Combination FIC Index

Compute checkerboard FIC components and apply caller-provided interpretation cutoffs; results are assay-specific, not a treatment recommendation.

Description

A fractional inhibitory concentration compares each combination concentration with that drug's alone MIC.

A fractional inhibitory concentration compares each combination concentration with that drug's alone MIC. Summing the two ratios yields an assay index whose interpretation depends on declared cutoffs.^[1]

Compute checkerboard FIC components and apply caller-provided interpretation cutoffs; results are assay-specific, not a treatment recommendation.

When to use Combination FIC Index

Use this tool to compute checkerboard FIC components and apply caller-provided interpretation cutoffs; results are assay-specific, not a treatment recommendation.

Prepare the sample, count, growth or assay measurements requested by this tool. Confirm units, sampling periods and reporting bases; supply independently established comparison criteria where requested.

How to use Combination FIC Index and understand its results

Prepare the sample, count, growth or assay measurements requested by this tool. Confirm units, sampling periods and reporting bases; supply independently established comparison criteria where requested.
Enter aloneMicA, aloneMicB, combinationConcentrationA, combinationConcentrationB, synergyMaximum, antagonismAbove using the field table and calculation rules, then run the tool.
Read ficA, ficB, ficIndex, suppliedInterpretation. The explanation below defines the units, signs, ordering and any flags or placeholder values.
If the tool reports an input issue, correct the values and run it again. Apply the stated assumptions when interpreting a successfully returned result.

Input field	Meaning	Value type; see rules below	Requirement
aloneMicA	Alone mic a	number	Required
aloneMicB	Alone mic b	number	Required
combinationConcentrationA	Combination concentration a	number; minimum: 0	Required
combinationConcentrationB	Combination concentration b	number; minimum: 0	Required
synergyMaximum	Synergy maximum	number; minimum: 0	Required
antagonismAbove	Antagonism above	number	Required

Enter positive alone MICs and nonnegative combination concentrations in matching units per drug. Supply nonnegative synergy maximum and a strictly higher antagonism boundary. The tool returns both FICs, their sum, and synergy for ≤lower, antagonism for >upper or between-cutoffs otherwise.

FIC_A=combinationA/MICaloneA; FIC_B=combinationB/MICaloneB; index=FIC_A+FIC_B

Units, indexing, boundary conditions and exceptional cases are defined in the calculation rules above.

Output field	Meaning	Value type
ficA	Fic a	number
ficB	Fic b	number
ficIndex	Fic index	number
suppliedInterpretation	Supplied interpretation	string

json

{
  "input": {
    "aloneMicA": 1,
    "aloneMicB": 2,
    "combinationConcentrationA": 0.25,
    "combinationConcentrationB": 0.5,
    "synergyMaximum": 0.5,
    "antagonismAbove": 4
  },
  "output": {
    "ficA": 0.25,
    "ficB": 0.25,
    "ficIndex": 0.5,
    "suppliedInterpretation": "synergy"
  }
}

Limitations and assumptions

Cutoffs and checkerboard conditions are caller-supplied. The result is assay-specific and does not recommend combination treatment, validate controls or model clinical exposures.

Numeric inputs must be finite. Follow the calculation-specific rules for overflow, underflow and exceptional cases. Missing required fields, extra fields, wrong value types and violations of the calculation rules are input errors.

Alternative or Complementary methods

Use Growth Inhibition MIC Endpoint: Derive the observed MIC from a monotone dilution-growth series, preserving below- and above-range censoring. For a complementary task, use Bactericidal Survival Endpoint: Find the first dilution meeting a caller-defined bactericidal kill fraction based on survivor counts, rather than merely recording MBC.

References

Combination FIC Index — tool.io

Similar or alternative tools

Growth Inhibition MIC Endpoint
Derive the observed MIC from a monotone dilution-growth series, preserving below- and above-range censoring.
Bactericidal Survival Endpoint
Find the first dilution meeting a caller-defined bactericidal kill fraction based on survivor counts, rather than merely recording MBC.