Predictors of positive blood cultures in critically ill patients: a retrospective evaluation

AIM: To identify predictors of bacteremia in critically ill patients, to evaluate the impact of blood cultures on the outcome, and to define conditions for breakthrough bacteremia despite concurrent antibiotic treatment. METHODS: A descriptive retrospective study was performed over a two-year period (2007-2008) in the medico-surgical Intensive Care Unit (ICU) of the San Giovanni Hospital in Bellinzona, Switzerland. RESULTS: Forty-five out of 231 patients (19.5%) had positive blood cultures. Predictors of positive blood cultures were elevated procalcitonin levels (>2 µg/L, P < 0.001), higher severity scores (Simplified Acute Physiology Score II>43, P = 0.014; Sequential Organ Failure Assessment >4.0, P < 0.001), and liver failure (P = 0.028). Patients with bacteremia had longer hospital stays (31 vs 21 days, P = 0.058), but their mortality was not different from patients without bacteremia. Fever (t > 38.5°C) only showed a trend toward a higher rate of blood culture positivity (P = 0.053). The rate of positive blood cultures was not affected by concurrent antibiotic therapy. CONCLUSIONS: The prediction of positive blood culture results still remains a very difficult task. In our analysis, blood cultures were positive in 20% of ICU patients whose blood was cultured, and positive findings increased with elevated procalcitonin levels, liver failure, and higher severity scores. Blood cultures drawn >4 days after the start of antibiotic therapy and >5 days after surgery could detect pathogens responsible for a new infection complication.