@susiellen Well, hello and welcome to the world of posters on Mayo Connect after more than 5 years here, I'm glad to hear from you. That diagnosis would bring me on-line as well.
Here is the difference between MAC (Mycobacterium avium complex) and Abcessus (Myobcateria abcessus) - MAC is slow-growing, and delaying treatment for a few months is generally not a problem unless you are very ill or have cavities in your lungs. So doctors wait for weeks while the culture is grown out in the lab and tested to see what antibiotics it responds to before starting treatment. Abcessus, on the other hand, can progress quite rapidly, especially if you are older or have other underlying conditions or a weakened immune system. Doctors are inclined to begin antibiotics immediately, and adjust as the results come back.

All that said, I have a few questions for you - is your pulmonologist experienced in treating MAC? This is not a reflection on their abilities - just that MAC quite rare - under 100,000 cases nationwide per year, and far fewer cases of Abcessus. So many pulmonologists never, or hardly ever see it, and may not be following the latest protocols.
How often have you had sputum cultures, and why was this one ordered? Routine followup, or a change in your health? Do you know where it was sent for culturing?
Finally, how are you actually feeling? Cough, fatigue, weight loss, fever?

I look forward to talking to you.
Sue