Antifungal susceptibility testing in Candida species: current methods and promising new tools for shortening the turnaround time

ABSTRACT

Introduction

Invasive fungal diseases (IFDs) have received attention as an emerging public health threat, are difficult to diagnose and to treat, and are associated with substantial morbidity and mortality. The standard of care in IFD management requires an early and targeted antifungal treatment, hence covers – amongst others – species identification and antifungal susceptibility testing (AFST).

Areas covered

This review gives an overview of methods currently applied in AFST and highlights promising new tools for shortening the turnaround time focusing on Candida species.

Expert opinion

The performance of the broth microdilution reference methods for AFST is not suitable for daily laboratory practice as they are too labor-intensive and time-consuming. Other conventional approaches such as disk diffusion assays, epsilometer tests, colorimetric or automated approaches are easier in handling, and in part, show good correlations with the reference methods. Promising results for shortening the turnaround time in providing MIC data or resistance detection include matrix-assisted laser desorption/ionization-time of flight mass spectrometer (MALDI-TOF MS) assisted AFST, molecular-based techniques and modified conventional approaches applying direct inoculation methods. These underlying AFST concepts are promising but in part completely different, have their own advantages and disadvantages, and need further clinical validation.

KEYWORDS:

• Antifungal susceptibility testing
• Candida
• direct inoculation
• MALDI-TOF MS assisted AFST
• shortening turnaround time in AFST

Article highlights

• Nowadays, standard reference and/or commercial AFST methods are part of standard of care in patient management.

• Disk diffusion assays, epsilometer tests, colorimetric or automated approaches are easy in handling, and in part show good correlations with the reference methods.

• Providing MIC data usually takes 24-48 h from the timepoint a culture is available.

• Disk diffusion and the direct inoculation method (positive blood culture bottles) reduce the time to result by more than 12 h, are cheap and easy to set up. The validation for a broad range of yeasts and novel antifungals is largely missing.

• The direct inoculation method with E-test strips resulted in contradictory data.

• Most promising are MALDI-TOF MS assisted AFST methods (3 h minimum), but they are available for only few drug-bug combinations.

• Molecular-based resistance detection may provide results within few hours, and is drug-class specific. Potential limitations are given by the fact that various underlying mechanisms may account for resistant phenotypes, resistant strains may lack target mutations, and a significant genetic diversity of resistance determinants may be present.

Declaration of interest

C Lass-Flörl reports financial support from Gilead Sciences, Astellas Pharma, Merck Sharp and Dohme, Basilea, Angelini, outside the submitted work. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

Reviewer disclosures

Peer reviewers on this manuscript have no relevant financial or other relationships to disclose.

Additional information

Funding

This paper was funded by the Medical University of Innsbruck.