According to a recent six-state study of adults with emergency department visits

8.2% of the patients revisited an emergency department within 3 days of their initial visit, with 32% of those revisits occurring at a different institution.

Source: "Revisit Rates and Associated Costs After an Emergency Department Encounter: A Multistate Analysis," Annals of Internal Medicine, abstract only, June 2, 2015, http://annals.org/article.aspx?articleid=2299853