Citing UpToDate in APA:
According to the instruction from UpToDate, "Cite the UpToDate® topic as a chapter in a book titled UpToDate, edited by Denise S. Basow, 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." Since UpToDate is an online resource, treat it as a chapter in an e-book. The rule for citing E-book chapter in APA is:
Author, A. (date). Title of chapter. In E. Editor (Ed.), Title of book (pp. xx–xx). Retrieved from http://xxxxxxxxx
Here is an example:
File, T.M. (2013). Treatment of hospital-acquired, ventilator-associated, and healthcare-associated pneumonia in adults. In D.S. Basow (Ed.), UpToDate. Retrieved from http://www.uptodate.com/home/index.html