Under-diagnosis of smear-positive pulmonary tuberculosis in Nairobi, Kenya

SETTING: Nairobi City Council Chest Clinic, Nairobi, Kenya. OBJECTIVE: TO determine if under-reading of sputum smears is a contributing factor in the disproportionate increase in smear-negative tuberculosis in Nairobi, Kenya. METHODOLOGY: Between October 1997 and November 1998, patients fulfilling th e local programme definition of smear-negative presumed pulmonary tuberculo sis were enrolled in the study. Two further sputum specimens were collected for examination in a research laboratory by fluorescence microscopy. RESULTS: Of 163 adult subjects enrolled, 55% were seropositive for the huma n immunodeficiency virus type 1 (HIV-1). One hundred subjects had had two p re-study sputum smears assessed before recruitment and produced two further sputum specimens for re-examination in the research laboratory; of these 1 9 (19%) were sputum smear-positive on re-examination and a further seven (7 %) became smear-positive on second re-examination. CONCLUSIONS: Of those patients with smear-negative presumed pulmonary tuber culosis by the local programme definition, 26% were smear-positive when ree xamined carefully with two repeat sputum smears. This suggests that the hig h rates of smear-negative tuberculosis being seen may in part be due to und er-reading. This is probably as a result of the overwhelming burden of tube rculosis leading to over rapid and inaccurate sputum examination. Retrainin g of existing technicians and training of more technicians is likely to red uce under-reading and increase the yield of smear-positive tuberculosis. Th is finding also stresses the need for regular quality assurance.