Comparison of the E-test with the NCCLS M38-P method for antifungal susceptibility testing of common and emerging pathogenic filamentous fungi

The National Committee for Clinical Laboratory Standards (NCCLS) M38-P method describes standard parameters for testing the fungistatic antifungal activities (MICs) of established agents against filamentous fungi (molds). The present study evaluated the in vitro fungistatic activities of itraconazole and amphotericin B by the E-test and the NCCLS M38-P microdilution method against 186 common and emerging pathogenic molds (123 isolates of Aspergillus spp. [five species], 16 isolates of Fusarium spp. [two species], 4 Paecilomyces lilacinus isolates, 5 Rhizopus arrhizus isolates, 15 Scedosporium spp., 18 dematiaceous fungi, and 5 Trichoderma longibrachiatum isolates). The agreement between the methods for amphotericin B MICs ranged from 70% for Fusarium solani to > or =90% for most of the other species after the first reading; agreement was dependent on both the incubation time and the species being evaluated. Major discrepancies between the amphotericin B MICs determined by the E-test and the NCCLS M38-P method were demonstrated for three of the five species of Aspergillus tested and the two species of Fusarium tested. This discrepancy was more marked after 48 h of incubation; the geometric mean MICs determined by the E-test increased between 24 and 48 h from between 1.39 and 3.3 microg/ml to between 5.2 and >8 microg/ml for Aspergillus flavus, Aspergillus fumigatus, and Aspergillus nidulans. The agreement between the itraconazole MICs determined by the E-test and the NCCLS M38-P method ranged from 83.3% for A. nidulans to > or =90% for all the other species tested; the overall agreement was higher (92.7%) than that for amphotericin B (87.9%). The agreement was less dependent on the incubation time. Clinical trials need to be conducted to establish the role of the results of either the E-test or the NCCLS M38-P method in vitro for molds with the two agents as predictors of clinical outcome.     

Comparison of broth macrodilution, broth microdilution, and E test antifungal susceptibility tests for fluconazole.

A comparison of the E test, the broth microdilution test, and the reference broth macrodilution susceptibility test of the National Committee for Clinical Laboratory Standards for fluconazole susceptibility testing was performed with 238 clinical isolates of Candida species and Torulopsis (Candida) glabrata. An 80% inhibition endpoint MIC was determined by the reference broth macrodilution method after 48 h of incubation. The MICs obtained by the two study methods were read after 24 and 48 h of incubation. Overall, excellent agreement within 2 doubling dilutions was obtained between the broth microdilution and the broth macrodilution methods for the combined results for all species at both 24 h (93%) and 48 h (94%). The correlation of 24-h MIC endpoints between the E test and the broth macrodilution methods was 37% for T. glabrata, 56% for Candida tropicalis, 93% for Candida albicans, and 90% for other Candida species. The percent agreement at 48 h ranged from 34% for T. glabrata to 97% for Candida species other than C. albicans and C. tropicalis. These initial results support the further evaluation of the E test as an alternative method for fluconazole susceptibility testing of Candida species.     

Comparison study of broth macrodilution and microdilution antifungal susceptibility tests.

An evaluation of broth dilution antifungal susceptibility tests was performed by determining both the micro- and macrodilution MICs of amphotericin B, flucytosine, fluconazole, ketoconazole, and cilofungin against 38 isolates of Candida albicans, Candida lusitaniae, Candida parapsilosis, Candida tropicalis, Cryptococcus neoformans, and Torulopsis glabrata. The following preliminary antifungal working group recommendations of the National Committee for Clinical Laboratory Standards for broth macrodilution tests with antifungal agents were used: inocula standardized to 1 x 10(4) to 5 x 10(4) CFU/ml with a spectrophotometer, RPMI 1640 medium buffered with morpholinopropanesulfonic acid (pH 7.0), incubation at 35 degrees C for 24 to 48 h, and an additive drug dilution procedure. Broth microdilution MICs were higher (two or more dilutions) than broth macrodilution MICs for all isolates tested with amphotericin B and for most isolates tested with ketoconazole, fluconazole, and cilofungin. MICs of flucytosine were the same by both techniques or lower by the broth microdilution test except in tests with C. neoformans. However, the only statistically significant differences between the two tests were observed with amphotericin B against all isolates (P = 0.01 to 0.07), ketoconazole against C. neoformans (P = 0.01 to 0.02), and cilofungin against C. albicans (P = 0.05 to 0.14). Tests performed with less dense inocula (1 x 10(3) to 5 x 10(3] produced similar results.     

