How to Select FIC in Checkerboard Assays: A Visual Guide

To calculate FIC values, we first need to identify the Minimum Inhibitory Concentration, or MIC, for each drug individually.The checkerboard shows decreasing concentrations of Drug A along the x-axis and Drug B along the y-axis, both ranging from 64 to 0.5 micrograms per milliliter.Let's visualize the growth pattern. Green indicates bacterial growth, while red shows areas where growth is inhibited.For Drug A, we look along the x-axis where Drug B is zero. The MIC is the lowest concentration that prevents growth, which is 8 micrograms per milliliter.Similarly for Drug B, we look along the y-axis where Drug A is zero. Here, the MIC is 4 micrograms per milliliter.These MIC values are crucial as they serve as our baseline measurements. They will be used as denominators when calculating the Fractional Inhibitory Concentration for each drug combination.Any concentration below the MIC allows bacterial growth, while concentrations at or above the MIC prevent it. This clear transition point is what defines our MIC values.Now that we have our checkerboard assay, we'll identify where bacterial growth is inhibited.Let's understand how to interpret the results. Red indicates bacterial growth, while green shows where growth is inhibited.We'll examine each well and mark it based on whether bacterial growth is present or inhibited.The boundary between growth and no-growth zones creates what we call the inhibition curve.These points along the boundary represent the critical combinations where the drugs transition from effective to ineffective.These boundary points will be crucial for calculating the Fractional Inhibitory Concentration Index in our next step.Now that we've identified our inhibition points, let's calculate the FIC values for each combination.The FIC formula combines the relative concentrations of both drugs compared to their individual MIC values.Let's start with point A, where we have 4 micrograms per milliliter of each drug.For point B, we have 2 micrograms per milliliter of Drug A and 8 of Drug B.Finally at point C, we have 8 micrograms per milliliter of Drug A and 2 of Drug B.Each point along the inhibition boundary gives us a different FIC value. We'll use these values in the next section to determine the optimal FIC index.After calculating individual FIC values for each effective combination, we need to compare them to find the optimal FIC index.Here we have several combinations with their calculated FIC values. We're looking for the lowest value, as this represents the most effective combination.The FIC index is interpreted using standard breakpoints that indicate the type of drug interaction.Proper documentation of FIC results is crucial for research records and clinical applications.A complete report should include study identification, drug names, the optimal FIC index value, and its interpretation.In this example, our optimal FIC index is 0.38, indicating synergistic interaction between the drugs.Remember to always document both the numerical FIC index value and its interpretation for complete reporting.