Scanty AFB smears: what's in a name?

Abstract

Setting: A tuberculosis control project in Bangladesh.

Objective: To document the frequency and diagnostic value of smears with scanty acid-fast bacilli (AFB) (IUATLD/WHO scale, < 10/100 high power fields), and to assess the appropriateness of the current positivity threshold.

Design: Analysis of databases of laboratory registers, patient records and the diagnostic yield of sputum collection strategies.

Results: Scanty smears constituted about 10% of suspect and almost 50% of follow-up smears. In suspect series, 10% of scanty 1-9/100 were not confirmed by another positive or scanty AFB sputum, compared to 7.5% of results at the current cut-off value of 10/100. Considering such results as positive by adopting a lower cut-off as low as the 1/100 used in the ATS scale added 1.5% false positives at the most. In return, the gain in confirmed positive cases was up to 10%, and that in positive results exceeded the incremental yield of the third diagnostic sputum. Significance of scanty follow-up smears at the end of the intensive phase was suggested by their association with treatment failure and unfavourable outcome overall.

Conclusions: Scanty results (IUATLD/WHO scale) are not rare and should not be ignored. Adoption of a considerably lower positivity threshold would be appropriate in control programmes where basic conditions for reliable AFB microscopy, including regular quality assessment, are present.