VITEK 2 Yeast AST System: Introduction, Name of Card, Principle, Procedure, Result-Interpretation, and Keynotes
VITEK 2 AST-YS08-Yeast Indentification Card
Introducton
Table of Contents
The VITEK 2 Yeast AST system is a fully automated solution for the quantitative determination of antifungal susceptibility in clinically significant yeasts, such as Candida species. It provides rapid and accurate results, often in as little as 13 to 24 hours.
Name of Card
AST-YS09: This is the primary card used for Yeast Susceptibility Testing.
Fig. VITEK 2 AST-YS08-Yeast Identification Card
ID-YST: Though not for AST, this card is often used in tandem for species-level identification of up to 63 types of yeast.
Principle
The system is based on the broth microdilution minimum inhibitory concentration (MIC) technique.
Miniaturization: The AST card is a miniaturized version of the doubling dilution technique, containing 64 microwells.
Growth Detection: It utilizes repetitive turbidimetric monitoring of fungal growth.
Optical Monitoring: The system monitors the attenuation of light in each well using an optical scanner (photometer) to determine the MIC based on the growth rate compared to a control well.
Procedure
The workflow is designed to be highly automated with minimal manual handling.
Inoculum Preparation: Select morphologically similar, pure colonies from a primary culture (18–24 hours old).
Standardization: Suspend the yeast in sterile saline (0.45% to 0.50% NaCl) to reach a turbidity of 1.80 to 2.20 McFarland, verified using the DensiCHEK instrument.
Loading: Place the inoculum tube and AST card into the VITEK 2 Cassette.
Automation: The instrument automatically fills the card (using a vacuum), seals it, and moves it to the incubator/reader module.
Incubation: The card is incubated at 35.5°C, with optical readings taken every 15 minutes.
Result-Interpretation
MIC Determination: The system generates a report with the MIC value for each antifungal agent.
Breakpoints: Results are interpreted as Susceptible (S), Intermediate (I), or Resistant (R) based on integrated CLSI, EUCAST, or FDA guidelines.
PDF: AST YS08
Advanced Expert System (AES): Every result is cross-checked against a database of over 3,800 phenotypes to ensure biological consistency and identify specific resistance mechanisms.
Keynotes
Rapid Turnaround: Most yeast results are available within 13 to 27 hours, significantly faster than the 48-hour reference broth microdilution method.
Closed System: The sealed card format minimizes aerosols, spills, and personal contamination, reducing biohazardous waste by up to 80%.
Fig. Yeast Breakpoints
Inoculum Stability: The age of the suspension must not exceed 30 minutes before card inoculation.
Limitations: It may not be recommended for certain rare or difficult-to-grow species like Candida auris without specific validation, as results can be clade-dependent.