Understanding ambulance utilization patterns is essential to assessing prehospital system capacity and preparedness at the national level.

To describe the characteristics of patients transported to US emergency departments (EDs) by ambulance and to determine predictors of ambulance utilization.

Data were obtained from the National Hospital Ambulatory Medical Care Survey using mode of arrival, demographic and visit information, ICD-9-CM E and V-codes, and classified reasons for the visit.

The rates for ED visits of persons conveyed by ambulence were stable between 1997 and 2003, consisting of approximately one in every seven ED visits (14%). In 2003, there were 16.2 million ED visits for which an ambulance was used in the US. However, for patients with mental health visits, nearly one in three ED presentations (31%) arrived by ambulance. Significantly higher rates of ambulance use were associated with: (1) mental health visits; (2) older age; (3) African-Americans; (4) Medicare or self-pay insurance status; (5) urban ED location; (6) US regions outside of the South; (7) presentation between 12 midnight to 0800 hours; (8) injury-related visits; (9) urgent visit status; and/or (10) those resulting in hospital admission. Among mental health patients, older age, self-pay insurance status, urban ED location, regions outside the southern US, and urgent visit classification predicted ambulance use. Ambulance usage within the mental health group was highest for suicide and lowest for mood and anxiety disorder-related visits.

Reliance on ambulance services varies by age, insurance status, geographic factors, time of day, urgency of visit, subsequent admission status, and type of mental health disorder. Even after controlling for many confounding factors, mental health problems remain an important predictor of ambulance use.