Has antifungal susceptibility testing come of age?

Evaluation by data mining techniques of fluconazole breakpoints established by the Clinical and Laboratory Standards Institute (CLSI) and comparison with those of the European Committee on Antimicrobial Susceptibility Testing (EUCAST)

The EUCAST and the CLSI have established different breakpoints for fluconazole and Candida spp. However, the reference methodologies employed to obtain the MICs provide similar results. The aim of this work was to apply supervised classification algorithms to analyze the clinical data used by the CLSI to establish fluconazole breakpoints for Candida infections and to compare these data with the results obtained with the data set used to set up EUCAST fluconazole breakpoints, where the MIC for detecting failures was >4 mg/liter, with a sensitivity of 87%, a false-positive rate of 8%, and an area under the receiver operating characteristic (ROC) curve of 0.89. Five supervised classifiers (J48 and CART decision trees, the OneR decision rule, the naïve Bayes classifier, and simple logistic regression) were used to analyze the original cohort of patients (Rex's data set), which was used to establish CLSI breakpoints, and a later cohort of candidemia (Clancy's data set), with which CLSI breakpoints were validated. The target variable was the outcome of the infections, and the predictor variable was the MIC or dose/MIC ratio. For Rex's data set, the MIC detecting failures was >8 mg/liter, and for Clancy's data set, the MIC detecting failures was >4 mg/liter, in close agreement with the EUCAST breakpoint (MIC > 4 mg/liter). The sensitivities, false-positive rates, and areas under the ROC curve obtained by means of CART, the algorithm with the best statistical results, were 52%, 18%, and 0.7, respectively, for Rex's data set and 65%, 6%, and 0.72, respectively, for Clancy's data set. In addition, the correlation between outcome and dose/MIC ratio was analyzed for Clancy's data set, where a dose/MIC ratio of >75 was associated with successes, with a sensitivity of 93%, a false-positive rate of 29%, and an area under the ROC curve of 0.83. This dose/MIC ratio of >75 was identical to that found for the cohorts used by EUCAST to establish their breakpoints (a dose/MIC ratio of >75, with a sensitivity of 91%, a false-positive rate of 10%, and an area under the ROC curve of 0.90).

Epidemiology of invasive mycoses in North America

The incidence of invasive mycoses is increasing, especially among patients who are immunocompromised or hospitalized with serious underlying diseases. Such infections may be broken into two broad categories: opportunistic and endemic. The most important agents of the opportunistic mycoses are Candida spp., Cryptococcus neoformans, Pneumocystis jirovecii, and Aspergillus spp. (although the list of potential pathogens is ever expanding); while the most commonly encountered endemic mycoses are due to Histoplasma capsulatum, Coccidioides immitis/posadasii, and Blastomyces dermatitidis. This review discusses the epidemiologic profiles of these invasive mycoses in North America, as well as risk factors for infection, and the pathogens' antifungal susceptibility.

In vitro susceptibility testing in fungi: a global perspective on a variety of methods

Candida and Aspergillus species are the most common causes of invasive fungal infections in immunocompromised patients. The introduction of new antifungal agents and recent reports of resistance emerging during treatment have highlighted the need for in vitro susceptibility testing. For some drugs, there is a supporting in vitro-in vivo correlation available from studies of clinical efficacy. Both intrinsic and emergent antifungal drug resistance are encountered. Various testing procedures have been proposed, including macrodilution and microdilution, agar diffusion, disk diffusion and Etest. Early recognition of infections caused by pathogens that are resistant to one or more antifungals is highly warranted to optimise treatment and patient outcome.

Correlation between Etest, disk diffusion, and microdilution methods for antifungal susceptibility testing of Candida species from infection and colonization

The correlation between the microdilution (MD), Etest (ET), and disk diffusion (DD) methods was determined for amphotericin B, itraconazole and fluconazole. The minimal inhibitory concentration (MIC) of those antifungal agents was established for a total of 70 Candida spp. isolates from colonization and infection. The species distribution was: Candida albicans (n=27), C. tropicalis (n=17), C. glabrata (n=16), C. parapsilosis (n=8), and C. lusitaniae (n=2). Non-Candida albicans Candida species showed higher MICs for the three antifungal agents when compared with C. albicans isolates. The overall concordance (based on the MIC value obtained within two dilutions) between the ET and the MD method was 83% for amphotericin B, 63% for itraconazole, and 64% for fluconazole. Considering the breakpoint, the agreement between the DD and MD methods was 71% for itraconazole and 67% for fluconazole. The DD zone diameters are highly reproducible and correlate well with the MD method, making agar-based methods a viable alternative to MD for susceptibility testing. However, data on agar-based tests for itraconazole and amphotericin B are yet scarce. Thus, further research must still be carried out to ensure the standardization to other antifungal agents.

Spectrum of fungal keratitis in central China

PURPOSE

To investigate the epidemiology, risk factors, fungal spectrum and to test antifungal drug susceptibility of these isolates at a tertiary eye care referral centre in central China.

METHODS

The medical and microbiology records of 2064 culture-proven cases (2064 eyes) of fungal keratitis diagnosed at Henan Eye Institute between January 2000 to March 2009 was retrospectively reviewed. The fungal isolates were identified and a subgroup of 103 isolates were subjected to drug susceptibility tests for amphotericin B, fluconazole and ketoconazole by broth microdilution method.

RESULTS

A total of 2064 cases of confirmed fungal keratitis were identified. The predominant fungal species isolated was Fusarium spp. followed by Aspergillus spp. Alternaria spp. were another most common fungi in central China. Fungal keratitis was more common among men. A large proportion of the patients were middle-aged adults, and most were farmers. Ocular trauma was a highly significant risk factor and vegetative injuries were identified as a significant cause for fungal keratitis. Greatest number of cases of fungal keratitis was higher between September and December. Fusarium was mostly sensitive to amphotericin B, next to ketoconazole. Aspergillus was sensitive to amphotericin B and ketoconazole. Relatively, both Fusarium and Aspergillus were insensitive to fluconazole.

CONCLUSION

Fusarium and Aspergillus are always the most isolated pathogens of fungal keratitis in central China, followed by Alternaria. Document available on the epidemiological features of a large series would greatly help ophthalmologists at primary and second health care centres in the management of this disease.