Preliminary evaluation of a semisolid agar antifungal susceptibility test for yeasts and molds

This report presents a semisolid agar antifungal susceptibility (SAAS) method for the rapid susceptibility screening of yeasts and molds. The reproducibility and accuracy of the SAAS method were assessed by comparing the MICs of amphotericin B and fluconazole obtained for 10 candidate quality control (QC) American Type Culture Collection yeast strains in >/=15 replicates with those found by six independent laboratories using the National Committee for Clinical Laboratory Standards (NCCLS) M27-P broth macrodilution method (M. A. Pfaller et al., J. Clin. Microbiol. 33:1104-1107, 1995). Overall, 96% of MICs for both drugs fell within 1 log(2) dilution of the modal MIC for each strain. The MICs for amphotericin B showed 99% agreement with the NCCLS proposed QC ranges within 1 log(2) dilution. Likewise, the MICs for fluconazole at >/=75% growth reduction showed 99% agreement for seven strains. Three strains, Candida albicans ATCC 24333 and ATCC 76615 and Candida tropicalis ATCC 750, showed a less sharp fluconazole endpoint at >/=75% growth reduction, but at >50% growth reduction, the agreement was 98% within 1 log(2) dilution of the proposed range. The MIC agreement within the proposed range for the suggested QC strains Candida parapsilosis ATCC 22019 and Candida krusei ATCC 6258 was 100% for fluconazole and 100% within 1 log(2) dilution of the proposed range for amphotericin B. The SAAS method demonstrated the susceptibility or resistance of 25 clinical isolates of filamentous fungi such as Aspergillus fumigatus to amphotericin B, itraconazole, and fluconazole, usually within 48 h. Although the results are preliminary, this SAAS method is promising as a rapid and cost-effective screen and is worthy of concerted investigation.     

Collaborative investigation of broth microdilution and semisolid agar dilution for in vitro susceptibility testing of Candida albicans.

A study was performed in two laboratories to evaluate the effect of growth medium and test methodology on inter- and intralaboratory variations in the MICs of amphotericin B (AMB), flucytosine (5FC), fluconazole (FLU), itraconazole (ITRA), and the triazole Sch 39304 (SCH) against 14 isolates of Candida albicans. Testing was performed by broth microdilution and semisolid agar dilution with the following media, buffered to pH 7.0 with morpholinepropanesulfonic acid (MOPS): buffered yeast nitrogen base (BYNB), Eagle's minimal essential medium (EMEM), RPMI 1640 medium (RPMI), and synthetic amino acid medium for fungi (SAAMF). Inocula were standardized spectrophotometrically, and endpoints were defined by the complete absence of growth for AMB and by no more than 25% of the growth in the drug-free control for all other agents. Comparative analyses of median MICs, as determined by each test method, were made for all drug-medium combinations. Both methods yielded similar (+/- 1 twofold dilution) median MICs for AMB in EMEM and RPMI, 5FC in all media, and FLU in EMEM, RPMI, and SAAMF. In contrast, substantial between-method variations in median MICs were seen for AMB in BYNB and SAAMF, FLU In BYNB, and ITRA and SCH in all media. Interlaboratory concordance of median MICs was good for AMB, 5FC, and FLU but poor for ITRA and SCH in all media. Endpoint determinations were analyzed by use of kappa statistical analyses for evaluating the strength of observer agreement. Moderate to almost perfect interlaboratory agreement occurred with AMB and 5FC in all media and with FLU in EMEM, RPMI, and SAAMF, irrespective of the test method. Slight to almost perfect interlaboratory agreement occurred with ITRA and SCH in EMEM, RPMI, and SAAMF when tested by semisolid agar dilution but not broth microdilution. Kappa values assessing intralaboratory agreement between methods were high for 5FC in all media, for AMB in BYNB, ENEM, and RPMI, and for FLU in EMEM, RPMI, and SAAMF. One laboratory, but not the other, reported substantial to almost perfect agreement between methods for ITRA, and SCH in EMEM, RPMI, and SAAMF. Both laboratories reported poor agreement between methods for the azoles in BYNB. Discrepancies noted in azole-BYNB combinations were largely due to the greater inhibitory effect of these agents in BYNB than in other media. These results indicate that the semisolid agar dilution and broth microdilution methods with EMEM or RPMI yield equivalent and reproducible MICs for AMB, 5FC, and FLU but not ITRA and SCH.     

