What's the purpose of having an acid-fast staining performed on a sputum sample from a patient exhibiting clinical signs of tuberculosis?

Active disease?: The presence of acid fast bacilli means that there is active disease requiring treatment. Three negative good sample sputums are necessary to assure cure for mycobacteria ( tuberculosis in this case). Acid fast staining is a simple way to detect the mycobacteria.

A confirmation...: Tuberculosis is a very significant comminicable infection and the treatments are often complex and long with potential of serious side effects, thus you don't want to treat unless the diagnosis is certain. Acid-fast staining is a biochemical process allowing the identification of this bug to confirm diagnosis. Treatment and prevention efforts are expensive/time consuming. Consult doc. Good luck.