Mycobacteria is a slow-growing bacteria, which is why 2 tests are run. A "negative smear" just means there was not enough bacteria in the "smear" - that is on the slide - to grow out quickly in the lab - that is 24-48 hours. But there was enough in the sample to grow out on a culture plate, which is grown for up to 6-8 weeks in a special lab.
Usually the next step is to run "sensitivity testing" - applying different antibiotics to the culture - to see what will kill the MAC. In a perfect world, the "colony count" is also used on the culture to determine how much infection is present - but many labs are not equipped to do this.
Have you discussed the results with your doctors yet?
Sue