Does pre-treatment microbiological testing translate into lower multidrug-resistant organism infection risk? A patient-level target-trial emulation of hospitalisation episodes.

OBJECTIVES: To assess whether pre-treatment microbiological testing is associated with subsequent multidrug-resistant organism (MDRO) hospital-acquired infection. METHODS: We conducted a retrospective target-trial emulation using linked hospital data from a tertiary hospital in China (May 2018-December 2025). Episodes were classified by whether qualifying microbiological testing had been recorded by the first antimicrobial use. The primary outcome was incident post-index MDRO hospital-acquired infection. Propensity-score overlap weighting estimated risk differences per 10,000 episodes and risk ratios in the overlap population. RESULTS: Among 859,135 episodes, 234,100 (27.2%) had qualifying testing recorded by index; 1,029 events occurred. After overlap weighting, risks were 11.29 versus 11.27 per 10,000 episodes, with a risk difference of 0.02 (95% CI, -1.63 to 1.67) and a risk ratio of 1.002 (95% CI, 0.866-1.159). ATO-aligned doubly robust and prespecified sensitivity analyses were broadly consistent with the primary estimate. A post hoc analysis restricted to infection-directed episodes similarly showed no protective association. CONCLUSION: In the primary overlap population, qualifying microbiological testing recorded by antimicrobial initiation had a near-null association with subsequent MDRO hospital-acquired infection. This finding concerns an index-time diagnostic process rather than downstream result-guided stewardship action.