Evaluation of disk diffusion and Etest compared to broth microdilution for antifungal susceptibility testing of posaconazole against clinical isolates of filamentous fungi

We performed Etest, disk diffusion, and broth microdilution susceptibility testing of posaconazole against 146 clinical isolates of filamentous fungi. By using provisional breakpoints for comparison purposes only, categorical agreement between the results of the agar-based methods and those of broth microdilution were 96 to 98%, with no very major errors. These agar-based methods hold promise as simple and reliable methods for determining the posaconazole susceptibilities of filamentous fungi.     

Comparison of E-test and broth microdilution methods for antifungal drug susceptibility testing of molds

We compared the E test with a broth microdilution method, performed according to National Committee for Clinical Laboratory Standards document M27-A guidelines, for determining the in vitro susceptibilities of 90 isolates of pathogenic molds (10 Absidia corymbifera, 10 Aspergillus flavus, 10 Aspergillus fumigatus, 10 Aspergillus niger, 10 Aspergillus terreus, 10 Exophiala dermatitidis, 10 Fusarium solani, 10 Scedosporium apiospermum, 5 Scedosporium prolificans, and 5 Scopulariopsis brevicaulis). Overall, there was 71% agreement between the results of the two methods for amphotericin B (E-test MICs within +/-2 log2 dilutions of broth microdilution MICs) and 88% agreement with the results for itraconazole. The overall levels of agreement (within +/-2 log2 dilutions) were >/=80% for 5 of the 10 species tested against amphotericin B and 8 of the 10 species tested against itraconazole. The best agreement between the results was seen with A. fumigatus and A. terreus (100% of results for both agents within +/-2 log2 dilutions). The poorest agreement was seen with S. apiospermum, S. prolificans, and S. brevicaulis tested against amphotericin B (20% of results within +/-2 log2 dilutions). In every instance, this low level of agreement was due to isolates for which the broth microdilution MICs were low but for which the E-test MICs were much higher. The E test appears to be a suitable alternative procedure for testing the susceptibility of Aspergillus spp. and some other molds to amphotericin B or itraconazole.     

Two-site comparison of broth microdilution and semisolid agar dilution methods for susceptibility testing of Cryptococcus neoformans in three media.

This study evaluated the inter- and intralaboratory agreement between results of the semisolid agar dilution and broth microdilution methods of antifungal susceptibility testing of Cryptococcus neoformans. Three media were tested in two laboratories. The drugs tested were amphotericin B, flucytosine, itraconazole, fluconazole, and Schering 39304. Analysis by kappa statistics revealed good agreement between the laboratories for the two methods. The highest level of inter- and intralaboratory agreement was observed in RPMI 1640 with L-glutamine followed by Eagle's minimum essential medium and yeast nitrogen broth. The broth microdilution method appears more suitable than the semisolid agar dilution method for testing cryptococci because of its ease in performance, cost, and simplicity.     

Evaluation of broth microdilution antifungal susceptibility testing conditions for Trichophyton rubrum

Fifty clinical isolates of Trichophyton rubrum were selected to test with ketoconazole, fluconazole, itraconazole, griseofulvin, and terbinafine by following the National Committee for Clinical Laboratory Standards susceptibility testing guidelines for filamentous fungi (M38-A). In addition, other susceptibility testing conditions were evaluated: (i) three medium formulations including RPMI 1640 (standard medium), McVeigh & Morton (MVM), and Sabouraud dextrose broth (SDB); (ii) two incubation temperatures (28 and 35 degrees C); and (iii) three incubation periods (4, 7, and 10 days). The strains Candida parapsilosis (ATCC 22019), Candida krusei (ATCC 6258), T. rubrum (ATCC 40051), and Trichophyton mentagrophytes (ATCC 40004) were included as quality controls. All isolates produced clearly detectable growth only after 7 days of incubation. MICs were significantly independent of the incubation temperature (28 or 35 degrees C) (P < 0.05). Different incubation periods resulted in MICs which were consistently different for each medium when azoles and griseofulvin were tested (P < 0.05). MICs obtained from different media at the same incubation time for the same isolate were significantly different when azoles and griseofulvin were tested (P < 0.05). MICs were consistently higher (usually 1 to 2 dilutions) with RPMI than with MVM or SDB (P < 0.05). When terbinafine was tested, no parameter had any influence on MICs (P < 0.05). RPMI standard medium appears to be a suitable testing medium for determining the MICs for T. rubrum. MICs obtained at different incubation times need to be correlated with clinical outcome to demonstrate which time has better reliability.     

