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Original Research: CRITICAL CARE MEDICINE |

Transferring Critically Ill Patients Out of Hospital Improves the Standardized Mortality Ratio*: A Simulation Study

Jeremy M. Kahn, MD, MS; Andrew A. Kramer, PhD; Gordon D. Rubenfeld, MD, MSc
Author and Funding Information

*From the Division of Pulmonary & Critical Care (Drs. Kahn and Rubenfeld), Harborview Medical Center, University of Washington, Seattle WA; and Cerner Corporation (Dr. Kramer), Kansas City, MO.

Correspondence to: Jeremy M. Kahn, MD, MS, Division of Pulmonary, Allergy, and Critical Care, University of Pennsylvania School of Medicine, 874 Maloney Building, 3600 Spruce St, Philadelphia, PA 19104; e-mail: jkahn@cceb.med.upenn.edu



Chest. 2007;131(1):68-75. doi:10.1378/chest.06-0741
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Background: Transferring critically ill patients to other acute care hospitals may artificially impact benchmarking measures. We sought to quantify the effect of out-of-hospital transfers on the standardized mortality ratio (SMR), an outcome-based measure of ICU performance.

Methods: We performed a cohort study and Monte Carlo simulation using data from 85 ICUs participating in the acute physiology and chronic health evaluation (APACHE) clinical information system from 2002 to 2003. The SMR (observed divided by expected hospital mortality) was calculated for each ICU using APACHE IV risk adjustment. A set number of patients was randomly assigned to be transferred out alive rather than experience their original outcome. The SMR was recalculated, and the mean simulated SMR was compared to the original.

Results: The mean (± SD) baseline SMR was 1.06 ± 0.19. In the simulation, increasing the number of transfers by 2% and 6% over baseline decreased the SMR by 0.10 ± 0.03 and 0.14 ± 0.03, respectively. At a 2% increase, 27 ICUs had a decrease in SMR of > 0.10, and two ICUs had a decrease in SMR of > 0.20. Transferring only one additional patient per month was enough to create a bias of > 0.1 in 27 ICUs.

Conclusions: Increasing the number of acute care transfers by a small amount can significantly bias the SMR, leading to incorrect inference about ICU quality. Sensitivity to the variation in hospital discharge practices greatly limits the use of the SMR as a quality measure.

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