Interpretation of Fractional Inhibitory Concentration Index (FICI)

Interpretation of Fractional inhibitory concentration index (FICI):  Observe the pore of about 50% growth in the combination of the two drugs, and record the respective concentrations of the two drugs in this pore. If there were multiple pores with 50% growth, the following two methods were used for interpretation: (1) If the drug concentration at 50% growth point of the two drugs combination was higher than the MICs of respective single drug, it indicated that ITR had a desensitizing effect on FCA. At this time, the pore with the highest FICI value was selected as the interpretation point, and the respective single drug concentration and FICI value of this pore were recorded. ② If the drug concentration at 50% growth point of the two drug combination pore was lower than the MICs of each single drug, it indicated that a sensitization effect. At this time, the pore with the lowest FICI value was selected as the interpretation point, and the respective single drug concentration and FICI value of this pore were recorded. FICI= (MIC of drug A when combined with drug B/MIC of drug A when only used) + (MIC of drug B when combined with drug A/MIC of drug B when only used). In this experiment, the MIC value of 50% growth was used as the standard for the calculation of FICI value. FICI ≤ 0.5, synergistic effects, that is, ITR has sensitizing effect on FCA; (2) 0.5 < FICI ≤ 1, additive effects; (3) 1 < FICI < 4, no interactions; (4) FICI ≥ 4.0, antagonistic effects, that is, ITR has a hypoallergenic effect on FCA. In vitro drug sensitivity test was conducted based on the Yeast drug sensitivity test method M27-A4 microbroth dilution method recommended by Clinical Laboratory and Standards Institute (CLSI).