The US EPA suggests a default value of M

Both substance concentrations which cause x% effect in combination are individually related to their corresponding effect concentration representing an individual effect of x% (formula see supplementary material S3). The threshold value of 1 indicates an additive effect while < 1 and > 1 characterize synergism or antagonism, respectively. Notwithstanding the foregoing, for reasons of conformity the decision criteria on synergistic effects for CI will be adapted from the TU approach.3.Fractional inhibitory concentration (FIC): The definition of the FIC equals the CI in terms of calculation and decision criteria (Berenbaum, 1978) using the minimum inhibitory concentration of single and mixed drugs (see supplementary material S3).