UptoDate recommends that you cite the UpToDate topic as a chapter in a book titled UpToDate. There are no page numbers to cite, and the publication year for any topic should be the current year.
The example below is provided by UptoDate:
The UptoDate corporate recommendation does not necessarily follow the citation style guidelines that you may need for your class assignments or for publication of a manuscript or book. We recommend you cite Uptodate as a chapter in an ebook, modified to the appropriate style guidelines.
The APA's Publication Manual, 6th Edition should be consulted first in all matters concerning APA Style®.
According to the Purdue Online Writing Lab, to cite a Chapter/Section of a Web Document or Online Book Chapter in APA style when the chapter is in an Edited Book you would follow this template:
Author, A. A., & Author, B. B. (Year of publication). Title of chapter. In A. A. Editor & B. B. Editor (Eds.), Title of book (pages of chapter). Location: Publisher.
Using the below UptoDate topic below as an example, here is the recommended APA citation format you would follow when you need to cite three to seven authors:
Borody, T.J., Leis, S., Pang, G., & Wettstein, A.R. (2013). Fecal microbiota transplantation in the treatment of recurrent Clostridium difficile infection. In T.W. Post, P. Rutgeerts, & S. Grover (Eds.), UptoDate. Available from http://www.uptodate.com/contents/fecal-microbiota-transplantation-in-the-treatment-of-recurrent-clostridium-difficile-infection?source=search_result&search=fecal+transplant&selectedTitle=1~7
Finally, in regards to the recommended editor, while UptoDate lists a specific individual on their website that they would like to credit, this individual is not necessarily listed as an editor on the UptoDate topic pages. In deference to the company, we recommend listing this individual as one of the editors in your citation. But we believe that authorship and editorship should be attributed to all names on an UptoDate topic. Our example citation reflects this guideline.
For more information, the rules for citing editors are listed in Section 6.27 Author and Editor Information of the APA's Publication Manual, 6th Edition.
Note: The Purdue OWL site also states that in the case of electronic books, "If the work is not directly available online or must be purchased, use "Available from," rather than "Retrieved from", and point readers to where they can find it. For books available in print form and electronic form, include the publish date in parentheses after the author's name." Since UptoDate is a licensed resource, not available to the general public, the library example reflects this recommendation.