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ICU-acquired bacteraemia and ICU mortality and discharge: addressing time-varying confounding using appropriate methodology

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Koen B. Pouwels, Stijn Vansteelandt, Rahul Batra, Jonathan D. Edgeworth, Timo Smieszek, Julie V. Robotham

Original languageEnglish
JournalJournal of Hospital Infection
Early online date23 Nov 2017
Accepted/In press17 Nov 2017
E-pub ahead of print23 Nov 2017


King's Authors


Background Studies often ignore time-varying confounding or use inappropriate methodology to adjust for time-varying confounding. Aim We estimated the effect of ICU-acquired bacteraemia on ICU mortality and discharge using appropriate methodology. Methods Marginal structural models with inverse probability weighting were used to estimate the ICU mortality and discharge associated with ICU-acquired bacteraemia among patients who stayed more than 2 days at the general ICU of a London teaching hospital and remained bacteraemia-free during those first two days. For comparison, the same associations were evaluated with 1) a conventional Cox model, adjusting for only baseline confounders and 2) a Cox model adjusting for baseline and time-varying confounders. Findings Using the marginal structural model with inverse probability weighting, bacteraemia was associated with an increase in ICU mortality (cause-specific hazard ratio [CSHR] 1.29, 95% CI 1.02-1.63) and a decrease in discharge (CSHR 0.52, 95% CI 0.45-0.60). By 60 days, among patients still in the ICU after 2 days and without prior bacteraemia, 8.0% of ICU-deaths could be prevented by preventing all ICU-acquired bacteraemia cases. The conventional Cox model adjusting for time-varying confounders gave substantially different results: CSHR 1.08 (95% CI 0.88-1.32) for ICU mortality and CSHR 0.68 (95% CI 0.60-0.77) for discharge. Conclusion In our study, even after adjusting for the timing of acquiring bacteraemia and time-varying confounding using inverse probability weighting for marginal structural models, ICU-acquired bacteraemia was associated with a decreased daily ICU discharge risk and an increased risk of ICU mortality.

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