Variation in susceptibility of bloodstream isolates of Candida glabrata to fluconazole according to patient age and geographic location in the United States in 2001 to 2007

We examined the susceptibilities to fluconazole of 642 bloodstream infection (BSI) isolates of Candida glabrata and grouped the isolates by patient age and geographic location within the United States. Susceptibility of C. glabrata to fluconazole was lowest in the northeast region (46%) and was highest in the west (76%). The frequencies of isolation and of fluconazole resistance among C. glabrata BSI isolates were higher in the present study (years 2001 to 2007) than in a previous study conducted from 1992 to 2001. Whereas the frequency of C. glabrata increased with patient age, the rate of fluconazole resistance declined. The oldest age group (> or = 80 years) had the highest proportion of BSI isolates that were C. glabrata (32%) and the lowest rate of fluconazole resistance (5%).

In vitro activity of seven systemically active antifungal agents against a large global collection of rare Candida species as determined by CLSI broth microdilution methods

Five Candida species (C. albicans, C. glabrata, C. tropicalis, C. parapsilosis, and C. krusei) account for over 95% of invasive candidiasis cases. Some less common Candida species have emerged as causes of nosocomial candidiasis, but there is little information about their in vitro susceptibilities to antifungals. We determined the in vitro activities of fluconazole, voriconazole, posaconazole, amphotericin B, anidulafungin, caspofungin, and micafungin against invasive, unique patient isolates of Candida collected from 100 centers worldwide between January 2001 and December 2007. Antifungal susceptibility testing was performed by the CLSI M27-A3 method. CLSI breakpoints for susceptibility were used for fluconazole, voriconazole, anidulafungin, caspofungin, and micafungin, while a provisional susceptibility breakpoint of < or = 1 microg/ml was used for amphotericin and posaconazole. Of 14,007 Candida isolates tested, 658 (4.7%) were among the less common species. Against all 658 isolates combined, the activity of each agent, expressed as the MIC50/MIC90 ratio (and the percentage of susceptible isolates) was as follows: fluconazole, 1/4 (94.8%); voriconazole, 0.03/0.12 (98.6%); posaconazole, 0.12/0.5 (95.9%); amphotericin, 0.5/2 (88.3%); anidulafungin, 0.5/2 (97.4%); caspofungin, 0.12/0.5 (98.0%); and micafungin, 0.25/1 (99.2%). Among the isolates not susceptible to one or more of the echinocandins, most (68%) were C. guilliermondii. All isolates of the less common species within the C. parapsilosis complex (C. orthopsilosis and C. metapsilosis) were susceptible to voriconazole, posaconazole, anidulafungin, caspofungin, and micafungin. Over 95% of clinical isolates of the rare Candida species were susceptible to the available antifungals. However, activity did vary by drug-species combination, with some species (e.g., C. rugosa and C. guilliermondii) demonstrating reduced susceptibilities to commonly used agents such as fluconazole and echinocandins.

Antifungal drug resistance mechanisms

Antifungal resistance is a prominent feature in the management of invasive mycoses, with important implications for morbidity and mortality. Microbiological resistance, the most common cause of refractory infection, is associated with a fungal pathogen for which an antifungal MIC is higher than average or within the range designated as the resistant breakpoint. Four major mechanisms of resistance to azoles have been described in Candida spp.: decreased intracellular drug concentration by activation of efflux systems or reduction of drug penetration, modification of the target site, upregulation of the target enzyme and development of bypass pathways. Conversely, echinocandins are a poor substrate for multidrug efflux transporters, and their mechanisms of resistance are associated with point mutations and/or overexpression of FKS1 and FKS2 genes. Acquired resistance to flucytosine results from defects in its metabolism through enzymatic mutations, whereas resistance to amphotericin B may be mediated by increased catalase activity or defects in ergosterol biosynthesis.

Fungal Spectrum Identified by a New Slide Culture andIn VitroDrug Susceptibility Using Etest in Fungal Keratitis

PURPOSE

To investigate the spectrum of fungal species causing keratitis and to test antifungal drug susceptibility to each isolate using Etest.

METHODS

Microbial cultures were performed for patients who were clinically diagnosed with fungal keratitis between September 2002 and July 2004. Modified slide culture was established to identify the fungal species of the isolates. Etest (AB BIODISK, Solna, Sweden) was applied to determine the antifungal agent susceptibility of each isolate to itraconazole, fluconazole, and amphotericin B in vitro, respectively.

RESULTS

Among 73 eyes of 73 patients with clinical diagnosis of fungal keratitis, 61 strains of fungi were isolated from 61 eyes. The rate of positive culture was 81.3% of all cases. The spectrum of fungal species involved: 58 (95.1%) isolates of filamentous fungi, including the two most common genera-Fusarium (n = 33, 54.1%) and Aspergillus (n = 9, 14.8%),-followed by 16 (26.2%) isolates of other genera of filamentous fungi such as Alternaria (n = 3, 4.9%), Trichophyton (n = 3, 4.9%), Curvularia (n = 2, 3.3%), Chrysosporium (n = 2, 3.3%), Acremonium (n = 2, 3.3%), and Scedosporium (n = 1, 1.6%), 1 (1.6%) yeast of Candida, as well as two (3.3%) dimorphic fungi of Blastomyces and Sporothrix isolate each. Three filamentous fungi of the isolates failed to be identified according to the information provided by slide culture. The results of Etest showed that 20 (60.6%) isolates of Fusarium were susceptible to amphotericin B, whereas all of them were resistant to itraconazole and fluconazole. All nine (100%) isolates of Aspergillus were sensitive to itraconazole, whereas four (44.4%) of them were sensitive to amphotericin B, and only two (22.2%) of them were sensitive to fluconazole. Seventeen (89.5%), 13 (68.4%), and 10 (52.6%) isolates of the remaining 19 organisms were sensitive to amphotericin B, itraconazole, and fluconazole, respectively.

CONCLUSIONS

Fusarium and Aspergillus are the most frequent pathogenic organisms in causing fungal keratitis, whereas other species of fungi can also cause corneal infection. In vitro Etest for assessing antifungal drug susceptibility is a simple and practical method and may provide referential information for clinical consideration of choosing antifungal agents to treat fungal keratitis.

