With the little bit of knowledge I have it is my understanding that to test for a type of MAC infection there are two ways.
First is a good release of 'sputum' into a medical sputum vile that will get sent into a lab for testing for a infection in the lungs. You release, hopefully, sputum into the vile by coughing/huff coughing your sputum into the vile, possibly done after an airway clearance technique, into the medical vile. It is best done first thing in the morning before any food intake.
Second is what you are scheduled for, the bronchoscopy which will help you and the doctor to understand all somewhat better.

If there is another way of testing for a lung infection or any corrections or additions to this post , hopefully another one of our posting BE buddies who are long time BE patients etc. will post this information.

Your being misdiagnosed is not uncommon as you will see on the threads/posts in Mayo Clinic Connect.
I was misdiagnosed, luckily for only about a year and finally due to my having lost weight the C Scan was ordered and told the whole story, BE, Bronchiectasis. The MAC/MAI infection was diagnosed with my visit to NJH a little over a year after the local C Scan test that said BE.
Barbara