Validation of length of hospital stay as a surrogate measure for injury severity and resource use among injury survivors

Abstract

Objectives: While hospital length of stay (LOS) has been used as a surrogate injury outcome when more detailed outcomes are unavailable, it has not been validated. This project sought to validate LOS as a proxy measure of injury severity and resource use in heterogeneous injury populations.

Methods: This observational study used four retrospective cohorts: patients presenting to 339 California emergency departments (EDs) with a primary International Classification of Diseases, Ninth Revision (ICD-9), injury diagnosis (years 2005-2006); California hospital injury admissions (a subset of the ED population); trauma patients presenting to 48 Oregon EDs (years 1998-2003); and injured Medicare patients admitted to 171 Oregon and Washington hospitals (years 2001-2002). In-hospital deaths were excluded, as they represent adverse outcomes regardless of LOS. Duration of hospital stay was defined as the number of days from ED admission to hospital discharge. The primary composite outcome (dichotomous) was serious injury (Injury Severity Score [ISS] >or= 16 or ICD-9 ISS <or= 0.90) or resource use (major surgery, blood transfusion, or prolonged ventilation). The discriminatory accuracy of LOS for identifying the composite outcome was evaluated using receiver operating characteristic (ROC) analysis. Analyses were also stratified by age (0-14, 15-64, and >or=65 years), hospital type, and hospital annual admission volume.

Results: The four cohorts included 3,989,409 California ED injury visits (including admissions), 236,639 California injury admissions, 23,817 Oregon trauma patients, and 30,804 Medicare injury admissions. Composite outcome rates for the four cohorts were 2.1%, 29%, 27%, and 22%, respectively. Areas under the ROC curves for overall LOS were 0.88 (California ED), 0.74 (California admissions), 0.82 (Oregon trauma patients), and 0.68 (Medicare patients). In general, the discriminatory value of LOS was highest among children, tertiary trauma centers, and higher volume hospitals, although this finding differed by the injury population and outcome assessed.

Conclusions: Hospital LOS may be a reasonable proxy for serious injury and resource use among injury survivors when more detailed outcomes are unavailable, although the discriminatory value differs by age and the injury population being studied.

Figure 1

ROC curves for the discriminatory accuracy of hospital LOS to identify patients of all ages with serious injury or resource use. Serious injury was defined as ISS ≥ 16 (Oregon Trauma Registry) or ICISS ≤ 0.90 (California all-ED patients, California admitted patients). Resource use was defined as any major surgery (brain or spine, neck, thoracic, abdominal, vascular), ventilation > 96 hours, or blood transfusion. ROC AUC for each of the data sets was 0.88 (California all-ED patients), 0.74 (California all admissions), and 0.82 (Oregon Trauma Registry). AUC = area under the curve; ISS = Injury Severity Score; ICISS = ICD-9 Injury Severity Score; LOS = length of stay; ROC = receiver operating characteristic.

Figure 2

ROC curves for the discriminatory accuracy of hospital LOS to identify patients with serious injury or resource use, by age group. (A) ROC curves for overall LOS in children (0–14 years). ROC AUC for each of the data sets was 0.67 (California all-ED patients), 0.81 (California all admissions), and 0.82 (Oregon Trauma Registry). (B) ROC for overall LOS in adults (15–64 years). ROC AUC for each of the data sets was 0.86 (California all-ED patients), 0.74 (California all admissions), and 0.84 (Oregon Trauma Registry). (C) ROC for overall LOS in elders (≥65 years). ROC AUC for each of the data sets was 0.91 (California all-ED patients), 0.69 (California all admissions), 0.75 (Oregon Trauma Registry), and 0.68 (Medicare). The Medicare data set included patients ≥67 years. Serious injury was defined as ISS ≤ 16 (Oregon Trauma Registry) or ICISS ≤ 0.90 (California all-ED patients, California admitted patients, Medicare patients). Resource use was defined as any major surgery (brain or spine, neck, thoracic, abdominal, vascular), ventilation > 96 hours, or blood transfusion. AUC = area under the curve; ISS = Injury Severity Score; ICISS = ICD-9 Injury Severity Score; LOS = length of stay; ROC = receiver operating characteristic.