Data mining validation of fluconazole breakpoints established by the European Committee on Antimicrobial Susceptibility Testing

European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints classify Candida strains with a fluconazole MIC < or = 2 mg/liter as susceptible, those with a fluconazole MIC of 4 mg/liter as representing intermediate susceptibility, and those with a fluconazole MIC > 4 mg/liter as resistant. Machine learning models are supported by complex statistical analyses assessing whether the results have statistical relevance. The aim of this work was to use supervised classification algorithms to analyze the clinical data used to produce EUCAST fluconazole breakpoints. Five supervised classifiers (J48, Correlation and Regression Trees [CART], OneR, Naïve Bayes, and Simple Logistic) were used to analyze two cohorts of patients with oropharyngeal candidosis and candidemia. The target variable was the outcome of the infections, and the predictor variables consisted of values for the MIC or the proportion between the dose administered and the MIC of the isolate (dose/MIC). Statistical power was assessed by determining values for sensitivity and specificity, the false-positive rate, the area under the receiver operating characteristic (ROC) curve, and the Matthews correlation coefficient (MCC). CART obtained the best statistical power for a MIC > 4 mg/liter for detecting failures (sensitivity, 87%; false-positive rate, 8%; area under the ROC curve, 0.89; MCC index, 0.80). For dose/MIC determinations, the target was >75, with a sensitivity of 91%, a false-positive rate of 10%, an area under the ROC curve of 0.90, and an MCC index of 0.80. Other classifiers gave similar breakpoints with lower statistical power. EUCAST fluconazole breakpoints have been validated by means of machine learning methods. These computer tools must be incorporated in the process for developing breakpoints to avoid researcher bias, thus enhancing the statistical power of the model.

Recent experience with fungaemia: change in species distribution and azole resistance

Owing to a rise in frequency and change in pattern of cases with fungaemia at our tertiary care centre, we conducted a prospective study for 4 months to understand the epidemiology and outcome of this infection. Detailed case histories including management protocol and outcomes were noted. Records of 140 cases with fungaemia (27.1% adult and 72.9% paediatric patients) were analysed. Although C. tropicalis was the most common yeast isolated, significantly higher isolation of C. guilliermondii (30.4%) and C. pelliculosa (17.6%) was noted in paediatric patients; and C. albicans (26.3%) and C. glabrata (10.5%) in adult patients. Rare species isolated included C. ustus (0.7%) and Trichosporon asahii (2.1%). Mortality was high (56.9% and 47.4%, respectively), in both groups of patients. Resistance to azoles (fluconazole, itraconazole, voriconazole) emerged in C. albicans (12.5-18.8%) and C. tropicalis (10.2-13.6%). Antifungal susceptibility testing report modified the therapy from fluconazole to amphotericin B in 8 patients; 5 survived. In conclusion, the study highlighted the rise of non-albicans Candida species in our hospital with differential distribution in paediatric and adult wards and emergence of azole resistance.

Activity of antibiotics against Fusarium and Aspergillus

BACKGROUND/AIMS

To study the susceptibility of Fusarium and Aspergillus isolated from keratitis to amoxicillin, cefazolin, chloramphenicol, moxifloxacin, tobramycin and benzalkonium chloride (BAK).

METHODS

10 isolates of Fusarium and 10 isolates of Aspergillus from cases of fungal keratitis at Aravind Eye Hospital in South India were tested using microbroth dilution for susceptibility to amoxicillin, cefazolin, chloramphenicol, moxifloxacin, tobramycin and BAK. The minimum inhibitory concentration (MIC) median and 90th percentile were determined.

RESULTS

BAK had the lowest MIC for both Fusarium and Aspergillus. Chloramphenicol had activity against both Fusarium and Aspergillus, while moxifloxacin and tobramycin had activity against Fusarium but not Aspergillus.

CONCLUSIONS

The susceptibility of Fusarium to tobramycin, moxifloxacin, chloramphenicol and BAK and of Aspergillus to chloramphenicol and BAK may explain anecdotal reports of fungal ulcers that improved with antibiotic treatment alone. While some of the MICs of antibiotics and BAK are lower than the typically prescribed concentrations, they are not in the range of antifungal agents such as voriconazole, natamycin and amphotericin B. Antibiotics may, however, have a modest effect on Fusarium and Aspergillus when used as initial treatment prior to identification of the pathological organism.

Rationale for reading fluconazole MICs at 24 hours rather than 48 hours when testing Candida spp. by the CLSI M27-A2 standard method

We investigated if CLSI M27-A2 Candida species breakpoints for fluconazole MIC are valid when read at 24 h. Analysis of a data set showed good correlation between 48- and 24-h MICs, as well as similar outcomes and pharmacodynamic efficacy parameters, except for isolates in the susceptible dose-dependent category, such as Candida glabrata.

Validation of 24-hour fluconazole MIC readings versus the CLSI 48-hour broth microdilution reference method: results from a global Candida antifungal surveillance program

We performed 24- and 48-h MIC determinations and disk diffusion testing of fluconazole against more than 11,000 clinical isolates of Candida species. By using the reference MIC breakpoints, the categorical agreement between the 24-h and reference 48-h broth microdilution results ranged from 93.8% (all Candida species) to 94.9% (all Candida species minus Candida krusei), with only 0.1% very major errors (VME). The essential agreement (within 2 log(2) dilutions) between the 24-h and 48-h results was 99.6%. The categorical agreement between the 24-h disk diffusion results and the 24-h MIC results, using the previously established breakpoints, was 94.4%, with 0.1% VME. Both the MIC and the disk diffusion results obtained for fluconazole after only 24 h of incubation may be used to determine the susceptibilities of Candida spp. to this widely used antifungal agent.

Antifungal susceptibility for common pathogens of fungal keratitis in Shandong Province, China

PURPOSE

To analyze common pathogens of fungal keratitis and results of antifungal drug sensitivity test in Shandong Province, China and provide guidance for appropriate choice of antifungal drugs in clinic.

DESIGN

Retrospective, noncomparative study.

METHODS

The pathogens isolated from 674 fungal keratitis patients between January 1, 2001 and December 31, 2006 were cultured and identified in Shandong Eye Institute, of which some common strains were tested for sensitivity to antifungal drugs.

RESULTS

Fungi were positively cultured in 549 (81.5%) patients, in which the dominating pathogen was genus Fusarium (77.6%), with F. solani (37.3%), F. moniliforme (30.0%), and F. oxysporum (27.9%) being common species; Fusarium was mostly sensitive to natamycin, next to amphotericin B, and then to terbinafin. The second common pathogen was genus Aspergillus (10.8%), in which the main species were A. flavus (49.2%) and A. fumigatus (35.6%); Aspergillus was mostly sensitive to natamycin, next to terbinafin, and then to amphotericin B. Relatively, both Fusarium and Aspergillus were insensitive to ketoconazole, miconazole, itraconazole, fluconazole, and fluorocytosine.

