Hey there!
I've had MAC before (which is a combo of 2-3 different mycobacteria organisms), but now my main cultprit is M. Abscessus. My CF doc was the one who diagnosed it via sputum culture. They diagnose it via AFB smear (where they smear your sputum and do a special stain that stains Acid Fast bacteria...which is what mycobateria are). This test only tells that there are acid fast bacteria, there's a further sputum culture, I think, to determine which bacteria you actually have. Again, my CF doc was the one who ordered all of these. When M. abscessus came up every time I was sick (and every time meds wouldn't work), my CF doc involved an infectious disease doc. Now, they are in cahoots together. Sometimes my cultures are done by the ID doc, others are done by my CF doc. It might depend on your doc or hospital with how involved an infectious disease doc will get, at least if you're inpatient.
For me, the main diagnosing factor of mycobacterium were constant fevers. I was always a little feverish when I got sick, but one time I got SUPER sick with constant fevers above 102*F. There were some 104*F fevers, too, and boy did I feel awful! I've had M. abscessus since then, and am always feverish, but not as bad as that initial time. The only reason I ever have night sweats is when fevers break. I had some fevers with MAC, but again, not as bad or as frequent. The MAC was able to be cleared up pretty quickly with a couple IV courses and long term oral abx.
Feel free to PM me if you have other questions about all this, and I hope you get to feeling better! I also hope you aren't on the verge of joining the mycobacteria club! <img src="i/expressions/face-icon-small-wink.gif" border="0">