Diagnosing tuberculosis in a resource-poor setting: the value of sputum concentration

Diagnosis of tuberculosis in resource-poor settings is hampered by the insensitivity of the direct Ziehl-Neelsen (ZN) smear. Liquefaction and concentration of sputum before preparing a ZN smear has been proposed as a way of increasing diagnostic sensitivity. A field trial of this technique was done in a district hospital in South Africa among 166 consecutive tuberculosis suspects. Correlation between the 2 types of smear was high, but the extra cases diagnosed after concentration was offset by a similar number that, initially positive, were negative after concentration. Overall diagnostic sensitivity of smear microscopy was not increased by sputum liquefaction and concentration. The value of this technique may lie in combining it with direct microscopy. Limiting specimen examination to one per patient and making an initial direct smear with subsequent concentration only if the direct smear was negative increased sensitivity from 43% to 55% without any reduction in specificity. However, overall diagnostic sensitivity remained disappointing.