One thing I'd like to address here is that this question inherently implies that House MD is somehow representative of real medicine. This is a topic I've had to breach with hundreds of students over the past 10 years when I was still teaching chemistry...

House is about as far from real medicine as you can get. The number of times his medical license would have been revoked is very high. He uses questionable methods and skips over very basic diagnostics on a predictably regular scale. The show puts that in the light of him being outspoken and clever, but in real life that would just be bullheaded stupidity.

Don't expect there will always be a "real" explanation for what he's doing, because there often isn't. Some comments here have brought up subdermal birth control for the scene in question, and maybe that was the intent... But there's a dozen other ways that diagnosis could have been confirmed that are much more scientifically sound.