Improved sensitivity of direct microscopy for detection of acid-fast bacilli in sputum in developing countries

Diagnosis of pulmonary tuberculosis depends on the bacteriological examination of sputum. Sputum smear microscopy is efficient and can confirm the disease. However, direct microscopy of sputum, though rapid, has low sensitivity. There is, therefore, an urgent need to develop rapid, much more sensitive and specific methods. In a field study we collected sputum samples from 488 tuberculosis suspects and compared the results of examining smears prepared after treatment with sodium hypochlorite (NaOCl) and concentration of bacteria by centrifugation and direct staining with Ziehl-Neelsen stain. Direct smears stained with auramine-phenol were the reference standard. The use of NaOCl increased the sensitivity from 43.4% to 76.3%, with a specificity of 100% for both methods. The method is simple and cheap. As a potent disinfectant, NaOCl also reduces the risk of laboratory-acquired infection. Its application would increase the efficiency of tuberculosis control programmes.