Efficiency of serial smear examinations in excluding sputum smear-positive tuberculosis

This study determined the number of slides required to identify one additional case of sputum smear-positive tuberculosis (TB) from the third smear. The study hypothesis was that not more than 100 and 75 slides, respectively, in Mongolia and Zimbabwe, need to be examined to find one additional case of TB with a third serial diagnostic sputum smear examination.

In a retrospective, record-based study, data were abstracted from TB laboratory registers from all 31 laboratories in Mongolia and 23 randomly selected laboratories in Zimbabwe using a uniform EpiData collection instrument.

A total of 52,909 records of examinees were available. In Mongolia, of 15 103 suspects, 1717 (11.4%) were positive. Of these, 0.7% were positive for the first time on the third smear examination. In Zimbabwe, of 25 693 suspects, 3452 (13.4%) were positive and 4.5% were positive only on the third smear examination. The expected number of slides required to detect one additional case on the third examination was 1153.3 for Mongolia and 132.6 for Zimbabwe.

The requirement of routine examination of three serial smears before declaring a suspect as a 'non-case' (of sputum smear-positive TB) will need to be reviewed in both Mongolia and Zimbabwe.