Cat scan results

MAC grows in sputum and the definitive test for it is a sputum culture. If it's positive, sensitivity tests should be done to find out which drugs will be effective.
The CT can show suspicious signs of MAC infections, like tree-in-bud, mucoid impactions, etc, but sputum cultures are required to confirm it. I was thought to have MAC by local and Mayo's pulmonologists based on CT's, but the cultures (gold standard) were negative.
The CT's do show bronchiectasis.

CT scan can indicates the severity of MAC and further damage such as cavitation. The read can determine the course of treatment

2 out of 4 sputum samples submitted in March were positive for MAC. Additional sputum samples were submitted in May, awaiting results. Chest Xray was unremarkable. At this point I don't think CT scan necessary until latest sputum results are reported. Yet my pulmonologist suggested CT scan. (Had scan two years ago which was negative for MAC issues or bronchiectasis.)
Am I right in holding off on CT scan?

I'd be receptive to a CT after 2 years, esp since it was negative and you're having positive cultures now. Some places do them more frequently. Other opinions may vary, but I like knowing if things have changed and CT's show that better than x-rays.

Are you doing any airway clearance?

Thanks for your comment. Yes, 2x daily nebulizing with Saline and 2x day with cromolyn sodium. I'm super sensitive to steroids (inhaled or nebulized) which limits the meds I can tolerate. Of late, my doc suggests Fasenra due to the high level (900) of eosinophils that showed in blood work. I'm undecided because this injected biologic decreases white blood cells which in turn weakens immune system, leaving you prone to infections. I don't want to be liable for bugs taking up residence in my lungs!

If two of four samples are positive for MAC it seems you have met the lab requirement for diagnosis of MAC via sputum sampling. I would want a CT at this point to see how much if any damage has been done by the MAC, which is central in guiding the decision whether to treat and how to treat, as Rick points out. As someone who was asymptomatic, I would never have done the sputum sampling if I hadn’t had a CT in the ER. My chest X-ray in the ER showed infiltrates which is why we did the CT, but as I have posted here before, at the time I would have said there was nothing wrong with my lungs so it was all quite surprising. I have more advanced disease, which may be why something showed on my X-ray, but X-rays often, maybe usually, do not pick up BE. So, I would not assume no damage from the MAC with a normal xray. Again, I would get the CT.

Thanks for your input. Have August appointment when I'll ask doctor to order CT.

Personally, I would message and tell doc you're now amenable to getting CT. That way, you'll be ready to make a plan with doc at your Aug appt. MAC is slow growing, but that's a couple months away. (I tend to want a plan though.)
I just looked up Fasenra. It sounds promising. I wish you luck in making these decisions. Take care and keep us posted.