The AFB smear is the first look a lab takes at a sputum sample being tested for TB or NTM. TB and NTM are acid fast bacteria (AFB). A thin layer of the sputum sample is smeared on a slide and then colored with a dye that is taken up by the bacteria. Then the slide is treated with an acid which bleaches out most of the bacteria. The ones that hold the color are “acid-fast” and are the TB and NTM. When the smear is reported as positive, usually with the number of organisms seen on a high power field, it means the bacterial load is relatively high. When the smear is negative, it typically means the bacterial load is lower, although there may still be enough for a positive culture. The lab can’t tell from a positive smear which specific organism you have. That can only be determined when a large enough quantity of the bacteria have grown in the culture.