Comparison of a new colorimetric assay with the NCCLS broth microdilution method (M-27A) for antifungal drug MIC determination

We evaluated a new microtiter assay for antifungal susceptibility testing based on a colorimetric reaction to monitor fungal substrate utilization. This new method (rapid susceptibility assay [RSA]) provides quantitative endpoint readings in less than 8 h compared with visual determination of MIC by the National Committee for Clinical Laboratory Standards (NCCLS) broth microdilution method, which requires a minimum of 48 h of incubation. In this study, we tested clinical isolates from each of the following species: Candida albicans (20 isolates), C. glabrata (20 isolates), C. krusei (19 isolates), C. tropicalis (19 isolates), and C. parapsilosis (28 isolates). RSA and NCCLS broth dilution methods were used to determine the MICs of amphotericin B, fluconazole, itraconazole, and 5-flucytosine for all 106 isolates. RPMI 1640 medium buffered with morpholinopropanesulfonic acid was used for both methods; however, glucose and inoculum concentrations in the RSA were modified. RSA MICs were determined as the lowest drug concentration that prevented glucose consumption by the organism after 6 h of incubation. MICs obtained from the RSA were compared with those obtained from the NCCLS M-27A method read at 24 and 48 h. MIC pairs were considered in agreement when the difference between the pairs was within 2 twofold dilutions. For the 106 isolates tested, amphotericin B and 5-flucytosine demonstrated the highest agreement in MICs between the two methods (100 and 98%, respectively), whereas fluconazole and itraconazole produced less favorable MIC agreement (63.2 and 61.3%, respectively). The azole MIC differences between the two methods were significantly reduced when lower inocula were used with a prolonged incubation time. This preliminary comparison suggests that this rapid procedure may be a reliable tool for the in vitro determination of MICs of amphotericin B and 5-flucytosine and warrants further evaluation.     

Comparison of Neo-Sensitabs tablet diffusion assay with CLSI broth microdilution M38-A and disk diffusion methods for testing susceptibility of filamentous fungi with amphotericin B, caspofungin, itraconazole, posaconazole, and voriconazole

We compared the Neo-Sensitabs tablet assay to both reference M38-A broth microdilution and disk diffusion methods for testing the susceptibility of 183 filamentous isolates to amphotericin B, caspofungin, itraconazole, posaconazole, and voriconazole. Neo-Sensitabs and disk assay inhibition zone diameters, in millimeters, were obtained on nonsupplemented Mueller-Hinton agar at 16 to 48 h. The reproducibility of zone diameters (i.e., the percentage of replicate zone diameters that were within 2 standard deviations of the means), their correlation with either MICs or minimum effective concentrations (for caspofungin only), and the categorical agreement were similar between tablet and disk assays (93 to 100% [R, >0.70] and 79 to 96%, respectively) with four of the five agents. The exceptions were the results for posaconazole tablets (R, 0.686; disk, 0.757; 84% categorical agreement for tablet and 96% for disk). These data suggest the potential value of the Neo-Sensitabs assay for testing 5-microg caspofungin and 1-microg voriconazole posaconazole tablets against all mold isolates, 8-microg itraconazole and 5-microg tablets against all mold isolates except zygomycetes, and 10-microg amphotericin B tablets against zygomycete isolates only.     

Comparison of agar disk diffusion, microdilution broth, and agar dilution for testing antimicrobial susceptibility of coagulase-negative staphylococci.

A collection of 120 oxacillin-susceptible and 120 oxacillin-resistant coagulase-negative staphylococci (CNS) from six tertiary care hospital laboratories were tested by agar disk diffusion, three microdilution broth systems (Sensititre, Dynatech, and Alpkem), and the Vitek AutoMicrobic system for comparison with reference agar dilution results. The antimicrobial agents tested were oxacillin, cefazolin, cefotaxime, cefuroxime, cefamandole, fusidic acid, rifampin, and vancomycin. Incubation was at 30 or 35 degrees C for 24, 48, and 72 h. The broth media were supplemented with 2% NaCl for some antimicrobial agents, and the agar dilution method was used with and without the addition of 4% NaCl. The CNS were identified to species by the method of Kloos and Schleifer. The results showed a lack of concordance between two hospitals with respect to oxacillin susceptibility testing by agar dilution with no NaCl supplement. The reasons are not clear but may be related to variations in media. The 4% NaCl supplement or extended incubation to 48 h eliminated this difference. The cefazolin and cefotaxime susceptibility results in the agar disk diffusion test were unreliable if accepted at face value. Cefamandole testing correlated well with the reference method regardless of the method used, and salt supplementation is not recommended. Most of the oxacillin-resistant CNS were resistant to the other beta-lactam drugs except cefamandole. Of 22 CNS resistant to cefamandole, 21 were S. haemolyticus.     

