Sorry to break it to you, but there is no particular field that is 'Diagnostics'. All fields in medicine involve diagnosis of disease, even the surgical fields.
The thing that comes closest to 'Diagnostics' is probably General Internal medicine. However in the real world >90% of IM cases are things that are relatively commonplace, rather than the exotic things you see on House. Hypertension, diabetes, COPD, CHF. The emphasis in the real world is to see as many patients as possible in a day. 30-40. No physician could realistically spend days on a single case.
No physician can break into someones home to get a better history neurosurgeons don't do appendectomies. The things you see on 'House' might POSSIBLY represent the practice of medicine in a parallel universe, but it is not even close to reality.
If you really like the idea of diagnosing rare diseases, look into being a pathologist at the Armed Forces Institute of Pathology. It is the supreme court of pathology (perhaps the most 'diagnostic' of fields). They have the tissue samples from cases with contested diagnoses, and extremely rare cases with only a few published reports.
Ahhhh... my lucky day. A duplicate asanine post deserves a double helping of cyto's sarcasm...
You actually don't even need to go to medical school. A diagnostician is the person who comes to your medical school when you are a second year and sells you an otoscope. All you need is your GED... and you're in college. What a waste!
Yes, I have seen "diagnosticians"......but only with fairies, unicorns, and lepricons!
Sorry, had to say that. I know, I am bad.
On a serious note, no...there are no doctors like "House MD" in the real world. Medicine is very specialized..even sub-specialized. All MDs are diagnosticians in their own speciality.
The closest thing to what you describe is probably Family Medicine (maybe even Internal Medicine, and Emergency Medicine). But then again, you rarely see "zebras", mostly "horses".
Pathologists are the "doctor's doctor", and all they do is diagnose all day, but again...they are not clinicians, they usually do not see and treat live patients. They diagnose disease by looking at, and analysing, tissue samples in the lab or morgue.
In real world medicine, all MDs see and treat common conditions....Thank God for that. If this was not the case, the job of the MD as a healer will be much more difficult, and frustrating.