AMA does not give a specific format for UpToDate citations, however, UpToDate recommends citing topics "as a chapter in a book titled UpToDate, edited by Ted. W. Post, published by UpToDate in Waltham, MA. There are no page numbers to cite, and the publication year for any topic should be the current year."
Author(s). Title of topic review. In: Post TW, ed. UpToDate. UpToDate; Current Year. Accessed [date]. URL