A comparative study on the reliability of the fluorescence microscopy and Ziehl-Neelsen method in the diagnosis of pulmonary tuberculosis.

Pulmonary tuberculosis (PTB) is the most common presentation of tuberculosis (TB) in Kenya. For the diagnosis of PTB the sputum smear is used because it is technically simple, non-invasive and cheap. The reliability of direct smear examination for the diagnosis of TB has however frequently been questioned. To address this problem, a study comparing the reliability of fluorescence microscopy (FM) and Ziehl-Neelsen (ZN) staining method for examination of direct smear in the diagnosis of PTB was carried out at the Respiratory Disease Research Unit Laboratory, Nairobi, Kenya. A total of 1480 sputum specimens collected from patients with suspected PTB were analyzed. Two direct smears were prepared from each specimen, one stained using FM and the other using the ZN method. Culture results were used as the gold standard for assessment. Specificity was 97% and 96% for FM and ZN methods, respectively. The sensitivity of the FM method was 80% and that of the ZN method 65% (p < 0.001). Overall agreement was 86.8%. Positive smears which were missed on the ZN stained smears (15%) contained low density bacilli on both FM stained smears and on culture. The use of FM greatly improves the diagnostic value of the sputum smear especially in patients with a low density of bacilli who are likely to be missed on ZN stained smears. The method is economical in both time and expense and is recommended for laboratories handling large numbers of sputum specimens.