Rate and predictors of blood culture positivity after antibiotic administration: a prospective single-center study

Purpose: Blood culture obtainment prior to antibiotic administration, in patients with suspected infection, is considered the best practice in international guidelines. However, there is little data regarding the effect of a single dose of antibiotics on blood culture sterilization. Methods: We conducted a prospective study, enrolling consecutive patients with suspected infection, hospitalized in an internal medicine ward between December 2019 and January 2023. Included patients had a positive blood culture prior to antibiotic administration and a set of blood cultures taken within 24 h after a single dose of antibiotics. The rate of patients with pathogen isolation after antibiotic administration was assessed. Logistic regression was performed to examine factors associated with blood culture positivity. Results: A total of 155 patients were recruited for the study of which 131 (50.8% female 77.5 ± 13.4 years) met the inclusion criteria. The overall rate of patients with a positive blood culture after a single dose of antibiotics was 42.0% (55/131 patients). Increasing time between antibiotic administration and post-antibiotic culture was an independent predictor for blood culture sterilization (odds ratio 0.89 [95% confidence interval, 0.83–0.97; p = 0.006] for every 60 min). Blood culture volume was an independent predictor for blood culture positivity in a sensitivity analysis which included 82 patients (OR = 1.26 [95% CI 1.03–1.57] for every 1 ml increase; p = 0.024). Conclusion: Blood culture positivity is reduced by antimicrobial therapy but remains high after a single dose of antibiotics. If cultures are not obtained prior to antibiotic administration, they should be obtained as soon as possible afterwards.