Repeat antimicrobial susceptibility testing of serial bacterial isolates: clinical utility and diagnostic stewardship

To synthesize evidence on the interpretation and temporal stability of antimicrobial susceptibility profiles in serial bacterial isolates and to identify factors that inform repeat testing within diagnostic stewardship.

This narrative review integrates evidence from studies of serial clinical isolates with data on analytical variability, within-host resistance evolution, heteroresistance, antimicrobial exposure, and diagnostic stewardship.

Susceptibility profiles are generally stable over short intervals, although clinically relevant changes vary according to bacterial species, antimicrobial agent, specimen context, and elapsed time. Small MIC shifts, particularly near clinical breakpoints, may result in categorical discordance without indicating true resistance emergence. Pseudomonas aeruginosa exhibits greater short-term phenotypic variability than several other common pathogens. Repeat AST is most strongly supported when infection persists or recurs, the clinical response is inadequate, antimicrobial exposure may have selected for resistance, or the consequences of missed resistance are substantial. In selected lower-risk settings, referral to a recent susceptibility result may be appropriate, provided that laboratory and clinical override mechanisms remain available.

Short-term susceptibility stability may support conservative referral to a recent AST result, but the degree of instability varies by organism, antimicrobial agent, and specimen context. Where a single operational interval is required, laboratories may adopt the shortest interval supported by local or otherwise applicable evidence, with predefined overrides for higher-risk clinical situations. Prospective multicentre validation is required before specific intervals can be generalized across clinical and laboratory settings.