Background Sepsis remains one of the most important causes of morbidity and mortality worldwide. In approximately 30–50% of cases of suspected sepsis, no pathogen is isolated, disabling the clinician to treat the patient with targeted antimicrobial therapy. Studies investigating the differences in the patient outcomes between culture-positive and culture-negative sepsis patients have only been conducted in subgroups of sepsis patients and results are ambiguous. Methods This is a sub-analysis of the PHANTASi (Prehospital Antibiotics Against Sepsis trial), a randomized controlled trial that focused on the effect of prehospital antibiotics in sepsis patients. We evaluated the outcome of cultures from different sources and determined what the clinical implications of having a positive culture compared to negative cultures were for patient outcomes. Furthermore, we looked at the effect of antibiotics on culture outcomes. Results 1133 patients (42.6%) with culture-positive sepsis were identified, compared to 1526 (56.4%) patients with culture-negative sepsis. 28-day mortality (RR 1.43 [95% CI 1.11–1.83]) and 90-day mortality (RR 1.41 [95% CI 1.15–1.71]) were significantly higher in culture-positive patients compared to culture-negative patients. Culture-positive sepsis was also associated with ≥ 3 organ systems affected during the sepsis episode (RR 4.27 [95% CI 2.78–6.60]). Patients who received antibiotics at home more often had negative blood cultures (85.9% vs. 78%) than those who did not ( p < 0.001). Conclusions Our results show that culture-positive sepsis is associated with a higher mortality rate and culture-positive patients more often have multiple organ systems affected during the sepsis episode. Trial registration The PHANTASi trial is registered at ClinicalTrials.gov, number NCT01988428 . Date of registration: November 20, 2013. Electronic supplementary material The online version of this article (10.1186/s13054-019-2431-8) contains supplementary material, which is available to authorized users.

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