Resistance detection straight from the (blood culture) bottle
February 05 2016
Bloodstream Candida infections have a high mortality, and swift and appropriate treatment is vital as delays of just hours may significantly increase mortality (1). Excellent agreement was found between azole conventional disk susceptibility methods on bottle subcultures and a direct disk diffusion method on positive blood culture bottles, on 106 Candida isolates.
In many countries, including Pakistan, C. tropicalis is the most predominant species, with a notable rise of frequency more recently (2). Also, growing resistance to fluconazole has become a serious concern and rapid antifungal susceptibility testing is necessary for both the chances of survival of the patient and to prevent unnecessary use of antifungal medicines. The conventional method for determining fungal susceptibility introduces lengthy delays in treatment, as the subculture of yeast positive cultures takes up to 48h. The final reporting takes another 24 hours.
Jabeen and colleagues (2015) compared the gold standard conventional disk diffusion susceptibility test, to direct disk diffusion testing from positive bottles for fluconazole, voriconazole and amphotericin B. The direct and conventional methods were performed on all 106 candida species (isolated from the 104 blood culture bottles) while E-test for minimally inhibitory concentrations (MICs) was performed on a small subset. Excellent agreement was found between the conventional method and the direct disk diffusion method for all the azole drugs with Pearson correlations demonstrating moderate to strong agreement. There was agreement in MICs by E-test for fluconazole, amphotericin B and voriconazole; however, zone diameters for amphotericin B correlated only moderately with MICs, confirming that disk diffusion (both direct and conventional) is not an acceptable method for amphotericin B susceptibility testing.
E test for antifungal susceptibility
Previous work has also demonstrated that the use of E-test for direct susceptibility testing can provide rapid results with 24-48 hours. The present study confirms that the disk method on direct blood culture bottles as a rapid and cost effective method. However, the authors note that this technique is not suitable for amphotericin B testing.