Sputum culture yields after antibiotic therapy – a retrospective cohort study

<h2>Abstract</h2><h3>Objective</h3> Sputum cultures can guide antibiotic therapy, particularly in severe pneumonia or when resistant pathogens are suspected. However, data on how prior and ongoing antibiotic exposure affects microbial detection are limited. This study aimed to evaluate this impact, stratified by timing and route of administration. <h3>Methods</h3> In- and outpatients from nine hospitals in Skåne, Sweden, who submitted sputum for bacterial culture between 2011 and 2021 were included. Patients requiring specialized pulmonology care (e.g., cystic fibrosis) were excluded. Microbiological results (dichotomized as typical respiratory pathogens versus more opportunistic pathogens) were linked to antibiotic exposure within 30 days prior to sampling, based on hospital records and pharmacy data. Exposure was classified as none, ongoing (oral antibiotics within 7 days or intravenous within 24 hours), or recent (treatment concluded within 30 days and prior to the definition of ongoing). Odds ratios and 95% confidence intervals for microbial detection were calculated with logistic regression models. Time-series analyses plotted detection rates by time since treatment initiation, with linear regression applied. <h3>Results</h3> Among 21361 sputum cultures from 15366 individuals, 46.1% (n=9848) had documented antibiotic exposure. Respiratory pathogens were detected in 19.8% (n=4235) of samples, with significantly higher detection in those without prior antibiotics (26.4%; n=3035 vs. 12.2%; n=1200, p<0.001). Detection of respiratory pathogens was reduced also following recent antibiotic exposure (OR 0.60 [95%CI 0.55 to 0.65]) while opportunistic pathogens were more frequently detected after recent antibiotic exposure (OR 1.51 [95%CI 1.38 to 1.65]). <h3>Conclusion</h3> The results underline that sputum samples should ideally be collected before antibiotic administration. Post-treatment samples yield fewer pathogens and more opportunists, warranting cautious interpretation.