CONCLUSIONS

Fusarium is the most common pathogen of fungal keratitis, followed by Aspergillus, in Shandong Province, China. Natamycin is still the first choice in the treatment of hyphomycetic keratitis. Fusarium and Aspergillus are also sensitive to amphotericin B and terbinafin. Early diagnosis and treatments are vital to good prognosis in the treatment of fungal keratitis.

Correlation of MIC with outcome for Candida species tested against caspofungin, anidulafungin, and micafungin: analysis and proposal for interpretive MIC breakpoints

The CLSI Antifungal Subcommittee followed the M23-A2 "blueprint" to develop interpretive MIC breakpoints for anidulafungin, caspofungin, and micafungin against Candida species. MICs of < or = 2 microg/ml for all three echinocandins encompass 98.8 to 100% of all clinical isolates of Candida spp. without bisecting any species group and represent a concentration that is easily maintained throughout the dosing period. Data from phase III clinical trials demonstrate that the standard dosing regimens for each of these agents may be used to treat infections due to Candida spp. for which MICs are as high as 2 microg/ml. An MIC predictive of resistance to these agents cannot be defined based on the data from clinical trials due to the paucity of isolates for which MICs exceed 2 microg/ml. The clinical data set included only three isolates from patients treated with an echinocandin (caspofungin) for which the MICs were > 2 microg/ml (two C. parapsilosis isolates at 4 microg/ml and one C. rugosa isolate at 8 microg/ml). Based on these data, the CLSI subcommittee has decided to recommend a "susceptible only" breakpoint MIC of < or = 2 microg/ml due to the lack of echinocandin resistance in the population of Candida isolates thus far. Isolates for which MICs exceed 2 microg/ml should be designated "nonsusceptible" (NS). For strains yielding results suggestive of an NS category, the organism identification and antimicrobial-susceptibility test results should be confirmed. Subsequently, the isolates should be submitted to a reference laboratory that will confirm the results by using a CLSI reference dilution method.

Direct fluconazole susceptibility testing of positive Candida blood cultures by flow cytometry

The standard methods for yeast susceptibility testing require 24-48 h of incubation. As there has been an increase in incidence of non-albicans Candida species, the clinician is very often wary of initiating therapy with fluconazole until a final susceptibility report is generated, especially when treating very sick patients. A rapid reliable susceptibility testing method would enable the clinician to prescribe fluconazole, thus avoiding more toxic or expensive therapy. To determine the feasibility of direct susceptibility testing of Candida species to fluconazole by a rapid flow cytometric method, 50 Candida strains were seeded into blood culture bottles and were tested for susceptibility to fluconazole directly from the bottles after their being flagged as positive by the blood culture instrument. Minimal inhibitory concentration (MIC) determined by fluorescent flow cytometry (FACS) showed excellent agreement to that determined by macrodilution. Following the seeding experiments, 30 true patient specimens were tested directly from positive blood cultures, and MIC determined by both methods showed excellent agreement. Antifungal susceptibility testing by FACS directly from positive blood culture bottles is a reliable, rapid method for susceptibility testing of Candida to fluconazole. The method allows same-day results, does not require subculture to agar media, and can greatly assist in the selection of appropriate antifungal therapy.

Comparison of disk diffusion, E-test and broth microdilution test in determination of susceptibility of Aspergillus species to amphotericin B, itraconazole and voriconazole

In response to the recent increase in Aspergillus infections, new antifungal agents have become available accompanied by studies on antifungal susceptibility tests for epidemiological follow-up. The aim of this study was to compare the efficacy of Clinical Laboratory Standards Institute (CLSI) M 38-A broth microdilution test with the disk diffusion and E-test in determining the susceptibility of Aspergillus spp. to amphotericin B, itraconazole and voriconazole. The study was carried out on 18 A. fumigatus, 7 A. flavus, 5 A. niger and 2 A. versicolor strains isolated from clinical samples. The microdilution method was performed by following the instructions of CLSI M 38-A. The E-test and disk diffusion tests were performed according to the instructions of their manufacturers. The percent agreement between the E-test and CLSI M38-A broth microdilution test at 24 (48) h within +/- 2 dilutions was, respectively, 81% (69%) for amphotericin B, 75% (72%) for itraconazole and 85% (81%) for voriconazole. The disk diffusion test showed good correlation with the E-test but poor correlation with the broth microdilution test for the three antifungal agents we tested. In conclusion, E-test and disk diffusion test have their advantages such as ease of application and interpretation, but their correlation with the broth microdilution should be improved.

The evolution of fungal drug resistance: modulating the trajectory from genotype to phenotype

The emergence of drug resistance in pathogenic microorganisms provides an excellent example of microbial evolution that has had profound consequences for human health. The widespread use of antimicrobial agents in medicine and agriculture exerts strong selection for the evolution of drug resistance. Selection acts on the phenotypic consequences of resistance mutations, which are influenced by the genetic variation in particular genomes. Recent studies have revealed a mechanism by which the molecular chaperone heat shock protein 90 (Hsp90) can alter the relationship between genotype and phenotype in an environmentally contingent manner, thereby 'sculpting' the course of evolution. Harnessing Hsp90 holds great promise for treating life-threatening infectious diseases.

Antimicrobial resistance: resistance to antifungal agents: mechanisms and clinical impact

Despite advances in preventive, diagnostic, and therapeutic interventions, invasive fungal infections cause significant morbidity and mortality in immunocompromised patients. The burden of antifungal resistance in such high-risk patients is becoming a major concern. A better understanding of the mechanisms and clinical impact of antifungal resistance is essential to the prompt and efficient treatment of patients with invasive mycoses and to improving the outcome of such infections. Although recent guidelines have attempted to standardize antifungal susceptibility testing, limitations still exist as a result of the incomplete correlation between in vitro susceptibility and clinical response to treatment. Four major mechanisms of resistance to azoles have been identified, all of which rely on altered gene expression. Mechanisms responsible for polyene and echinocandin resistance are less well understood. In addition to discussing the molecular mechanisms of antifungal resistance, this article elaborates on the concept of clinical resistance, which is critical to the understanding of treatment failure in patients with invasive fungal infections.

