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There is no specific example for citing an UpToDate topic in the AMA Manual of Style 11th edition. However, UpToDate can be cited just like how you would cite a book chapter from an eBook, except there will be no page numbers. UpToDate would count as the title of the book.
Wheeler SG, Wipf JE, Staiger TO, Deyo RA, Jarvik JG. Evaluation of low back pain in adults. In: Atlas SJ, Kunins L, eds. UpToDate. UpToDate Inc; 2020. Updated June 25, 2019. Accessed April 23, 2020. https://www.uptodate.com