Q. If I am using APA, do websites such as UpToDate and Medline plus count as legitimate resources?

MedLine is created by the National Library of Medicine, which would be considered a legitimate source.

UpToDate is a subscription service, so I was not able to look at the content. Legitimacy, or as we used to call it "authority" is going to be based on the sources of the information you are receiving from UpToDate. The service is created by Wolters Kluwer, a respected publisher. My guess is that they are pulling and condensing for the subscriber various research on a particular topic, a bit like the old Reader's Digest magazine.

So, yes, I would think it is a legitimate source. When you read an update look at the authorship of the articles that are being referenced. Are they coming from journals or other legit formats of research? If so, then you should be fine.