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Comparison of Etest and microdilution method for antifungal susceptibility testing of C. neoformans to four azoles

Abstract number: 1733_1458

Aller A.I., Claro R., López-Oviedo E., Romero A., Martín-Mazuelos E.

Objective: The purpose of this study is to compare the results obtained by the reference broth microdilution method (MDM) with those obtained with Etest system (ET) (Ab Biodisk, Solna, Sweden) for antifungal susceptibility testing of C. neoformans to fluconazole (Pfizer Central Research, UK), itraconazole (Janssen Pharmaceutica, UCA), voriconazole (Pfizer Central Research, UK) and posaconazole (Shering-Plough Research Institute, USA).

Methods: We have studied retrospectively 80 clinical isolates of C. neoformans. The susceptibility to four azoles were performed by the MDM according to the CLSI guidelines (M27-A2 document) modified by Ghannoum et al (J Clin Microbiol 1992; 30:2881–86). The final concentrations were of 0.12–64 mg/L for fluconazole and 0.015–8 mg/L for the rest of antifungal agents. The final inoculum size was 104 CFU/mL. The MICs were defined as the lowest concentration in which a prominent decrease in turbidity was observed.

The ET was performed according to manufacturer's instructions. The inoculum suspensions of C. neoformans isolates matched the turbidity of n°1 McFarland standard. The incubation time was 48–72 h. The MIC was read where the border of the elliptical inhibition zone intersected the scale on the antifungal strip. C. krusei ATCC 6258, C. parapsilosis ATCC 22019, C. neoformans 90112 and C. neoformans 90113 were included as control strains. Essential agreement (EA) was defined as MIC results by ET and MDM in exact agreement or within two dilutions.

Results: The MICs obtained by the two methods after different incubation times varied by no more than one two-fold dilutions. The MIC ranges, MIC50 and MIC90 obtained at 72 h are summarised in table 1. The EA was 75% for fluconazole, 63.4% for posaconazole, 30% for voriconazole and 6.3% for itraconazole.

Table 1 

Antifungal agentMethodMIC (mg/L)
RangeMIC50MIC90  
FluconazoleMDM0.5–3248
ET0.032–>=256432  
ItraconazoleMDM0.06–10.51
ET<0.002–0.250.0640.25  
PosaconazoleMDM0.03–40.250.5
ET<0.02–10.0230.125  
VoriconazoleMDM0.25–10.50.5
ET0.008–1.50.0940.19  

Conclusions:

1. Fluconazole MICs by ET showed good correlation with MDM (75%)

2. Itraconazole showed the lowest agreement (6.3%)

3. It would be necessary to carry out further studies including interlaboratory agreement and correlation of MICs by different methods with ``in vivo'' response.

Session Details

Date: 31/03/2007
Time: 00:00-00:00
Session name: European Society of Clinical Microbiology and Infectious Diseases
Subject:
Location: ICC, Munich, Germany
Presentation type:
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