AFB refers to a test done to identify the bacterium that causes

An acid-fast bacteria (AFB) culture is done to find out if you have tuberculosis (TB) or another mycobacterial infection. Besides TB, the other main mycobacterial infections are leprosy and a TB-like disease that affects people with HIV/AIDS.

To do an AFB culture, a health care provider takes a sample of phlegm or sputum you're coughing up or a tiny bit of your tissue. They "culture" it by putting it in a special container with food the bacteria need to grow. They then check it over a few weeks to see whether the bacteria grow. If they do, you have a mycobacterial infection.