Proposal for screening cut-off values and use of tolerance disk test (TDtest) for detection of tolerance/persistence to ceftazidime-avibactam in Klebsiella pneumoniae

Journal article, 2026

Objective: We aimed to characterize ceftazidime-avibactam (CZA)-tolerant/persistent Klebsiella pneumoniae, previously categorized as CZA-susceptible, and propose screening cut-off values for CZA-tolerance/persistence. Moreover, we discuss the Tolerance Disk Test (TDtest) to detect CZA-tolerance/persistence in clinical microbiology laboratories. Methods: We studied 80 K. pneumoniae clinical isolates detected as CZA-resistant, truly CZA-susceptible, or CZA-tolerant/persistent by TDtest. CZA-tolerant and CZA-persistent isolates were differentiated by determination of minimal duration for killing (MDK) and minimum bactericidal concentration (MBC)/minimum inhibitory concentration (MIC) ratio. Screening cut-off values were proposed based on MIC values and inhibition zone diameters. Results: 33/80 isolates were differentiated as CZA-tolerant and 15/80 as CZA-persistent. For 48/80 CZA-tolerant/persistent isolates, the CZA inhibition zone was lower (from 14 to 20 mm) than for truly 28/80 CZA-susceptible bacteria (most of them above 21 mm). For CZA-tolerant/persistent isolates, CZA MIC values were higher (from 0.5 to 4 mg/L) than for truly CZA-susceptible isolates (most of them below 0.25 mg/L). 4/80 isolates were categorized as resistant (according to EUCAST breakpoints), and no heteroresistant isolates were detected. MBC/MIC ratio was statistically higher for CZA-tolerant/persistent isolates than for CZA-susceptible bacteria. There was 100% of accordance between TDtest and MDK results for CZA-tolerance/persistence detection. Conclusions: The distribution ranges of CZA MIC and inhibition zone for CZA-tolerant/persistent isolates differ significantly from those for truly CZA-susceptible isolates. MDK data and MBC/MIC ratio corroborated TDtest results. Thereby, we suggested screening cut-off values for CZA-tolerance/persistence in K. pneumoniae isolates (CZA MIC >0.5 mg/L and CZA inhibition zone <20 mm) as well as TDtest for laboratory detection.