be highly antibiotic resistant. The standard microdilution test served as the reference method. The overall essential agreement between the W/A system and microdilution was 92.7%; however, less than 90% agreement occurred for piperacillin, cefazolin, cefotetan, and ceftriaxone, although the results for cefazolin were suppressed for certain organism groups. Without exclusion of certain organism-antimicrobial combinations that were suppressed by the W/A system, there were 906 determinations of resistance, of which 658 were classified as resistant by the W/A system and 133 as moderately susceptible, leaving 115 very major discrepancies (12.7% of the 906 resistant organismantimicrobial combinations or 2.8% of the total tests). After exclusion of organism-antimicrobial combinations suppressed by the W/A system, there were 708 resistant determinations, of which 517 were called resistant by the W/A system and 93 were called moderately susceptible, leaving 98 very major discrepancies (13.8% of the 708 resistant determinations or 2.6% of the total determinations). The W/A had difficulty detecting narrow-spectrum cephalosporin resistance in Klebsiella spp. and broad-spectrum cephalosporin resistance in C. freundii, Enterobacter spp., M. morganii, and Serratia spp. Only 7 3 % of the resistant organism -cephalosporin combinations in this latter group of the Enterobacteriaceae were called resistant by the W]A system. Whether the small difference in incubation time between the W/A and Vitek systems is sufficient to explain the difference in accuracy in detection of resis-
tance to beta-lactams by species typically associated with Bush group 1 betalactamase is unclear. These two studies emphasize the need for evaluation of susceptibility testing systems, rapid or otherwise, with not only common clinical isolates but also isolates characterized by resistance to various classes of antimicrobial agents.
Detection of Methicillin Resistance The accurate detection of methiciUin or oxacillin resistance in staphylococci, particularly those heterogeneously resistant in expression class 1 or 2 (4), remains a concern. A recently published report of a provincial quality control program in Canada found that laboratories experienced particular difficulty in detecting resistance in an expression class 1 strain (5), Skulnick et al. (6) evaluated the MicroScan Positive MIC panel and the Vitek GPS-SA card, along with the oxacillin agar screen, in comparison with the standard broth microdilution method and a DNA probe encoding the mec gene. They reported 14.2% very major errors (i.e., false susceptible results) with Vitek and 6.7% very major errors with MicroScan. It was not clear, however, what Vitek software program was used in their study. In a comparable preliminarily reported (7) and recently completed study m our laboratory of the Vitek GPS-SA card with the 6.1 software upgrade we found no very major errors when results were compared with the standard broth microdilution test, the oxacillin agar screen, population analysis profiles, and DNA probe studies for the mec gene. Our studies did include isolates belonging to expression classes 1 arid 2, as well as 3 and 4. Reanalysis of our data with the 7.1 software upgrade had no effect on either very major or major errors. Whether the difference between our results and those of Skulnick et al. (6) can be ascribed to their use of the older 4.2 GPS-SA program is unknown since they did not specify which soft-
ware program they used. It is also not clear what expression class the resistant isolates studied by them belonged to. To summarize the discussion up to this point, it would appear that false susceptibility is a problem when testing gram-negative bacilli associated with Bush group 1 beta-lactamase against expanded spectrum cephalosporins and penicillins with rapid methods providing results within 5 or 6 h of incubation. Whether detection of this type of resistance can be made more accurate by incorporating a low concentration of an inducing agent (e.g., imipenem, 0.1 mg/l, as suggested by Reeves et al. (8) for agar dilution) in the medium for a rapid test would improve accuracy is uncertain but may be worth ex ploring Clinical C o n s i d e r a t i o n s Technical considerations aside, the big question is whether faster is better. From a theoretical standpoint, the more rapid availability of susceptibility results should, in turn, lead to the more rapid administration of appropriate antimicrobial therapy, whether that is defined by effective therapy based on an organism's susceptibility in vitro and/or the administration of an active but less costly antibiotic regimen. In most instances patients with suspected infections are started on antimicrobial therapy empirically, and in most instances today the antimicrobial agent started is active against the organism involved. Thus, inappropriate therapy involving the administration of an antimicrobial agent to which the organism is resistant is not a frequent occurrence except perhaps in instances of suspected polymicrobial infections, in which case two or three antimicrobial agents may have already been started. The patient's clinical course on empirical therapy, rather than the arrival of the susceptibility report, may then be the ultimate determinant of whether to modify therapy.
