About This Guide

Understanding how antibiotics interact is crucial in the fight against resistant bacteria. This guide explains the concepts behind the Antibiotic Synergy Index calculator, a tool that uses the Fractional Inhibitory Concentration Index (FICI) to classify drug interactions based on laboratory data.

What This Calculator Does

The calculator quantifies the interaction between two antibiotics. It determines if their combined effect is:

  • Synergistic: The combined effect is significantly greater than the sum of their individual effects.
  • Additive / Indifferent: The combined effect is equal to the sum of their individual effects, or there is no discernible interaction.
  • Antagonistic: The combination is less effective than one or both of the individual drugs.

This classification is based on the FICI, a standard metric derived from checkerboard assay results.

When to Use It

The FICI calculation is a standard method in preclinical microbiology research. It is typically used to:

  • Screen for promising new antibiotic combinations against multidrug-resistant pathogens.
  • Investigate the mechanisms of action and resistance.
  • Provide foundational data for further in vivo or clinical studies.

It is an investigational tool and is not intended for direct clinical decision-making at the point of care without expert interpretation in a broader clinical context.

Inputs Explained

The calculation requires four key values, all derived from a controlled laboratory experiment like a checkerboard dilution test:

  1. MIC of Drug A alone (MICₐ): The Minimum Inhibitory Concentration of Drug A when tested by itself. This is the lowest concentration that prevents visible growth of the bacteria.
  2. MIC of Drug B alone (MICₑ): The Minimum Inhibitory Concentration of Drug B when tested alone.
  3. Concentration of Drug A in combination: The concentration of Drug A in the first well of the checkerboard assay where the combination of A and B inhibits bacterial growth.
  4. Concentration of Drug B in combination: The concentration of Drug B in that same growth-inhibiting well.

Results Explained

The FICI score is a single number that summarizes the interaction. The calculator provides the components of this score and a final interpretation.

  • FIC of Drug A/B: This is the "Fractional Inhibitory Concentration." It represents the proportion of a drug's MIC needed to inhibit growth when used in combination. A FIC of 0.25 means only 25% of the original MIC was required.
  • FICI (ΣFIC): The sum of the individual FIC values. This index is compared against standard cutoffs for interpretation.
  • Interpretation:
    • Synergy: FICI ≤ 0.5
    • Additive/Indifference: 0.5 < FICI ≤ 4.0
    • Antagonism: FICI > 4.0

Formula / Method

The calculator uses the following standard formulas:

1. Calculate the FIC for each drug:
FIC of Drug A = (Concentration of A in Combination) / (MIC of Drug A alone)
FIC of Drug B = (Concentration of B in Combination) / (MIC of Drug B alone)

2. Sum the FIC values to get the FICI:
FICI = FIC of Drug A + FIC of Drug B

Step-by-Step Example

Let's test the combination of Ciprofloxacin and Meropenem against a strain of Pseudomonas aeruginosa.

1. Experimental Data:

  • MIC of Ciprofloxacin alone (MICₐ) = 2 µg/mL
  • MIC of Meropenem alone (MICₑ) = 8 µg/mL
  • In the checkerboard test, the most effective well that inhibited growth contained 0.25 µg/mL of Ciprofloxacin and 2 µg/mL of Meropenem.

2. Calculate FIC for Each Drug:

  • FIC Ciprofloxacin = 0.25 µg/mL / 2 µg/mL = 0.125
  • FIC Meropenem = 2 µg/mL / 8 µg/mL = 0.25

3. Calculate the Final FICI:

  • FICI = 0.125 + 0.25 = 0.375

4. Interpretation:

Since the FICI is 0.375 (which is ≤ 0.5), the interaction between Ciprofloxacin and Meropenem against this bacterial strain is classified as Synergy.

