A presumptive diagnosis is commonly based on the finding of AFB on microscopic examination of a diagnostic specimen. At least 10,000 acid fast bacilli should be present per ml of sputum to be readily demonstrable in direct smears. A negative repo should not be considered till at least 300 fields have been examined, taking about 10 minutes. A positive repo can be taken only if two or more typical bacilli have been seen. Ref: Textbook of Microbiology By Ananthanarayan And Paniker, 7th edition, Page 357.