III
N O T E : No rc~pousibility is assmned by the Publisher for any injury and/or damage to persons or property as a m a t e r of products liability, negligence or otherwise, or from any use or operation of any methods, products, instmutiotts or i d s ~ contained in the nmmrial ber¢in. No suggested test o r p ~ t ~ should be carried out unlms, in the r~d©r's judgment, its risk is justified. Because of rapid advances in medical sciences, w e m m e n d that the indqpeudmt vor~e~tinn of diagnoses and drug dosages should be made. Disc,tmsitus, views, and
reo~mmctadations as to medical procedures, choice of drags, and drug dosages are the responsibility of the authors.
Clinical Microbiology Newsletter (ISSN 0196-4399) is issued twice monthly in one indexed vohaneper year by Elsevier Science Publishing Co., Inc.. 655 Avenue of the Americas. N e w York. NY 10010. S/lbscription price per year. $145.00 including pottage and handling in the United States. Canada. and Mexico. Add $56.00 for postaSe in the rest of the world. Second*class postage paid at New York, NY and at additional mailing offices. Postmaster: Send address changes to Climcal Microbiology Newsletter. Elsevier Science Publishing Co.. Inc., 655 Avctlue of the Americas, New York. NY 10010.
154
0196-4399/93/$0.00 + 06.00
© 1993 Elsevier Science Publishing Co,. inc.
Clinical Microbiology Newsletter ! 5:20.199]
Another problem in defining appropriate therapy is the tendency of laboratory personnel to focus on the organism isolated from a particular site and the organism's antimicrobial susceptibility without knowledge or consideration of other potential sites of infection. In other words, while an Escherichia coli or Proteus mirabilis from the urine of a patient receiving imipenem is fully susceptible to ampicillin or trimethoprim/sulf:~methoxazole, the same patient may have what appears to be a nosocomial pneumonia with diverse gram-negative flora on Gram stain alad culture of respiratory secretions. Under these circumstances, the choice of therapy may have been a prudent one, despite the susceptibility of the E. coli or P. mirabilis to less expensive antimicrobial agents. In another example, a patient may be receiving vancomycin for an infection due to a methicillin-susceptible Staphylococcus spp.; however, this regimen may be completely appropriate ff the patient is on hemodialysis. The point of presenting these two examples is that a rapid result in these instances would probably have no major impact on the antibiotic regimens being administered.
Acceptance of Same-Day Reporting There are very few hard data documenting the clinical value of rapid testing, due at least in part to the fact that studies on the use of susceptibility resuits have repeatedly demonstrated physician ignorance of the availability or existence of susceptibility results in up to 50% of cases. In an early study of rapid testing, Matsen (9) found that only 51.7 % of physicians at the University of Utah were aware of the susceptibility results. Similar results were reported by Pestotnik et al. (10). These investigators found that intervention constituted the only effective way to modify antimicrobial therapy. Matsen (9) reported a study in which isolates from patients' specimens were tested by both a 3- to 5-h semiautomated disk elution method and the disk diffusion method. Results from the rapid method were reported the same afternoon, while those from the disk method were reported the next day. AfClinical Microbiology Newsletter 15:20,1993
ter receiving both results, physicians were questioned as to whether receiving the earlier result had influenced their choice of antimicrobial therapy. In 31.7% of cases physicians responded that receipt of the preliminary culture result with the rapid susceptibility report had led them to start therapy other than that which they might have started. In another 17% of cases physicians indicated that they "probably" would have made a different antibiotic selection on the basis of the earlier report. The only problems with this study were the biases introduced when the investigators called attention to susceptibility reports that had previously been ignored in about half the cases and then further called attention to the existence of two reports--one rapid and one conventional. By contrast, Vincent et at. (11) conducted a prospective, randomized study in a surgical unit in which half the patients' positive cultures were tested by a rapid disk elution method and the other half by a conventional method. They reported that therapy was modified in 14.5% of patients whose susceptibility results were reported