Tips + Common Errors

  • Consistent Units: Ensure all four input values use the same units (e.g., µg/mL, mg/L). Mixing units is a common source of error.
  • Accurate MICs: The entire calculation depends on accurately determined MICs. Any error in the initial MIC tests will lead to an incorrect FICI.
  • Select the Right Well: In a checkerboard assay, there may be multiple wells that inhibit growth. The FICI should be calculated for the well that shows the greatest reduction in concentrations. If multiple wells give the same minimal FICI, the highest value is often reported for a more conservative estimate.
  • Avoid MBC Values: The FICI is for inhibitory activity (MIC), not bactericidal activity (MBC). Using MBC values in this formula is incorrect.

Frequently Asked Questions

A checkerboard assay is a laboratory method used to assess the interaction between two antimicrobial agents. It involves preparing a microtiter plate with serial dilutions of Drug A along the x-axis and Drug B along the y-axis, creating a "checkerboard" of different concentration combinations to test against a target microorganism.
This cutoff is an empirically derived and widely accepted convention in microbiology. It signifies a four-fold or greater decrease in the MIC of both drugs in combination. While it's the standard, some researchers use slightly different cutoffs, so it's always important to state the criteria being used.
The terms are often used interchangeably as they fall within the same FICI range (0.5 to 4.0). "Additive" implies the combined effect is what you would expect from summing the individual effects. "Indifference" means the drugs act independently, with one having no effect on the other's activity. The FICI calculation doesn't distinguish between the two, so the combined term is used.
The relationship is complex. While a synergistic interaction (FICI ≤ 0.5) in the lab is promising, it does not guarantee clinical success. Factors like drug pharmacokinetics, site of infection, and host immune status play a major role. FICI is a valuable preclinical screening metric but not a direct predictor of patient outcomes.
The FICI method is a static, in vitro measurement that has limitations. It can be technically demanding, results can vary between labs, and interpretation of the broad "additive/indifference" range can be difficult. It also doesn't provide information about the rate of bacterial killing (pharmacodynamics).
Antagonism means the combination of drugs is less effective than the individual agents. This can happen if one drug induces an enzyme that degrades the other, or if they compete for the same target site. Such combinations are generally avoided in clinical practice.
No. The FICI calculation requires quantitative MIC values, which are obtained from broth microdilution or agar dilution methods. Disk diffusion (Kirby-Bauer) tests provide qualitative results (sensitive, intermediate, resistant) and are not suitable for calculating FICI.
For a combination to be considered synergistic or additive, the concentration of each drug required to inhibit growth should be lower than its own MIC. If, for example, the concentration of Drug A in the combination is equal to or greater than its MIC, it means Drug A is likely inhibiting growth on its own, and there is no beneficial interaction from Drug B.

References

  1. Eliopoulos, G. M., & Moellering, R. C. (1996). Antimicrobial combinations. In V. Lorian (Ed.), Antibiotics in laboratory medicine (4th ed., pp. 330–396). Williams & Wilkins.
  2. Odds, F. C. (2003). Synergy, antagonism, and what the chequerboard puts between them. Journal of Antimicrobial Chemotherapy, 52(1), 1. https://doi.org/10.1093/jac/dkg301
  3. Hsieh, M. H., Yu, C. M., Yu, V. L., & Chow, J. W. (1993). Synergy assessed by checkerboard. A critical analysis. Diagnostic microbiology and infectious disease, 16(4), 343–349. https://doi.org/10.1016/0732-8893(93)90087-n
  4. CLSI. M07-A10. Methods for Dilution Antimicrobial Susceptibility Tests for Bacteria That Grow Aerobically; Approved Standard—Tenth Edition. Clinical and Laboratory Standards Institute; 2015.

Disclaimer

This information is for educational and research purposes only. It is not intended as a substitute for professional medical advice, diagnosis, or treatment. The FICI is an in vitro metric and does not directly predict clinical efficacy. Always consult with a qualified healthcare professional or microbiologist for any health concerns or before making any decisions related to treatment.

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