Investing time in microscopy: an opportunity to optimise smear-based case detection of tuberculosis

St. Elizabeth Hospital, rural North-west Cameroon, an area with high tuberculosis (TB) and human immunodeficiency virus (HIV) incidence.

To measure the time spent during routine sputum smear microscopy and assess whether reexamining slides for 10 min translates into higher case detection of smear-positive cases.

A prospective observational study over a 6-month period with three components: 1) timing of routine sputum smear examination; 2) blinded reexamination of all slides for 10 min and results compared with initial readings; and 3) blinded re-examination, by the original microscopists, of a portion of false-negative slides mixed with true negatives for 10 min.

A total of 204 patients submitted 612 sputum specimens for screening. The median routine examination time was 2 min 6 seconds (interquartile range 1:30-2:30). A 10 min examination significantly increased the number of positive smears from 82 to 116 (P = 0.0083), and overall case detection from 28 to 48 patients (P = 0.011). On review by the original readers, more than half of the false-negative slides were reported as positive after 10 min.

Sputum smear microscopy has low sensitivity if performed too quickly, and 10 min re-examination significantly increases case detection. Ensuring that smears are examined for the recommended duration may be a simple and low-cost way to improve case detection.