[1] Univ Penn, Sch Med, Dept Pediat, Philadelphia, PA 19104 USA
[2] Univ Penn, Sch Med, Dept Emergency Med, Philadelphia, PA 19104 USA
[3] Univ Penn, Sch Med, Dept Biostat & Epidemiol, Philadelphia, PA 19104 USA
[4] Univ Penn, Sch Med, Ctr Clin Epidemiol & Biostat, Philadelphia, PA 19104 USA
[5] Childrens Hosp Philadelphia, Div Emergency Med, Philadelphia, PA 19104 USA
[6] Med Coll Wisconsin, Dept Pediat, Sect Pediat Emergency Med, Milwaukee, WI 53226 USA