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American Journal of Epidemiology Advance Access originally published online on September 17, 2009
American Journal of Epidemiology 2009 170(9):1186-1194; doi:10.1093/aje/kwp249
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American Journal of Epidemiology © The Author 2009. Published by the Johns Hopkins Bloomberg School of Public Health. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org.

PRACTICE OF EPIDEMIOLOGY

Using a Longitudinal Model to Estimate the Effect of Methicillin-resistant Staphylococcus aureus Infection on Length of Stay in an Intensive Care Unit

Adrian G. Barnett, Rahul Batra, Nicholas Graves, Jonathan Edgeworth, Julie Robotham and Ben Cooper

Correspondence to Dr. Adrian G. Barnett, School of Public Health and Institute of Health and Biomedical Innovation, Queensland University of Technology, 60 Musk Avenue, Kelvin Grove, Queensland 4059, Australia (e-mail: a.barnett{at}qut.edu.au).

Received for publication January 18, 2009. Accepted for publication July 20, 2009.

Health-care-associated methicillin-resistant Staphylococcus aureus (MRSA) infection may cause increased hospital stay, or sometimes death. Quantifying this effect is complicated because the exposure is time dependent: infection may prolong hospital stay, while longer stays increase infection risk. In this paper, the authors overcome these problems by using a multinomial longitudinal model to estimate the daily probability of death and discharge. They then extend the basic model to estimate how the effect of MRSA infection varies over time and to quantify number of excess days in the intensive care unit due to infection. They found that infection decreased the relative risk of discharge (relative risk ratio = 0.68, 95% credible interval: 0.54, 0.82). Infection on the first day of admission resulted in a mean extra stay of 0.3 days (95% credible interval: 0.1, 0.5) for a patient with an Acute Physiology and Chronic Health Evaluation II score of 10 and 1.2 days (95% credible interval: 0.5, 2.0) for a patient with a score of 30. The decrease in the relative risk of discharge remained fairly constant with day of MRSA infection but was slightly stronger closer to the start of infection. Results confirm the importance of MRSA infection in increasing stay in an intensive care unit but suggest that previous work may have systematically overestimated the effect size.

intensive care; longitudinal studies; risk factors; survival analysis; time

Abbreviations: APACHE, Acute Physiology and Chronic Health Evaluation; CI, credible interval; ICU, intensive care unit; MRSA, methicillin-resistant Staphylococcus aureus; RRR, relative risk ratio; TISS, therapeutic intervention scoring system


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