In a large tertiary hospital, the ideal time to perform antimicrobial susceptibility testing (AST) to detect changes in antimicrobial susceptibility patterns (cASP) was found to vary by type of specimen and bacterial species, according to results of a single-center retrospective analysis published in Clinical Microbiology and Infection.

Investigators extracted data on all antibiograms performed between 2015 and 2018 at a tertiary care hospital in Germany with a focus on Staphylococcus aureus, Enterococcus faecium, E faecalis, Enterobacter cloacae complex, Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, and Acinetobacter baumannii complex. They performed AST for all isolates of every specimen per patient using European Committee on Antimicrobial Susceptibility Testing (EUCAST) breakpoints. The investigators included only isolates with AST results for certain antibiotics: piperacillin-tazobactam, ciprofloxacin, meropenem, cefotaxime, ceftazidime, cefoxitin, levofloxacin, rifampicin, trimpethoprim-sulfamethoxazole, ampicillin, and vancomycin. A case was defined as 1 hospital admission, and multiple cases from a single patient were included.

For each isolate and antimicrobial agent, the investigators assessed changes incASP compared with those observed in previous antibiograms. They also measured the time interval between the date of initial species detection and the date cASP occurred within the same specimen.

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A total of 35,473 antibiograms were included in the final analysis, of which 5.6% had at least 1 cASP. The frequency of cASP ranged from 7.4% to 18.6% for all cases when stratified by specimen, and 0.1% to 21.0% when stratified by pathogens and antimicrobial agents.

Overall, 31.5% (628/1991) of cASP occurred on the first day of sampling, mostly due to phenotypic variants within the same specimen. After the exclusion of isolates with differing AST patterns on the first day of sampling, the median time to cASP was 12 (range, 1-364) days. After stratification by species and specimen, the investigators observed a significant variance in the median time to cASP. For example, for E coli, the median time to cASP was 5 (range, 1-48) days in blood culture specimens vs 16 (range, 1-364) days in urine specimens. Of note, the frequency of cASP was increased among Gram-negative pathogens compared with Gram-positive pathogens (range, 0.1%-21.0% vs 0.1%-6.4%, respectively).

The study was limited by its inclusion of only patients who had 2 or more ASTs performed, which may have created a bias toward the inclusion of those with complicated or severe infections in the study population. In addition, there was a lack of information regarding whether patients received prior antibiotic treatment.

“Further studies should ideally consider clinical information (eg, on antibiotic therapy) and use standardized sampling time intervals in order to evaluate whether recommendations for an ideal time interval to repeat AST can be established beyond local settings,” the investigators concluded.


Köck R, Schuler F, Idelevich EA, Schaumburg F. Variability of antibiograms: How often do changes in the antimicrobial susceptibility pattern occur in isolates from one patient? Clin Microbiol Infect. 2021;27(11):1638-1643. doi:10.1016/j.cmi.2021.02.012