In patients with severe sepsis, does the sensitivity of blood cultures decrease after the initiation of antibiotics?

The sensitivity of blood cultures is reduced by almost 50% after the initiation of empiric antibiotics for the treatment of severe sepsis. These results support the Surviving Sepsis Campaign best-practice recommendation to obtain blood cultures prior to the start of antimicrobial therapy. (LOE = 2b)

Cheng MP, Stenstrom R, Paquette K et al, for the FABLED Investigators. Blood culture results before and after antimicrobial administration in patients with severe manifestations of sepsis. Ann Intern Med 2019;171(8):547-554.  [PMID:31525774]

Diagnostic test evaluation

Foundation

Uncertain

Inpatient (any location)

The Surviving Sepsis guidelines recommend obtaining blood cultures prior to the start of antibiotics from patients presenting with sepsis, with a 45-minute delay considered acceptable. To determine whether the diagnostic sensitivity of blood cultures decreases when drawn after antibiotic administration, these investigators enrolled 330 patients who presented to the emergency department with severe sepsis and had 2 sets of blood cultures drawn before starting antibiotics. The researchers analyzed the 325 of them who were able to have additional sets of cultures drawn within 120 minutes after receiving antibiotics. Severe sepsis was defined as patients with systolic blood pressure less than 90 mmHg or a lactate level of 4 mmol/L or greater. The study population was 63% men, the mean age was 66 years, and 33% had sepsis due to respiratory infections. Compared with post-antimicrobial blood cultures, pre-antimicrobial blood cultures were positive in a greater number of patients (31.4% vs 19.4%; difference of 12%; 95% CI 5.4% - 18.6%; P < .001). Results were similar in the per-protocol population of 264 patients who had post-antimicrobial cultures drawn between 30 minutes and 120 minutes of initiation of antibiotics. The sensitivity of post-antimicrobial blood cultures was 52.9% (42.8% - 62.9%). If blood cultures had been drawn only after initiation of antimicrobrial treatment, 1 in every 6.7 patients would have a false-negative result. When results from other culture sites (urine, sputum, and wound cultures) were taken into account along with postantimicrobial blood cultures, sensitivity increased to 67.7% (57.7% - 76.6%).