Current status of antifungal susceptibility testing methods

Antifungal susceptibility testing is a very dynamic field of medical mycology. Standardization of in vitro susceptibility tests by the Clinical and Laboratory Standards Institute (CLSI) and the European Committee for Antimicrobial Susceptibility Testing (EUCAST), and current availability of reference methods constituted the major remarkable steps in the field. Based on the established minimum inhibitory concentration (MIC) breakpoints, it is now possible to determine the susceptibilities of Candida strains to fluconazole, itraconazole, voriconazole, and flucytosine. Moreover, utility of fluconazole antifungal susceptibility tests as an adjunct in optimizing treatment of candidiasis has now been validated. While the MIC breakpoints and clinical significance of susceptibility testing for the remaining fungi and antifungal drugs remain yet unclear, modifications of the available methods as well as other methodologies are being intensively studied to overcome the present drawbacks and limitations. Among the other methods under investigation are Etest, colorimetric microdilution, agar dilution, determination of fungicidal activity, flow cytometry, and ergosterol quantitation. Etest offers the advantage of practical application and favorable agreement rates with the reference methods that are frequently above acceptable limits. However, MIC breakpoints for Etest remain to be evaluated and established. Development of commercially available, standardized colorimetric panels that are based on CLSI method parameters has added more to the antifungal susceptibility testing armamentarium. Flow cytometry, on the other hand, appears to offer rapid susceptibility testing but requires specified equipment and further evaluation for reproducibility and standardization. Ergosterol quantitation is another novel approach, which appears potentially beneficial particularly in discrimination of azole-resistant isolates from heavy trailers. The method is yet investigational and requires to be further studied. Developments in methodology and applications of antifungal susceptibility testing will hopefully provide enhanced utility in clinical guidance of antifungal therapy. However, and particularly in immunosuppressed host, in vitro susceptibility is and will remain only one of several factors that influence clinical outcome.

Therapeutic potential of antifungal plant and insect defensins

To defend themselves against invading fungal pathogens, plants and insects largely depend on the production of a wide array of antifungal molecules, including antimicrobial peptides such as defensins. Interestingly, plant and insect defensins display antimicrobial activity not only against plant and insect pathogens but also against human fungal pathogens, including Candida spp. and Aspergillus spp. This review focuses on these defensins as novel leads for antifungal therapeutics. Their mode of action, involving interaction with fungus-specific sphingolipids, and heterologous expression, required for cost-effective production, are major assets for development of plant and insect defensins as antifungal leads. Studies evaluating their in vivo antifungal efficacy demonstrate their therapeutic potential.

In vitro activity of the synthetic lipopeptide PAL-Lys-Lys-NH(2) alone and in combination with antifungal agents against clinical isolates of Cryptococcus neoformans

The in vitro activity of the lipopeptide PAL-Lys-Lys-NH(2) (PAL), alone or combined with either fluconazole (FLU) or amphotericin B (AMB), was investigated against 14 Cryptococcus neoformans isolates. PAL MICs ranged from 1.0 to 4.0 microg/ml. Fungicidal activity was observed. Synergy, defined as a fractional inhibitory concentration (FIC) index of < or =0.5, was observed in 21.4% of PAL/AMB interactions. Antagonism (FIC index>4) was never observed. The broad antifungal activity and the positive interactions with AMB suggest that PAL can represent a promising candidate in infections due to C. neoformans.

Results from the ARTEMIS DISK Global Antifungal Surveillance study, 1997 to 2005: an 8.5-year analysis of susceptibilities of Candida species and other yeast species to fluconazole and voriconazole determined by CLSI standardized disk diffusion testing

Fluconazole in vitro susceptibility test results for 205,329 yeasts were collected from 134 study sites in 40 countries from June 1997 through December 2005. Data were collected for 147,776 yeast isolates tested with voriconazole from 2001 through 2005. All investigators tested clinical yeast isolates by the CLSI M44-A disk diffusion method. Test plates were automatically read and results recorded with a BIOMIC image analysis system. Species, drug, zone diameter, susceptibility category, and quality control results were collected quarterly. Duplicate (same patient, same species, and same susceptible-resistant biotype profile during any 7-day period) and uncontrolled test results were not analyzed. Overall, 90.1% of all Candida isolates tested were susceptible (S) to fluconazole; however, 10 of the 22 species identified exhibited decreased susceptibility (<75% S) on the order of that seen with the resistant (R) species C. glabrata and C. krusei. Among 137,487 isolates of Candida spp. tested against voriconazole, 94.8% were S and 3.1% were R. Less than 30% of fluconazole-resistant isolates of C. albicans, C. glabrata, C. tropicalis, and C. rugosa remained S to voriconazole. The non-Candida yeasts (8,821 isolates) were generally less susceptible to fluconazole than Candida spp. but, aside from Rhodotorula spp., remained susceptible to voriconazole. This survey demonstrates the broad spectrum of these azoles against the most common opportunistic yeast pathogens but identifies several less common yeast species with decreased susceptibility to antifungal agents. These organisms may pose a future threat to optimal antifungal therapy and emphasize the importance of prompt and accurate species identification.

Epidemiology of invasive candidiasis: a persistent public health problem

Invasive candidiasis (IC) is a leading cause of mycosis-associated mortality in the United States. We examined data from the National Center for Health Statistics and reviewed recent literature in order to update the epidemiology of IC. IC-associated mortality has remained stable, at approximately 0.4 deaths per 100,000 population, since 1997, while mortality associated with invasive aspergillosis has continued to decline. Candida albicans remains the predominant cause of IC, accounting for over half of all cases, but Candida glabrata has emerged as the second most common cause of IC in the United States, and several less common Candida species may be emerging, some of which can exhibit resistance to triazoles and/or amphotericin B. Crude and attributable rates of mortality due to IC remain unacceptably high and unchanged for the past 2 decades. Nonpharmacologic preventive strategies should be emphasized, including hand hygiene; appropriate use, placement, and care of central venous catheters; and prudent use of antimicrobial therapy. Given that delays in appropriate antifungal therapy are associated with increased mortality, improved use of early empirical, preemptive, and prophylactic therapies should also help reduce IC-associated mortality. Several studies have now identified important variables that can be used to predict risk of IC and to help guide preventive strategies such as antifungal prophylaxis and early empirical therapy. However, improved non-culture-based diagnostics are needed to expand the potential for preemptive (or early directed) therapy. Further research to improve diagnostic, preventive, and therapeutic strategies is necessary to reduce the considerable morbidity and mortality associated with IC.

Antifungal susceptibility testing

Antifungal susceptibility testing has been in routine use now for more than 15 years and has become a useful tool for clinicians who are faced with difficult treatment decision. Although most clinicians order susceptibility testing, much confusion still exists regarding the use of the results. Sufficient data have been generated to determine susceptibility trends for specific fungi against specific agents, but correlation data are minimal. Despite the lack of correlation data, antifungal susceptibility testing continues to provide useful information to assist with patient care.

