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

PurposeBlood 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.MethodsWe 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.ResultsA 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).ConclusionBlood 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.