Systematic review and meta-analysis of in vitro efficacy of antibiotic combination therapy against carbapenem-resistant Gram-negative bacilli

•In vitro models may inform on the efficacy of antibiotic regimens against carbapenem-resistant Gram-negative bacilli.•High or moderate synergism was shown for polymyxin/rifampicin vs. A. baumannii and polymyxin/fosfomycin vs. K. pneumoniae.•High or moderate synergism was also shown for imipenem/amikacin vs. P. aeruginosa.•Combinations including old antibiotics appear promising, while high-quality studies on novel antibiotics are lacking.•Correctly designed in vitro studies may highlight potential synergies to test in human trials or to optimise further tests.