The role of admission surveillance cultures in patients requiring prolonged mechanical ventilation in the intensive care unit

We undertook a prospective observational cohort study in intensive care unit (ICU) patients requiring mechanical ventilation for four days or more to evaluate normal and abnormal bacterial carriage on admission detected by surveillance cultures of throat and rectum. We assessed the importance of surveillance and diagnostic cultures for the early detection of resistance to third generation cephalosporins employed as the parenteral component of the selective decontamination of the digestive tract. Finally, we sought the risk factors of abnormal carriage on admission to the ICU. During the 58-month study 621 patients were included: 186 patients (30%) carried abnormal flora including methicillin-resistant Staphylococcus aureus (MRSA) and aerobic Gram negative bacilli (AGNB) on admission to the ICU Both MRSA and AGNB carriers were more commonly present in the hospital group of patients than in patients referred from the community (P < 0.001), although overgrowth was equally present both in community and in hospital patients. The incidence of infections during ICU stay was higher in abnormal (n=120, 64.5%) than in normal carriers (n=185, 42.5%) (P < 0.0001), with an odds ratio of 2.46 (95% confidence interval 1.72 to 3.51). Third generation cephalosporins covered ICU admission flora in 482 (78%) of the studied population. AGNB resistant to cephalosporins and MRSA were detected in surveillance cultures of 139 patients (22%), while the same resistant micro-organisms were identified only in 49 diagnostic samples (7.9%). Parenteral cephalosporins were modified in patients with abnormal flora (P < 0.0001). One hundred and ninety-six patients received antibiotics before admission to the ICU and 42% carried AGNB resistant to cephalosporins. Previous antibiotic use was the only risk factor for abnormal carriage in the multivariate analysis (OR 3.5; 95% confidence interval 2.1 to 5.8). The knowledge of carriage on admission using surveillance cultures may help intensivists to identify patients with abnormal carriage on admission and resistant bacterial strains at an early stage even when diagnostic samples are negative. Third generation cephalosporins covered admission flora in about 80% of the enrolled population and were modified in patients with abnormal flora who received antibiotic therapy before ICU admission. Our finding of overgrowth present on admission may justify the immediate administration of enteral antimicrobials.