Comparing antimicrobial activity against resistant Pseudomonas aeruginosa using an index for the absence of cross-resistance.

We devised an index to estimate the degree cross-resistance between piperacillin, ceftazidime, sulbactam/cefoperazone, amikacin, tobramycin, carumonam and imipenem against 139 separate clinical isolates of Pseudomonas aeruginosa. A negative value of the index indicated the cross-resistance whereas a positive value suggested the converse making the device an index for the absence of cross resistance (ACR). Using the ACR index, we identified pairs of antibiotics exhibiting the least degree of cross-resistance and therefore the highest potential for treating infections due to P. aeruginosa.