Impact of antibiotic administration on blood culture positivity at the beginning of sepsis: a prospective clinical cohort study.

OBJECTIVES: Sepsis guidelines recommend obtaining blood cultures before starting anti-infective therapy in patients with sepsis. However, little is known of how antibiotic treatment before sampling affects bacterial growth. The aim of this study was to compare the results of blood cultures drawn before and during antibiotic therapy.

METHODS: Prospective clinical cohort study of septic patients. Adult intensive care unit patients with two or three blood culture sets at the beginning of sepsis between 2010 and 2017 were included. Patients with blood culture samples obtained before antibiotic therapy were compared with patients with samples taken during antibiotic therapy. Blood culture positivity, defined as presence of a microbiological pathogen, was compared between the groups. Logistic regression was performed to adjust the impact of different factors with respect to blood culture positivity.

RESULTS: In total, 559 patients with 1364 blood culture sets at the beginning of sepsis were analysed. Blood culture positivity was 50.6% (78/154) among patients with sepsis who did not receive antibiotics and only 27.7% (112/405) in those who were already receiving antibiotics (p <0.001). Logistic regression revealed antibiotic therapy as an independent factor for less pathogen identification (odds ratio 0.4; 95% CI 0.3-0.6). Gram-positive pathogens (28.3% (111/392) versus 11.9% (116/972); p <0.001) and also Gram-negative pathogens (16.3% (64/392) versus 9.3% (90/972); p <0.001) were more frequent in blood culture sets drawn before antibiotic therapy compared with sets obtained during antibiotic therapy.

CONCLUSIONS: Obtaining blood cultures during antibiotic therapy is associated with a significant loss of pathogen detection. This strongly emphasizes the current recommendation to obtain blood cultures before antibiotic administration in patients with sepsis.