Prevalence of Candida Species and Comparison of Susceptibility Patterns Using Commercially Available Susceptibility Testing Methods

Background: Invasive candidiasis (IC) remains a significant cause of inpatient morbidity and mortality. The rising rates of non-albicans Candida and resistance complicate the management of IC, making accurate and timely antifungal susceptibility testing critical for directing therapy. Commercially available susceptibility testing products vary greatly in cost, agents tested, ease of use, and implementation. The purpose of this study was to compare available susceptibility testing methods using prospectively collected Candida isolates.

Methods: Clinical Candida isolates were collected from July 2014 to March 2015. Susceptibility testing was performed using broth microdilution (BMD) according to the Clinical and Laboratory Standards Institute method, Etest, Vitek2, and Sensititre BMD panels.  Essential agreement (EA) and categorical agreement (CA) were assessed using BMD as the reference method.  Categorical disagreements were classified as very major errors (VME), major errors (ME), or minor errors (MiE).

Results: 74 Candida isolates underwent susceptibility testing.  Agreement is presented in table 1. The E test method yielded 6 VME, all due to FLC, and 22 MiE, 16 due to CAS.  Low rates of agreement were observed for the caspofungin Etest among C. glabrata and C. krusei.  Despite CA, a trend towards increased fluconazole MIC was observed.  Using Vitek2, there were 6 VME, all due to FLC, and one CAS MiE.  Sensititre BMD yielded only 3 VME, 1 ME, and 2 MiE.  All VME and the ME were due to FLC, and there was 1 MiE with CAS and MIC. 

Conclusion:  The most accurate method evaluated was Sensititre BMD panels, though it was the most time and resource intensive.  Caspofungin and fluconazole MICs all trended higher when using Etest.  Vitek 2 demonstrated >90% agreement, but evaluation remains limited in both ranges of MICs reported and agents tested.  When implementing antifungal susceptibility testing, clinicians should be aware of the strengths and limitations of each testing method.

Table 1.  Susceptibility Agreement Compared to BMD
	Etest		Vitek2		Sensititre
	EA (n=74)	CA (n= 58)	EA (n=74)	CA (n= 58)	EA (n=71)	CA (n= 57)
Fluconazole	87%	86%	92%	90%	93%	93%
Caspofungin	78%	72%	99%	98%	99%	98%
Micafungin	97%	98%	-	-	99%	98%
Anidulafungin	93%	97%	-	-	96%	100%