rapidly in contrast with 8.8% of those whose results were reported conventionally (P< 0.001); however, there was no statistically significant difference between the two groups as regards length of stay. While both of these studies demonstrated that therapeutic changes were made, apparently in response to notation of a rapid susceptibility test result, it is unclear what reporting systems were in place in either institution at the time. The current availability of interfaces between automated instruments and laboratory information systems and the availability of these results to the medical staff via remote inquiry terminals has further facilitated the opportunity for rapid communication of results and potential modification of therapy. Moreover, interfaces between the microbiology laboratory and the pharmacy permit intervention by an infectious disease pharmacist, when appropriate (10). Evangelista (12) audited the impact of rapid computerized results generated by the Vitek system on antimicrobial therapy in patients with positive blood cultures. In contrast with the previously cited studies, this study did not corn-
Summary In the final analysis, the effectiveness of microbiology reports reflects a complex web of factors including proper specimen collection and transport, process control by the laboratory as regards the establishment of appropriate culturing and antimicrobial susceptibility testing protocols, reporting procedures, interaction with clinicians, and understanding of infectious diseases. Rapid turnaround time of tests may be helpful in certain critical cases
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pare the impact of rapid testing with that of conventional testing but instead assessed adequacy of therapy and the impact of computerized reporting on modification of therapy. The study confirmed the fact that the majority of patients (56%) were receiving adequate therapy based on in vitro susceptibility results and demonstrated that modifications in therapy were made in 22% of cases following the Gram stain report and in 28% of cases following the preliminary identification/susceptibility report. Of interest was the observation that therapy was modified only after a printed report was charted on 11% of patients. In examining respiratory, wound, and urine reports, Evangelista found that empirical therapy was adequate in only 25% of cases and that modifications in therapy were made in 8% of cases on the basis of Gram stain and in only I7% of cases following preliminary identification/susceptibility reports. In this portion of the study modifications were made in an additional 25% of cases following charting of the reports. Inappropriate or no change in therapy was noted in 21% of cases. It was also noted that fewer patients in this audit had been placed on empirical antimicrobial therapy than in the blood culture audit, which was ascribed to differences in disease severity between the two groups. One other explanation is that a positive wound or respiratory culture, which is often mixed, is less likely to equate with disease than a positive blood culture. At any rate, the results of this study send a mixed message but may provide greater support for the value of rapid computerized reporting of results, especially those of blood cultures.
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but may also be helpful only when linked with a prospective intervention program. While early studies in this area focused on the effects of rapid testing on length of stay, current DRG pressures to minimize length of stay may have superseded the impact of rapid testing since home parenteral and oral antimicrobial therapy have become commonplace. Nonetheless, one cannot argue that appropriate therapy is good medicine and that its administration should be sought by whatever means possible. Microbiologists, however, need to recognize that having the technical capability to produce rapid results is not sufficient in of itself to optimize therapy. References 1. Lampe, M.F., B.H. Minshew, and J.C. Sherris. 1979. In vitro response of Enterobacter to ampicillin. Antimicrob. Agents Chemother. 16:458--462. 2. Washington, J. A. 1I, C. C. Knapp, and C. C. Sanders. 1938. Accuracy ofmicrodilution and the AutoMicrobie System in detection of beta-lactam
resistance in gram-negative bacterial mutants with derepressed beta-lactamase. Rev. Infect. Dis. 10:.824--829. 3. York, M. K., G. F. Brooks, and E. H. Fiss. 1992. Evaluation of the autoSCAN-W/A rapid system for identification and susceptibility testing of gram-negative fermentative bacilli. J. Clin. Microbiol. 30:2903--2910. 4.