Comparative evaluation of Etest and sensititre yeastone panels against the Clinical and Laboratory Standards Institute M27-A2 reference broth microdilution method for testing Candida susceptibility to seven antifungal agents

To assess their utility for antifungal susceptibility testing in our clinical laboratory, the Etest and Sensititre methods were compared with the Clinical and Laboratory Standards Institute (CLSI) M27-A2 reference broth microdilution method. Fluconazole (FL), itraconazole (I), voriconazole (V), posaconazole (P), flucytosine (FC), caspofungin (C), and amphotericin B (A) were tested with 212 Candida isolates. Reference MICs were determined after 48 h of incubation, and Etest and Sensititre MICs were determined after 24 h and 48 h of incubation. Overall, excellent essential agreement (EA) between the reference and test methods was observed for Etest (95%) and Sensititre (91%). Etest showed an >or=92% EA for MICs for all drugs tested; Sensititre showed a >or=92% EA for MICs for I, FC, A, and C but 82% for FL and 85% for V. The overall categorical agreement (CA) was 90% for Etest and 88% for Sensititre; minor errors accounted for the majority of all categorical errors for both systems. Categorical agreement was lowest for Candida glabrata and Candida tropicalis with both test systems. Etest and Sensititre provided better CA at 24 h compared to 48 h for C. glabrata; however, CA for C. glabrata was <80% for FL with both test systems despite MIC determination at 24 h. Agreement between technologists for both methods was >or=98% for each agent against all organisms tested. Overall, Etest and Sensititre methods compared favorably with the CLSI reference method for determining the susceptibility of Candida. However, further evaluation of their performance for determining the MICs of azoles, particularly for C. glabrata, is warranted.

Meningitis due toPrototheca wickerhamii: rare case in China

A rare case of Prototheca wickerhamii meningitis is reported in a patient without any underlying immunodeficient condition. Wet-mount microscopy and culture of cerebral spinal fluid specimens, along with temperature resistance, cycloheximide tolerance, carbohydrates assimilation, including API 20C AUX tests of the isolated etiologic agent were performed. In addition, transmission and scanning electron microscopy studies and in vitro antifungal susceptibility tests were conducted. Through the combination of these investigations, the isolate was identified as P. wickerhamii and the patient was successfully treated with intravenous amphotericin B and itraconazole. This is the first detailed report of meningitis caused by P. wickerhamii in China.

Role of antifungal susceptibility testing in patient management

PURPOSE OF REVIEW

The review focuses on the evolving role of antifungal susceptibility testing in the clinical management of patients with invasive fungal infections. It will clarify how to clinically interpret available susceptibility testing methods.

RECENT FINDINGS

Fluconazole susceptibility testing for yeasts has now shown a relationship between fluconazole resistance and therapeutic failure. Of increasing concern are the reported cases of fluconazole cross-resistance to the newer triazoles. The correlation between amphotericin B and echinocandin susceptibility testing for yeasts, resistance, and outcomes is unknown. Although there are reports of clinical failures with echinocandins, resistance is uncommon and susceptibility testing is unhelpful. There are standardized methods of testing for the triazoles and amphotericin B against molds; however, resistance has not been correlated with outcomes due to host and laboratory factors. Knowing the intrinsic resistance of molds to particular antifungals is of greater importance. Posaconazole is a potent new antifungal agent against many difficult-to-treat molds including fusariosis and zygomycosis.

SUMMARY

Antifungal susceptibility testing of yeasts can assist in treating patients with prior antifungal exposure in determining resistance or cross-resistance. Mold susceptibility testing is of limited clinical benefit, while posconazole is an exciting new agent for treating difficult invasive fungal infections.

Candida guilliermondii, an opportunistic fungal pathogen with decreased susceptibility to fluconazole: geographic and temporal trends from the ARTEMIS DISK antifungal surveillance program

Although a rare cause of invasive candidiasis, Candida guilliermondii has been reported to exhibit decreased susceptibility to antifungal agents. Aside from case reports and small surveys, there is little information regarding the epidemiology and antifungal susceptibility profile of C. guilliermondii. We report geographic and temporal trends in the isolation and antifungal susceptibilities of 1,029 C. guilliermondii clinical isolates collected from 127 medical centers as part of the ARTEMIS DISK Antifungal Surveillance Program. In addition, we report the in vitro susceptibility of 132 bloodstream isolates of C. guilliermondii to caspofungin. C. guilliermondii represented 1.4% of the 75,761 isolates collected from 2001 to 2003 and was most common among isolates from Latin America (3.7% versus 0.6 to 1.1%). Decreased susceptibility to fluconazole was noted (75% susceptible; range, 68 to 77% across regions), and voriconazole was more active in vitro against C. guilliermondii than fluconazole (91% susceptible; range, 88 to 93% across regions). Fluconazole was least active against isolates from dermatology (58%) and surgical (69%) services and against isolates associated with skin and soft tissue infection (68%, compared to 85% susceptible for bloodstream isolates). There was no evidence of increasing azole resistance over time among C. guilliermondii isolates tested from 2001 to 2003. Of 132 bloodstream isolates of C. guilliermondii tested against caspofungin, most were inhibited by < or =2 microg/ml (96%; MIC50/MIC90, 0.5/1.0 microg/ml). C. guilliermondii, a species that exhibits reduced susceptibility to fluconazole, is the sixth most frequently isolated Candida species from this large survey and may be an emerging pathogen in Latin America.

Candida rugosa, an emerging fungal pathogen with resistance to azoles: geographic and temporal trends from the ARTEMIS DISK antifungal surveillance program

Candida rugosa is a fungus that appears to be emerging as a cause of infection in some geographic regions. We utilized the extensive database of the ARTEMIS DISK Antifungal Surveillance Program to describe the geographic and temporal trends in the isolation of C. rugosa from clinical specimens and the in vitro susceptibilities of 452 isolates to fluconazole and voriconazole. C. rugosa accounted for 0.4% of 134,715 isolates of Candida, and the frequency of isolation increased from 0.03% to 0.4% over the 6.5-year study period (1997 to 2003). C. rugosa was most common in the Latin American region (2.7% versus 0.1 to 0.4%). Decreased susceptibility to fluconazole (40.5% susceptible) was observed in all geographic regions; however, isolates from Europe and North America were much more susceptible (97 to 100%) to voriconazole than those from other geographic regions (55.8 to 58.8%). C. rugosa was most often isolated from blood and urine in patients hospitalized at the Medical and Surgical inpatient services. Notably, bloodstream isolates were the least susceptible to both fluconazole and voriconazole. C. rugosa should be considered, along with the established pathogens Candida krusei and Candida glabrata, as a species of Candida with reduced susceptibility to the azole antifungal agents.

