Reducing the number of sputum samples examined and thresholds for positivity: an opportunity to optimise smear microscopy

Urban health clinic, Nairobi.

To evaluate the impact on tuberculosis (TB) case detection and laboratory workload of reducing the number of sputum smears examined and thresholds for diagnosing positive smears and positive cases.

In this prospective study, three Ziehl-Neelsen stained sputum smears from consecutive pulmonary TB suspects were examined blind. The standard approach (A), > or = 2 positive smears out of 3, using a cut-off of 10 acid-fast bacilli (AFB)/100 high-power fields (HPF), was compared with approaches B, > or = 2 positive smears (> or = 4 AFB/100 HPF) out of 3, one of which is > or = 10 AFB/100 HPF; C, > or = 2 positive smears (> or = 4 AFB/100 HPF) out of 3; D, > or = 1 positive smear (> or = 10 AFB/100 HPF) out of 2; and E, > or = 1 positive smear (> or = 4 AFB/100 HPF) out of 2. The microscopy gold standard was detection of at least one positive smear (> or = 4 AFB/100 HPF) out of 3.

Among 644 TB suspects, the alternative approaches detected from 114 (17.7%) (approach B) to 123 cases (19.1%) (approach E) compared to 105 cases (16.3%) for approach A (P < 0.005). Sensitivity ranged between 82.0% (105/128) for A and 96.1% (123/128) for E. The single positive smear approaches reduced the number of smears by 36% compared to approach A.

Reducing the number of specimens and the positivity threshold to define a positive case increased the sensitivity of microscopy and reduced laboratory workload.