Carbapenem-resistant Acinetobacter baumannii (CRAB) infections are a major public health problem worldwide that have been increasing in prevalence in recent years. Antimicrobial combination therapy is the current treatment approach against carbapenem resistance. Objectives: The aim of this study was to investigate the efficacy of colistin (CO) in combination with tigecycline (TGC) and doripenem (DOR) against CRAB isolates in relation to the carbapenem resistance genes. Methods: Thirty CRAB isolates were collected from patients admitted to Theodor Bilharz Research Institute (TBRI). Species identification and antimicrobial susceptibility testing were performed by VITEK 2 system. Minimum inhibitory concentration (MIC) value of CO was detected by broth microdilution (BMD) and by E-test for TGC and DOR. Interactions of antimicrobial combination of CO, TGC and DOR were detected by E-test fixed ratio method. Carbapenemase producing genes (blaOXA-23, blaOXA-24, blaOXA-58 and blaNDM), in addition to the intrinsic blaOXA-51 were determined by polymerase chain reaction (PCR). Results: All isolates were sensitive to colistin and susceptibility to TGC was 77%, while all isolates were resistant to DOR by E test. CO/TGC combination revealed synergy and additive effect in 43.3% and 13.3% of the isolates respectively. For CO/DOR combination, synergy rate was 23.3% while additive effect was 16.7%. Carbapenemase producing genes were detected in 24 (80%) of the isolates; blaOXA-23 was the most predominant gene (60%; 18/30) followed by blaNDM gene in 43.3% (13/30). The blaOXA-23 was determined alone in 37% (11/30) and simultaneously with blaNDM gene in 23.3% (7/30), while blaNDM was detected alone in 20% (6/30) of the isolates. The blaOXA-24 and blaOXA-58 genes were not detected. Among the11 isolates positive for only blaOXA-23, the synergistic effect was observed for CO/TGC combination in 64% and in 18% for CO/DOR combination. Synergy was detected in 33% for CO/TGC combination and 17% for CO/DOR combination among isolates harboring blaNDM gene only. In isolates harboring blaOXA23 and blaNDM simultaneously, synergy was detected in 42.9% and 28.7% for CO/DOR and CO/TGC combinations respectively. In conclusion: Limited options in the treatment of CRAB infections have made the combination antibioticis a strategic therapy. The best synergy rate and the highest antibacterial activity were reached with the combination of CO and TGC especially against OXA-23 producing CRAB isolates . NDM-producing isolates are still the more difficult to treat even by the combination therapy.