Association of fluconazole area under the concentration-time curve/MIC and dose/MIC ratios with mortality in nonneutropenic patients with candidemia

The present study tested in vitro susceptibility of Candida bloodstream isolates to fluconazole to determine if the ratio of the fluconazole area under the concentration-time curve (AUC) or weight-normalized daily dose (dose(wn)) to MIC correlated with mortality. Fluconazole susceptibility and outcome data were determined for 77 patients with a positive Candida blood culture between 2002 and 2005. The most commonly isolated Candida species were C. albicans (64%), C. glabrata (14%), C. parapsilosis (8%), C. tropicalis (6%), and C. lusitaniae (4%). Only two isolates were classified as fluconazole resistant by the CLSI M27-A2 method. Fluconazole MICs were highest against C. glabrata relative to other Candida species. Overall the crude mortality assessed at hospital discharge was 19.4% (n = 15). Mortality rates by species were as follows: C. albicans, 16.3%; C. glabrata, 36.4%; C. parapsilosis, 0%; C. tropicalis, 0%; C. lusitaniae, 33.3%. A mortality rate of 50% was noted among patients infected with nonsusceptible isolates (MIC > or = 16 microg/ml) compared to 18% for patients infected with susceptible (MIC < or = 8 microg/ml) isolates (P = 0.17). The fluconazole dose(wn)/MIC (24-h) values were significantly higher for the 62 survivors (13.3 +/- 10.5 [mean +/- standard deviation]) compared to the 15 nonsurvivors (7.0 +/- 8.0) (P = 0.03). The fluconazole AUC/MIC (24 h) values also trended higher for survivors (775 +/- 739) compared to nonsurvivors (589 +/- 715) (P = 0.09). These data support the dose-dependent properties of fluconazole. Underdosing fluconazole against less-susceptible Candida isolates has the potential to increase the risk of mortality associated with candidemia.

Use of fluconazole as a surrogate marker to predict susceptibility and resistance to voriconazole among 13,338 clinical isolates of Candida spp. Tested by clinical and laboratory standards institute-recommended broth microdilution methods

Clinical laboratories frequently face the problem of delayed availability of commercially prepared approved reagents for performing susceptibility testing of new antimicrobials. Although this problem is encountered more often with antibacterial agents, it is also an issue with antifungal agents. A current example is voriconazole, a new triazole antifungal with an expanded spectrum and potency against Candida spp., Aspergillus spp., and other opportunistic fungal pathogens. The present study addresses the use of fluconazole as a surrogate marker to predict the susceptibility of Candida spp. to voriconazole. Reference broth microdilution MIC results for 13,338 strains of Candida spp. isolated from more than 200 medical centers worldwide were used. Voriconazole MICs and interpretive categories (susceptible, < or =1 microg/ml; susceptible dose dependent, 2 microg/ml; resistant, > or =4 microg/ml) were compared with those of fluconazole by regression statistics and error rate bounding analyses. For all 13,338 isolates, the absolute categorical agreement was 91.6% (false susceptible or very major error [VME], 0.0%). Since voriconazole is 16- to 32-fold more potent than fluconazole, the performance of fluconazole as a surrogate marker for voriconazole susceptibility was improved by designating those isolates with fluconazole MICs of < or =32 microg/ml as being susceptible to voriconazole, resulting in a categorical agreement of 97% with 0.1% VME. Clinical laboratories performing antifungal susceptibility testing of fluconazole against Candida spp. can reliably use these results as surrogate markers until commercial FDA-approved voriconazole susceptibility tests become available.

Results obtained with various antifungal susceptibility testing methods do not predict early clinical outcome in patients with cryptococcosis

The in vitro susceptibilities of Cryptococcus neoformans isolates from consecutive human immunodeficiency virus-positive and -negative patients to the antifungal agents fluconazole, amphotericin B, and flucytosine were determined by different techniques, including the CLSI method, Etest, and broth microdilution in yeast nitrogen base (YNB) medium, during a multicenter prospective study in France. The relationship between the in vitro data and the clinical outcome 2 weeks after the initiation of antifungal therapy was assessed. In addition, the correlation between the strain serotype and the in vitro activities of the antifungals was determined, and the susceptibility results obtained with the different techniques were also compared. Thirty-seven patients received a combination of amphotericin B with flucytosine as first-line therapy, 22 were treated with amphotericin B alone, and 15 received fluconazole alone. Whatever the antifungal tested, there was no trend toward higher MICs for strains isolated from patients who failed to respond to a given therapy compared to those from patients who did not with either the CLSI method, Etest, or broth microdilution in YNB medium. The MICs obtained by the CLSI or Etest method were significantly lower for serotype D strains than for serotype A strains for both fluconazole and amphotericin B, while flucytosine MICs were not different according to serotype. These findings suggest that the in vitro antifungal susceptibility of C. neoformans, as determined with the techniques used, is not able to predict the early clinical outcome in patients with cryptococcosis.

In vitro pharmacodynamics of anidulafungin and caspofungin against Candida glabrata isolates, including strains with decreased caspofungin susceptibility

The activities of anidulafungin and caspofungin against Candida glabrata were evaluated. MICs, 50% inhibitory concentrations (IC(50) values), and IC(90) values for anidulafungin were lower than those for caspofungin for 16 of 18 strains tested. Anidulafungin has potent in vitro activity against C. glabrata that is maintained against isolates with elevated caspofungin MICs.

Candida resistance and its clinical relevance

Since the 1970s, the death rate from Candida infection has risen in conjunction with increasing numbers of patients at risk for serious fungal infections, such as those immunocompromised because of tissue or organ transplantation, chemotherapy, acquired immunodeficiency syndrome, or advanced age. Candidal infections are not only prevalent but also associated with considerable mortality and morbidity. In 2005, the overall mortality rate was 44% within 30 days of the first blood culture positive for any Candida species. Given the substantial morbidity and mortality, clinicians must include fungal infection in the differential diagnosis for at-risk patients, and they must quickly select appropriate antifungal therapy for those with identified infection. However, clinicians cannot use the minimum inhibitory concentration to select antifungal therapy in the same way they use it to treat bacterial infections. The relationship between in vitro susceptibility and clinical effect is not as direct with antifungals as it is with antibiotics. As long as Candida species continue to be the major causes of fungal infections, improving outcomes remains an important therapeutic goal. A key element is preventing and managing drug resistance. Further study of treatment duration, dosage, intermittent-versus-continuous administration schedules, and other treatment options are needed to determine their effect on resistance. Therapy that combines agents with complementary mechanisms of action may increase potency and broaden the spectrum of antifungal efficacy while decreasing the number of resistant organisms.

