Evaluation of in vitro methods for testing tigecycline combinations against carbapenemase-producing K.pneumoniae isolates

Abstract Objective Therapy of infections caused by carbapenemase-producing K. pneumoniae (CPKP) frequently involves combination therapy with various agents in the hope of achieving synergistic effects. Routine laboratory antimicrobial synergy testing is a service currently unavailable, due to the laborious nature of the reference Time Kill Assay (TKA), as well as the commonly held Checkerboard (CB) method. We explored whether easier methods, based on the Etest technique, might offer an adequate substitute. Methods We evaluatedin vitro interactions of tigecycline with colistin, gentamicin, fosfomycin or meropenem against 26 CPKP isolates employing the TKA, the CB and three Etest methodologies (the MIC/MIC ratio, the cross formation and the agar/Etest method). We determined the rates of consequent synergy and concordance of studied methods. Results All antimicrobial combinations demonstrated some degree of synergy againstK. pneumoniae isolates tested. No antagonism was found in any of the combinations. All methods showed poor synergy concordance with the TKA, producing non-significant Kappa (κ) results. Etest methods (MIC/MIC ratio and agar/Etest) exhibited a fair agreement (κ: 0.29 and 0.38, respectively) with the checkerboard method. Conclusion There is a poor correlation between synergy testing methods of tigecycline combinations, which may be associated with their different endpoints. To elucidate method comparability and reliability, their correlation to clinical outcomes seems important.