Reproducibility of sputum smear examination far acid-fast bacilli: practical problems met during cross-checking

SETTING: A TB control programme in Bangladesh with proficiency testing of s putum smears for acid-fast bacilli (AFB) using cross-checking of routine sm ears. OBJECTIVE: To document factors that may adversely affect repeatability of t he AFB smear, and which should be taken into account for interpretation of cross-checking. DESIGN: A number of simple experiments falling within the scope of small ro utine laboratories in a developing country. RESULTS: Fuchsin staining is not stable, fading quickly in direct sunlight and combinations of high temperature and humidity. Diffuse daylight, immers ion oil or xylene did not have the same effect. Contamination of smears by saprophytic mycobacteria may occur during staining or rinsing, with contami nants becoming visible only after restaining and cross-checking. Finally, A FB may be dislodged from smears during soaking in xylene or restaining, esp ecially if the smears are made from thin, liquefied sputum. CONCLUSIONS: These possible interfering factors should be taken into accoun t when organizing proficiency testing and interpreting its results.