The fractional inhibitory concentration is an interaction coefficient indicating whether the combined inhibitory/bacteriostatic effect of drugs is synergistic, additive or antagonistic.
fractional inhibitory concentration = A + B
where:
• A = (MIC of combination X + Y) / (MIC of drug X alone)
• B = (MIC of combination X + Y) / (MIC of drug Y alone)
Interpretation:
• synergistic: FIC <= 0.5
• additive: FIC = 1
• antagonistic: FIC >= 4
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