Molds E-Test: Introduction, Principle, Clinical Significance, and Keynotes

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Introduction

Mold infections caused by species such as Aspergillus, Fusarium, Scedosporium, and Mucorales are increasingly common, especially in immunocompromised and oncology patients. Effective treatment requires accurate antifungal susceptibility testing (AFST). The E-test (Epsilometer test) is a practical method used in clinical microbiology to determine the minimum inhibitory concentration (MIC) of antifungal agents against filamentous fungi. Although broth microdilution (CLSI/EUCAST) is the reference standard, the E-test offers a simpler, gradient diffusion approach that is suitable for routine laboratories.

Principle

  • The E-test is performed using a plastic strip impregnated with a continuous gradient of antifungal drug concentrations.
  • The strip is placed on the surface of an agar plate (commonly RPMI 1640 agar with glucose and MOPS buffer) that has been previously inoculated with a standardized suspension of mold conidia.
  • As the antifungal diffuses into the agar, it creates a concentration gradient.
  • After incubation (24–48 hours, sometimes longer for slow-growing molds), an elliptical inhibition zone forms around the strip.
  • The MIC is determined by reading the scale on the strip at the point where the ellipse intersects the strip.

Clinical Significance

  1. Guides Therapy – Provides MIC values for molds like Aspergillus fumigatus, A. terreus, Fusarium spp., and Scedosporium spp.
  2. Rapid and Practical – Easier to perform than broth microdilution, requiring no specialized equipment.
  3. Resistance Detection – Identifies azole resistance in Aspergillus (e.g., due to CYP51A mutations).
  4. Supports Clinical Decisions – Helps in adjusting antifungal therapy, particularly in refractory or breakthrough infections.
  5. Validation Tool – Useful for confirming results from automated or reference systems.

Keynotes

  • The E-test is a gradient diffusion method providing quantitative MIC results for molds.
  • Common antifungals tested include amphotericin B, itraconazole, voriconazole, posaconazole, isavuconazole, and echinocandins.
  • Advantages: Simpler, user-friendly, and provides visual results.
  • Limitations:
    • More expensive than disk diffusion.
    • Endpoints may be difficult to read due to fuzzy mold growth.
    • Lower reproducibility compared to CLSI/EUCAST broth microdilution.
  • Despite limitations, it is widely used in routine labs and clinical studies where reference methods are not feasible.
  • Plays a crucial role in high-risk patients (oncology, transplant, ICU) where invasive mold infections have high mortality.

Further Readings

  1. https://www.mdpi.com/2309-608X/5/4/108
  2. https://www.researchgate.net/file.PostFileLoader.html?id=53f218c3d039b1bc6b8b45c3&assetKey=AS%3A273580883939331%401442238173115
  3. https://pmc.ncbi.nlm.nih.gov/articles/PMC84808/
  4. https://microbenotes.com/epsilometer-test-e-test/
  5. https://microbiologynotes.com/e-epsilometer-test-principle-purpose-procedure-result-interpretation-with-precaution-advantages-and-disadvantages/
  6. https://www.biomerieux.com/corp/en/our-offer/clinical-products/etest.html
  7. https://microbeonline.com/e-test-epsilometer-test-principle-purpose-procedure-results-and-interpretations/
  8. https://en.wikipedia.org/wiki/Etest
  9. https://acmeresearchlabs.in/2024/03/28/minimum-inhibitory-concentration-mic-test-protocol/
  10. https://pmc.ncbi.nlm.nih.gov/articles/PMC11234478/
  11. https://onlinelibrary.wiley.com/doi/10.1002/pbc.25540
  12. https://veteriankey.com/antimicrobial-agents/

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