Hello Joann, I apologize for my delayed response to your question. A suseptibility test is usually done along with your sputem test. It is a test to determine which antibiotics will work on your particular specie of mac; there is now over 180 species. You need to ask your dr if such test was done and if it was, get a hard copy for your personal file. If it was not performed, then you need to insist on one with your next sputem test. Many private care pulmonologists do not take that extra step (suseptibility test) and just prescribe a broad spectrum antibiotic or the Big 3 as we call it. These meds won't work if your type of mac has a resistance to it.