Difficulties in assessing community-acquired infection as a risk factor for nosocomial infection at an intensive care unit

Objective: To analyze the relationship between presentation with a community-acquired infection (CAI) and the risk of subsequent nosocomial infection, and the assessment of a likely diagnostic bias in this association.Design: A prospective cohort study. APACHE II and TISS were used to assess severity and therapeutic intensity, respectively. Nosocomial infection (NI) was diagnosed according to SENIC and CDC diagnostic criteria. The relative risk and its 95% confidence interval were estimated.Setting: The intensive care unit (ICU) of the University of Granada Hospital (Spain). It is a ten-bed multidisciplinary unit.Patients: 448 patients admitted to the intensive care unit (ICU) between December-1986 and April-1988 who stayed at the ICU for at least 24 hours were included in the study.Results: The crude analysis suggests that CAI may prevent NI. When data were stratified by other variables a previous infection acted as a preventive factor in patients admitted to the ICU from emergency room, in patients with lower severity level and in those with shorter stay lengths at ICU. Patients with a CAI showed higher severity; they were treated more aggressively, they had a longer stay at ICU before a NI was diagnosed and they remained at ICU longer. In multivariate analysis the NI risk in patients with a CAI compared with those not infected previously and controlling for other variables was of 0.36.Conclusion: The presence of a CAI may introduce a differential information bias in the study of NI.Abbreviations APACHE acute physiologic and chronic health evaluation - CAI community-acquired infection - ICU intensive care unit - RTI respiratory tract infection - TISS therapeutic intensity score system