Comparison of disk diffusion method and broth microdilution method for antifungal susceptibility testing of dermatophytes.

The use of the agar diffusion Neo-Sensitabs method to determine antifungal susceptibility of 59 isolates of dermatophytes, namely Epidermophyton floccosum, Microsporum canis, M. gypseum, Trichophyton mentagrophytes, T. rubrum and T. tonsurans to Clotrimazole (CLZ), Itraconazole (ITZ) and Terbinafine (TBF) is described. Results obtained are compared to the minimum inhibitory concentrations (MIC) determined by an adaptation of the NCCLS-M38-A procedure. Using the diffusion method, all strains showed a broad zone of inhibition at the first available reading time (3 or 7 days). Using the broth microdilution method, the geometric mean MIC (microg/ml) with regard to all isolates was < or = 0.03 for TBF, < or = 0.069 for CLZ and < or = 0.919 for ITZ. In both methods, TBF was the most active antifungal agent against all isolates tested. The two methods evaluated were able to detect the resistance of the quality control strains of Aspergillus fumigatus to ITZ. Even though a reference method for testing dermatophytes still has not been developed, our data suggest that the Neo-Sensitabs diffusion method could provide a simple procedure for the antifungal susceptibility testing of dermatophytes in the routine clinical laboratory.     

Quality control limits for broth microdilution susceptibility tests of ten antifungal agents

Broth microdilution susceptibility tests of Candida species have now been standardized by the National Committee for Clinical Laboratory Standards (NCCLS). An eight-laboratory collaborative study was carried out in order to document reproducibility of tests of Candida parapsilosis ATCC 22019 and Candida krusei ATCC 6258 by the NCCLS method. Replicate broth microdilution tests were used to define control limits for 24- and 48-h MICs of amphotericin B, flucytosine, fluconazole, voriconazole, ketoconazole, itraconazole, caspofungin (MK 0991), ravuconazole (BMS 207147), posaconazole (SCH 56592), and LY 303366.     

Effect of increasing inoculum sizes of pathogenic filamentous fungi on MICs of antifungal agents by broth microdilution method.

Inoculum size is a critical variable in development of methods for antifungal susceptibility testing for filamentous fungi. In order to investigate the influence of different inoculum sizes on MICs of amphotericin B, 5-fluorocytosine, itraconazole, and miconazole, 32 clinical isolates (8 Aspergillus fumigatus, 8 Aspergillus flavus, 5 Rhizopus arrhizus, 8 Pseudallescheria boydii, and 3 Fusarium solani isolates) were studied by the broth microdilution method. Four inoculum sizes were studied: 1 x 10(2) to 5 x 10(2), 1 x 10(3) to 5 x 10(3), 1 x 10(4) to 5 x 10(4), and 1 x 10(5) to 5 x 10(5) CFU/ml. The National Committee for Clinical Laboratory Standards reference method for antifungal susceptibility testing in yeasts was modified and applied to filamentous fungi. The inoculum was spectrophotometrically adjusted, and all tests were performed in buffered medium (RPMI 1640) at pH 7.0 with incubation at 35 degrees C for 72 h. MICs were read at 24, 48, and 72 h. Amphotericin B showed a minimum effect of inoculum size on MICs for all species with the exception of P. boydii (P < 0.05). A significant effect of inoculum size on MICs was observed with 5-fluorocytosine, for which there was an increase of more than 10-fold in MICs against all Aspergillus spp. between inoculum concentrations of 10(2) and 10(4) CFU/ml (P < 0.001). For itraconazole, the results showed a more species-dependent increase of MICs, most strikingly for R. arrhizus and P. boydii. Miconazole, which was tested only with P. boydii, did not demonstrate a significant effect of inoculum size on MICs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Collaborative comparison of broth macrodilution and microdilution antifungal susceptibility tests.

