Clinical and microbiological implications of time to positivity of blood cultures in adult patients with blood stream infections

Blood stream infections (BSI) are one of the serious and life threatening complications associated with high morbidity and mortality. Identification of patients without bacterial infections is an important component of antimicrobial stewardship. With the advances in the automated blood culture systems especially with the continuous monitoring systems, time to positivity (TTP) of blood cultures has been reduced drastically thereby allowing faster de-escalation of the antibiotics. : In this study, we have analysed the TTP of different bacterial isolates, and the effects of initiation of antimicrobials and blood volume on TTP. Adult patients with monomicrobial bacteraemia in an academic hospital were included retrospectively over a four-year period. Time to positivity was recorded for each positive sample. Information about the timing of blood sample collection i.e. before or after start of antibiotics was collected from the blood culture requisition form. The blood volume in the blood culture bottle is the virtual blood volume given by the Bact-Alert Virtuo instrument. A total of 38,606 blood culture samples that flagged positive from adult patients with suspected BSIs were included. 79% of the samples had a TTP of less than 24 hours and 15% of the samples had TTP of 24 to 48 hrs. Only 6% of the samples had TTP of more than 48 hours. Gram negative pathogens (Average TTP- 12.5hours) have shorter TTP when compared to Gram positive pathogens (Average TTP- 15.4 hours). With the use of modern automated blood culture systems, TTP can be used as a tool to guide the antimicrobial therapy and early de-escalation of the empirical antibiotics thereby reducing the emergence of antimicrobial resistance.