For sputum smear to come positive on ZN staining there should be minimum:-
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.
100 bacilli per per ml sputum
1000 bacilli per ml sputum
2000 bacilli per ml sputum
10,000 bacilli per ml sputum
RNTCP