How many sputum smears are necessary for case finding in pulmonary tuberculosis?

We reviewed the laboratory registers of 42 tuberculosis (TB) diagnostic centres in the southern region of Ethiopia to determine the value of submitting serial sputum samples for the diagnosis of pulmonary TB (PTB) and estimate the proportion of suspects that are smear positive. A total of 15,821 TB suspects submitted three smears each (47,463 smears) in 2000 with a median of 228 per centre. The smear positivity rate (two or more positive smears) was 25%, with a range of 16.8-36.4% per zone. This exceeds the international recommendations of examining 10 suspects to identify one case. A total of 4099 (26%) of the suspects had at least one positive smear with 3753 (91.6%) of the first specimens being positive. A further 303 (7.4%) were negative in the first specimen but had a positive second specimen and 42 (1%) suspects had two negative specimens followed by a positive third smear. The value of the third sputum is negligible as 99% of the cases were identified from the first and second specimens. Reducing the number of specimens to two or even one would have multiple advantages in countries where laboratories are usually over-burdened and are not easily accessible to the population. Submission of two specimens on the same day could improve compliance in submitting samples and collecting results as the number of diagnostic visits would be reduced without significant loss of sensitivity.