Summation of fractional inhibitory concentration (FIC) index endpoints for combination testing of amphotericin B and natamycin against Fusarium spp. (n=10) and Aspergillus spp. (n=10). The FIC of a drug is the fraction of the minimum inhibitory concentration (MIC) required to inhibit fungal growth when used in combination with another drug. This is a way of normalising the respective MICs of each drug so they can be compared when used in combination in the checkerboard. If the drugs do not interact, the FIC remains at the normalised value of 1.00 (underlined in the figure). If the drugs interact synergistically, the FIC decreases. If the drugs interact antagonistically, the FIC increases. The FIC index (FICI) is the sum of the two FIC values from the x- and y-axes for the checkerboard square corresponding to the visual endpoint. The endpoint is the checkerboard square with the smallest FICI that inhibits growth. There is one endpoint (one FICI) for each isolate (each checkerboard). The figure is a graphical overlay of the 20 checkerboards with a numeric summation of the endpoint locations (white text) with corresponding FICI values (black text). All 20 isolates showed indifference to combination treatment. No isolate showed synergy or antagonism.