Synergy measurement by checkerboard analysis is used to determine the impact on potency of the combination of antibiotics in comparison to their individual activities. This comparison is then represented as the Fractional Inhibitory Concentration (FIC) index value. The FIC index value takes into account the combination of antibiotics that produces the greatest change from the individual antibiotic’s MIC.
To quantify the interactions between the antibiotics being tested (the FIC index), the following equation is used:
+= FICA + FICB = FIC Index
where A and B are the MIC of each antibiotic in combination (in a single well), and MICA and MICB are the MIC of each drug individually.
The FIC Index value is then used to categorize the interaction of the two antibiotics tested.