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.

Evaluation of amphotericin B interpretive breakpoints for Candida bloodstream isolates by correlation with therapeutic outcome

One hundred seven Candida bloodstream isolates (51 C. albicans, 24 C. glabrata, 13 C. parapsilosis, 13 C. tropicalis, 2 C. dubliniensis, 2 C. krusei, and 2 C. lusitaniae strains) from patients treated with amphotericin B alone underwent in vitro susceptibility testing against amphotericin B using five different methods. Fifty-four isolates were from patients who failed treatment, defined as death 7 to 14 days after the incident candidemia episode, having persistent fever of >or=5 days' duration after the date of the incident candidemia, or the recurrence of fever after two consecutive afebrile days while on antifungal treatment. MICs were determined by using the Clinical Laboratory Standards Institute (formally National Committee for Clinical Laboratory Standards) broth microdilution procedure with two media and by using Etest. Minimum fungicidal concentrations (MFCs) were also measured in two media. Broth microdilution tests with RPMI 1640 medium generated a restricted range of MICs (0.125 to 1 microg/ml); the corresponding MFC values ranged from 0.5 to 4 microg/ml. Broth microdilution tests with antibiotic medium 3 produced a broader distribution of MIC and MFC results (0.015 to 0.25 microg/ml and 0.06 to 2 microg/ml, respectively). Etest produced the widest distribution of MICs (0.094 to 2 microg/ml). However, none of the test formats studied generated results that significantly correlated with therapeutic success or failure.

Correlation of MIC with outcome for Candida species tested against voriconazole: analysis and proposal for interpretive breakpoints

Developing interpretive breakpoints for any given organism-drug combination requires integration of the MIC distribution, pharmacokinetic and pharmacodynamic parameters, and the relationship between the in vitro activity and outcome from both in vivo and clinical studies. Using data generated by standardized broth microdilution and disk diffusion test methods, the Antifungal Susceptibility Subcommittee of the Clinical and Laboratory Standards Institute has now proposed interpretive breakpoints for voriconazole and Candida species. The MIC distribution for voriconazole was determined using a collection of 8,702 clinical isolates. The overall MIC90 was 0.25 microg/ml and 99% of the isolates were inhibited at < or = 1 microg/ml of voriconazole. Similar results were obtained for 1,681 Candida isolates (16 species) from the phase III clinical trials. Analysis of the available data for 249 patients from six phase III voriconazole clinical trials demonstrated a statistically significant correlation (P = 0.021) between MIC and investigator end-of-treatment assessment of outcome. Consistent with parallel pharmacodynamic analyses, these data support the following MIC breakpoints for voriconazole and Candida species: susceptible (S), < or = 1 microg/ml; susceptible dose dependent (SDD), 2 microg/ml; and resistant (R), > or = 4 microg/ml. The corresponding disk test breakpoints are as follows: S, > or = 17 mm; SDD, 14 to 16 mm; and R, < or = 13 mm.

Méthodologie d’évaluation de la sensibilité in vitro aux antifongiques

During the last years, a large amount of work has been completed to improve the methods used for in vitro antifungal susceptibility testing. Reference techniques are currently available both for yeasts and filamentous fungi, but in some instances, technical improvement are needed. Etest is another well standardized method that can be used as an alternative on a routine basis in the clinical microbiology laboratory. Studies of in vitro-in vivo correlations have led to the definition of susceptibility breakpoints for yeasts for fluconazole, itraconazole, and flucytosine.

Interpretive breakpoints for fluconazole and Candida revisited: a blueprint for the future of antifungal susceptibility testing

Developing interpretive breakpoints for any given organism-drug combination requires integration of the MIC distribution, pharmacokinetic and pharmacodynamic parameters, and the relationship between in vitro activity and outcome from both in vivo and clinical studies. Previously, the Subcommittee for Antifungal Testing of the Clinical and Laboratory Standards Institute (CLSI [formerly National Committee for Clinical Laboratory Standards]) proposed MIC interpretive breakpoints for fluconazole and Candida spp. These breakpoints were considered to be somewhat weak, because the clinical data supporting them came largely from mucosal infections and there were very few infections involving strains with elevated fluconazole MICs. We readdress the issue of fluconazole breakpoints for Candida by using published clinical and microbiologic data to provide further validation of the breakpoints proposed by the CLSI in 1997. We also address interpretive breakpoints for agar disk diffusion testing of fluconazole. The MIC distribution for fluconazole was determined with a collection of 13,338 clinical isolates. The overall MIC at which 90% of the isolates were inhibited was 8 microg/ml: 91% were susceptible (S) at a MIC of or= 64 microg/ml). Similar results were obtained for 2,190 isolates from randomized clinical trials. Analysis of available data for 1,295 patient-episode-isolate events (692 represented mucosal infections and 603 represented invasive infections) from 12 published clinical studies demonstrated an overall success rate of 77%, including 85% for those episodes in which the fluconazole MIC was or= 64 microg/ml) isolates. Pharmacodynamic analysis demonstrated a strong relationship between MIC, fluconazole dose, and outcome. A dose/MIC ratio of approximately 25 was supportive of the following susceptibility breakpoints for fluconazole and Candida spp.: S, MIC or= 64 microg/ml. The corresponding disk test breakpoints are as follows: S, >or=19 mm; SDD, 15 to 18 mm; R, Collapse

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MESH Headings

• Antifungal Agents/administration & dosage
• Antifungal Agents/pharmacology
• Antifungal Agents/therapeutic use
• Candida/drug effects
• Candidiasis/drug therapy
• Clinical Trials as Topic
• Dose-Response Relationship, Drug
• Drug Resistance, Fungal
• Fluconazole/pharmacokinetics
• Fluconazole/pharmacology
• Fluconazole/therapeutic use
• Humans
• Microbial Sensitivity Tests/standards
• Treatment Outcome

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Affiliation(s)

M A Pfaller Department of Pathology, Medical Microbiology Division, C606 GH, University of Iowa College of Medicine, Iowa City, 52242, USA.
D J Diekema
D J Sheehan

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