This study performed a retrospective analysis on the relationship between blood culture time-to-positivity (TP) and type of isolated microorganism, antibiotic administration, and immunological status of the patients. We analyzed the data related to 1,218 positive blood cultures. When compared to Gram positive bacteraemia, the percentage of Gram negative growth was higher and the mean TP significantly shorter (p < 0.0001). In patients receiving antibiotics, median and mean TPs of blood culture were different for Gram positive bacteraemia (log-rank p = 0.0022, Wilcoxon p < 0.0001) but not for Gram negative (log-rank p = 0.4011, Wilcoxon p = 0.1585). No statistically significant effect on TP was found for sampling site, interaction between sampling site and antibiotic administration, and immunological status of the patient. In conclusion, TP is independent of antibiotic therapy in cases of Gram negative bacteraemia, while for Gram positive bacteraemia a prolongation of TP occurs.