Tomasz, A., S. Naclunan, and H. l.#~f. 1991 Stable classes of phenotypic expression in methicillin-resistant clinical isolates of staphylococci. Antimicrob. Agents Chemother. 35:124--129.
5.
Mackenzie,A.M.R. et al. 1993. Accuracy of reporting of meliaicillin-resistant Staphylococcus aureus in provincial quality control program: A 9-year study. J. Clim Microbiol. 31:1275-1279.
6.
Skuinick, M. et al. 1992. Evaluation of commercial and standard methodology for determination of oxacillin susceptibility in Staphylococcus aureus. J. Clin. Microbiol. 30:1985--1988.
7.
Knapp, C. C., M. D. Ludwig, and J. A. Washington. 1992. Evaluation of screening disk diffusion method and Vitek GPS-SA card for detection of ox-
acillin-resistant staphylococci. Abstract C274. Abstracts. 92rid General Meeting of the American Society for Microbiology. 8.
Reeves, D. S., M. J. Bywater, and H. A. Holt. 1993. Activity of eefpirome and ten other antimicrobial agents against 2,858 clinical isolates collected from twenty centres. J. Antimicrob. Chemother. 31:345-362.
9,
Matsen, J. M. 1985. Means to facilitate physician acceptance and use of rapid test results. Diagn. Microbiol. Infect. Dis. 3:773S-778S.
10. Pestotnik, S. L. et aL 1990. Therapuetic antibiotic monitoring: Surveillance using computerized expert system. Am. J. Med. 88:43-48, 11. Vincent. P. et al. 1985. Int6r~t cliniques des resuitats rapides de bacteriologic au de l'infection nosocomiales: Comparaison avec les methods traditionelles. Presse M6d. 14:1697-1700. 12. Evangelista, A.T. 1989. The clinical impact of automated susceptibility rctx~ing using a computer interface. In D. Klegcr (cal.), Rapid Methods in Clinical Microbiology. New York, Plenum Press.
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Editorial
In Search of the Golden Kile Mueller.Hinton Agar
The Development of.Reference Standards for
George L. Evans, Ph.D. Research and Development Becton Dickinson Microbiology Systems Cockeysville, MD 21030
Mueller-Hinton agar since it was a Class II in vitro diagnostic product and a written NCCLS standard was available for the single disk diffusion (BauerKirby) procedure. A draft outfine of a written standard for Mueller-Hinton agar and a study proposal were prepared in 1979. The new sroup became a subcommittee of the NCCLS Area Committee on Microbiology and Harry Frankel and Helen Pollock were appointed co-
The National Committee for Clinical Laboratory Standards (NCCLS) appointed a Working Group on Microbiology Culture Media in 1977, with Harry Frankel as Chairholder. The original p r e l a t e was to prepare written standards for culture media, Which were be-
ing reviewed at that time by the F D A Advisory Subcommittee for Microbiology Products. It was decided that those culture media to be put in Class II,and therefore that require written standards, would be addressed f'wst. Performance standards were the original goal of the group, with emphasis on testing methodology. It was decided by late 1977 to concentrate on 156
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chairholders. The concept of areagcnt
standard that would be stored at the Centers for Disease Control was decided upon in 1980. The project was announced at the ASM meeting that year and a budget of $63,000 was estimated. Initially, funding was expected from the FDA, but later, the media manufacturers were solicited for support. The pro© 1993 Elsevier Science Publishing Co., Inc.
tocol for the study was drafted in 1981. but due to delays in funding, the project did not begin until 1982. At the heart of the Mueller-Hinton project, as it was known in the early days, was the concept of two standards, a primary and a secondary reference standard. The secondary standard was for media manufacturers to use in conjunction with a protocol for the testing o f production lots of dehydrated Muelter-Hinton agar. The primary reference standard was to be used only for the selection of the secondary standard and for the selection of new reference media. In 1982, a multicenter study was conducted to select the first primary
and secondary standards. Seven manufacturers of dcl~ydrated media each provided one lot ofMueUer-Hinton agar for the project.The medium (25 kg) Clinical MJc~biology Newsletter 15:20.1993