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.