Background: Two out of the three recently approved β-lactam (BL)/β-lactamase inhibitors (BLIs) have higher CLSI susceptibility breakpoints (ceftazidime/avibactam 8 mg/L; meropenem/vaborbactam 4 mg/L) compared with the BL alone (ceftazidime 4 mg/L; meropenem 1 mg/L). This can lead to a therapeutic gray area on susceptibility reports depending on resistance mechanism. For instance, a meropenem-resistant OXA-48 isolate (MIC 4 mg/L) may appear as meropenem/vaborbactam-susceptible (MIC 4 mg/L) despite vaborbactam's lack of OXA-48 inhibitory activity. Methods: OXA-48-pos. (n = 51) and OXA-48-neg. (KPC, n = 5; Klebsiella pneumoniae WT, n = 1) Enterobacterales were utilized. Susceptibility tests (broth microdilution) were conducted with ceftazidime/avibactam, imipenem/relebactam and meropenem/vaborbactam, as well as their resp. BL partner. Antimicrobial activity of all six agents was evaluated in the murine neutropenic thigh model using clin. relevant exposures. Efficacy was assessed as the change in bacterial growth at 24 h, compared with 0 h controls. Results: On average, the three BL/BLI agents resulted in robust bacteria killing among OXA-48-neg. isolates. Among OXA-48-pos. isolates, poor in vivo activity with imipenem/relebactam was concordant with its resistant phenotypic profile. Variable meropenem/vaborbactam activity was observed among isolates with a 'susceptible' MIC of 4 mg/L. Only 30% (7/23) of isolates at meropenem/vaborbactam MICs of 2 and 4 mg/L met the ≥1 log bacterial reduction threshold predictive of clin. efficacy in serious infections. In contrast, ceftazidime/avibactam resulted in marked bacterial d. reduction across the range of MICs and 73% (37/51) of isolates exceeded the ≥1 log bacterial reduction threshold. Conclusions: Data demonstrate that current imipenem/relebactam and ceftazidime/avibactam CLSI breakpoints are appropriate. Data also suggest that higher meropenem/vaborbactam breakpoints relative to meropenem can translate to potentially poor clin. outcomes in patients infected with OXA-48-harbouring isolates.