This test is done to diagnose tuberculosis and other diseases caused by Mycobacterium species. Mycobacteria are called acid-fast bacilli because they are a group of rod-shaped bacteria (bacilli) that can be seen under the microscope following a staining procedure where the bacteria retain the color of the stain after an acid wash (acid-fast). This test is ordered if an individual has symptoms of lung infection, like cough, fever with chills, night sweats and loss of appetite. Symptoms of extra pulmonary disease include back pain, anemia, headache or abdominal pain.