For example, drug A's MIC is reported as >16microg/ml and that of B as >12.5microg/ml. The combined MIC that brings about synergy is 0.5microg/ml for drug A and 0.78microg/ml for drug B. How do you calculate the FICi? I do not know what to input as the MICs for both A and B in the following formula: (C A/MIC A) + (C B/MIC B), where MIC A and MIC B are the MICs of drugs A and B alone, respectively, and C A and C B are the concentrations of the drugs in combination, respectively.
I think I have inserted the parameters required. May be if there's a different method for calculating the FIC index. The paper in this link had the similar results and got the FICi, I still have not been able to identify how they calculated the value. http://aac.asm.org/content/47/3/956.full
Have you considered using other methods of analyses for your drug combination effects? For example, the CompuSyn program is free, and it determines drug interactions by the combination index (Chou-Talalay) method. http://www.combosyn.com/feature.html
I read a couple of papers where they used the next higher concentration if the highest one tested in the checkerboard assay was not MIC. In your example, the MIC values to be used would be 32ug/mL for drug A and 25ug/mL for B. I would suggest thought to perform another MIC test for each drug using higher concentration ranges for validation. You can, in this new MIC test use staggered concentrations, so that you avoid the exponential effect of 2 fold dilutions (see 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)
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