Susceptibility testing for piperacillin-tazobactam has undergone rapid evolution, largely driven by some surprising results from the MERINO trial, which compared the efficacy of piperacillin-tazobactam and meropenem for treatment of patients with ceftriaxone-resistant E. coli or Klebsiella pneumoniae bacteremia. We discuss how the results of the MERINO trial led to reconsideration of breakpoints for pipercillin-tazobactam at the Clinical Laboratory Standards Institute, or CLSI, how the breakpoints were changed, and how well commercial platforms perform piperacillin-tazobactam testing using FDA or CLSI breakpoints. Spoiler alert: the news isn’t great. If you are interested in beta-lactam/beta lactamase combinations, you should check out the previous episode of this podcast, hosted by our friend Dr. Cesar Arias, the Editor-in-Chief of Antimicrobial Agents and Chemotherapy. Cesar and the guests did a great review of recent developments in this fast-moving area. Guests: Dr. Trish Simner - Director of the Medical Bacteriology and Infectious Disease Sequencing Laboratories, Johns Hopkins Hospital Dr. Romney Humphries - Director of the Division of Laboratory Medicine and the Medical Director of the Microbiology Laboratory at Vanderbilt University Medical Center.