Reducing the number of sputum samples examined and thresholds for positivity: an opportunity to optimise smear microscopy
OBJECTIVE: 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.
DESIGN: In this prospective study, three Ziehl-Neelsen stained sputum smears from consecutive pulmonary TB suspects were examined blind. The standard approach (A), ≥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, ≥2 positive smears (≥4 AFB/100 HPF) out of 3, one of which is ≥10 AFB/100 HPF; C, ≥2 positive smears (≥4 AFB/100 HPF) out of 3; D, ≥1 positive smear (≥10 AFB/100 HPF) out of 2; and E, ≥1 positive smear (≥4 AFB/100 HPF) out of 2. The microscopy gold standard was detection of at least one positive smear (≥4 AFB/100 HPF) out of 3.
RESULTS: 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.
CONCLUSION: Reducing the number of specimens and the positivity threshold to define a positive case increased the sensitivity of microscopy and reduced laboratory workload.
Keywords: diagnosis; microscopy; tuberculosis
Document Type: Regular Paper
Affiliations: 1: Epicentre, Paris, France 2: Liverpool School of Tropical Medicine, Liverpool, UK 3: Centre for Respiratory Diseases Research, Kenya Medical Research Institute, Nairobi Kenya 4: Médecins Sans Frontières, Paris, France
Publication date: 01 September 2007
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