Contamination of stored sputum AFB smears with environmental mycobacteria.

To check the quality of stains and the acid-fast bacilli (AFB) staining procedure, a sample of 73 AFB-negative sputum smears selected systematically from a microscopy centre was rechecked before and after restaining with the same AFB staining method at a reference laboratory. Respectively 0 and 30 showed AFB before and after restaining. Detection of AFB after restaining in 30 of 73 negative smears triggered suspicion of contamination with environmental mycobacteria, which was confirmed by reexamination of these slides. Restaining before rechecking is a major limiting factor, and the results of external quality assessment should be ignored when contamination of slides is suspected.