A collaborative comparison of macro- and microdilution antifungal susceptibility tests was performed in five laboratories. MICs of amphotericin B, fluconazole, flucytosine, and ketoconazole were determined in all five centers against 95 coded isolates of Candida spp., Cryptococcus neoformans, and Torulopsis glabrata. A standard protocol with the following National Committee for Clinical Laboratory Standards Subcommittee on Antifungal Susceptibility Testing recommendations was used: an inoculum standardized by spectrophotometer, buffered (RPMI 1640) medium (pH 7.0), incubation at 35 degrees C, and an additive drug dilution procedure. Two inoculum sizes were tested (1 x 10(4) to 5 x 10(3) to 2.5 x 10(3) CFU/ml) and three scoring criteria were evaluated for MIC endpoint determinations, which were scored as 0 (optically clear), < or = 1 (slightly hazy turbidity), and < or = 2 (prominent decrease in turbidity compared with that of the growth control). Overall intra- and interlaboratory reproducibility was optimal with the low-density inoculum, the second-day readings, and MICs scored as either 1 or 2. The microdilution MICs demonstrated interlaboratory agreement with most of the four drugs higher than or similar to that of the macrodilution MICs. In general, there was good interlaboratory agreement with amphotericin B, fluconazole, and flucytosine; ketoconazole gave more variable results.     

Antimicrobial susceptibility testing of Acinetobacter spp. by NCCLS broth microdilution and disk diffusion methods

Although both broth microdilution (BMD) and disk diffusion (DD) are listed by NCCLS as acceptable methods for testing Acinetobacter spp. for antimicrobial susceptibility, few studies have compared the results generated by the two methods. We tested 196 isolates of Acinetobacter spp. from nine U.S. hospitals and from the Centers for Disease Control culture collection by using BMD and DD and clinically appropriate antimicrobial agents. Categorical results for amikacin, ciprofloxacin, gatifloxacin, gentamicin, imipenem, levofloxacin, meropenem, tobramycin, and trimethoprim-sulfamethoxazole were comparable for the two methods: there was only one very major (VM) error, with tobramycin, and only one major (M) error, with meropenem, when DD results were compared with BMD results. However, VM errors were frequent with the beta-lactams and beta-lactam-beta-lactam inhibitor combinations, while M errors were often observed with tetracyclines. For BMD, tests frequently exhibited subtle growth patterns that were difficult to interpret, especially for beta-lactams. If subtle growth (i.e., granular, small button, or "starry" growth) was considered positive, error rates between BMD and DD were unacceptably high for ampicillin-sulbactam (VM error, 9.8%; minor [m] error, 16.1%), piperacillin (VM error, 5.7%; m error, 13.5%), piperacillin-tazobactam (VM error, 9.3%; m error, 12.9%), ceftazidime (VM error, 6.2%; m error, 11.4%), cefepime (VM error, 6.2%; m error, 13.0%), cefotaxime (m error, 21.2%), ceftriaxone (m error, 23.3%), tetracycline (M error, 11.4%; m error, 32.1%), and doxycycline (M error, 2.6%). When subtle growth patterns were ignored, the agreement still did not achieve acceptable levels. To determine if the problems with BMD testing occurred in other laboratories, we sent frozen BMD panels containing beta-lactam drugs and nine isolates to six labs with experience in performing BMD and DD. Among these laboratories, cefepime MICs ranged from < or =8 to > or =32 microg/ml for four of the nine strains, confirming the problem in interpreting BMD results. Discrepancies between the categorical interpretations of BMD and DD tests were noted primarily with cefepime and piperacillin, for which the BMD results were typically more resistant. Clinical laboratories should be aware of these discrepancies. At present, there are no data to indicate which method provides more clinically relevant information.     

Comparison of the agar dilution, tube dilution, and broth microdilution susceptibility tests for determination of teicoplanin MICs.

The purpose of this study was to evaluate the National Committee for Clinical Laboratory Standards agar dilution, tube dilution, and broth microdilution susceptibility tests for the measurement of teicoplanin MICs. The three standardized tests gave equivalent (within a twofold dilution) results with 98.8 to 99.0% of the 508 gram-positive clinical isolates tested, indicating that either method may be used for teicoplanin MIC determination.     

Comparison of agar dilution, tube dilution, and broth microdilution susceptibility tests for determination of ramoplanin MICs.

Standard broth microdilution (with and without bovine serum albumin [BSA] supplementation), tube dilution, and agar dilution susceptibility tests were compared for determining ramoplanin MICs. With a data base of 246 clinical isolates of gram-positive bacteria from 33 U.S. sites, it was shown that (i) agar and tube dilution susceptibility tests gave essentially the same results (93.9% of the test results were within 1 doubling dilution of equivalence), (ii) broth microdilution susceptibility tests gave results up to 5 doubling dilutions higher than agar or tube assays, and (iii) this data skewing could be reversed by BSA supplementation (final concentration, 0.02%) of the broth microdilution test medium.