Table 3.

Association Between Transportation Method and the Outcome of Survival to Discharge

	OR (95% CI)	P Value	ARR (95% CI), %a
Level I centers (n = 159 511)
Unadjusted mortality	0.88 (0.85–.90)	<.001	1.6 (1.4–1.7)b
Logistic regression
Standard	1.31 (1.27–1.38)	<.001	2.9 (2.6–3.6)
Controlled for clusteringc	1.32 (1.20–1.45)	<.001	3.0 (1.9–4.2)
After propensity score matchingd	1.16 (1.14–1.17)	<.001	1.5 (1.4–1.6)
Level II centers (n = 63 964)
Unadjusted mortality	0.86 (0.83–0.91)	<.001	1.7 (1.4–1.8)b
Logistic regression
Standard	1.37 (1.28–1.48)	<.001	4.3 (2.6–4.5)
Controlled for clusteringc	1.37 (1.23–1.53)	<.001	3.4 (2.2–5.0)
After propensity score matchingd	1.15 (1.13–1.17)	<.001	1.4 (1.3–1.6)

Abbreviations: ARR, absolute risk reduction; OR, odds ratio.

^aAbsolute RR calculations were made after calculating the number needed to treat (NNT), based on the adjusted ORs.²⁵ NNT=1− (PEER × [1−absolute OR])/(1−PEER) × PEER × (1−OR)]. PEER is the patient expected event rate (eg, the event rate in the ground transport group [control group]. ARR=1/NNT.

^bIndicates greater mortality for transportation by helicopter.

^cGeneralized estimating equations with robust variance calculations were performed. Independent variables included systolic blood pressure, heart rate, respiratory rate, motor component of Glasgow Coma Score, e-code, age, type of trauma, injury severity score, and sex. E-codes refer to types of external injuries according to the International Classification of Disease, Ninth Edition, Clinical Modification.

^dLogistic regression was performed following estimation of a propensity score with the subclassification matching method. The covariates used to estimate the propensity score were systolic blood pressure, heart rate, respiratory rate, motor component of Glasgow Coma Scale score, e-code, age, type of trauma, injury severity score, facility identifier, and sex.