For sputum smear to come positive on ZN staining there should be minimum:-
High-Yield Explanation
Sputum smear microscopy for tubercle bacilli is positive when there are at least 10,000 organisms present per ml of sputum. ZN staining in RNTCP: Acid fast bacilli of TB; 'Rod shaped' with 'beaded appearance'. >10,000 bacilli per ml sputum must be present for a positive result.