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Optimal tuberculosis case detection by direct sputum smear microscopy: how much better is more?
A Van Deun1, A Hamid Salim, E Cooreman
1Mycobacteriology Unit, Institute of Tropical Medicine, Antwerpen, Belgium. avdeun@microbiol.itg.be
Summary
Screening 100 microscopic fields per sputum smear is sufficient for tuberculosis diagnosis. A strategy using two morning sputum samples for negative suspects, with diagnosis based on a single positive result, is efficient and recommended.
Area of Science:
- Public Health
- Infectious Disease Control
- Microscopy
Background:
- Tuberculosis (TB) diagnosis relies heavily on sputum smear microscopy.
- Optimizing diagnostic efficiency is crucial for effective TB control programs, especially in resource-limited settings.
Purpose of the Study:
- To evaluate the diagnostic yield of screening different numbers of microscopic fields per sputum smear.
- To determine the efficiency of various sputum collection schemes for TB diagnosis.
Main Methods:
- A tuberculosis control project in Bangladesh utilized quality-controlled sputum smear microscopy.
- Cumulative acid-fast bacilli counts per 100 fields were recorded.
- Incremental diagnostic yield of different sputum sampling strategies was analyzed.
Main Results:
- Screening the first 100 fields detected 99.6% of positive and 79.3% of scanty slides.
- A third sputum specimen yielded only a 2.7% incremental increase in positive cases.
- The positive predictive value of a single positive or scanty smear was high (99.2%).
Conclusions:
- Screening beyond 100 fields per smear offers diminishing returns.
- Examining only morning sputum samples is more efficient and practical.
- A strategy of examining two morning sputum samples for negative suspects, with diagnosis based on one positive smear, is proposed.