Is a Large Number of Sputum Specimens Necessary for the Bacteriological Diagnosis of Tuberculosis?

The recent increase in tuberculosis (TB) incidence (7) has further focused interest on efforts to design more effective diagnostic procedures to improve classical diagnostic methodologies for the disorder (1, 2, 5, 6). The recent paper by Nelson and coworkers (3), concerning the contribution of the number of specimens to the efficiency of TB diagnosis, prompted us to retrospectively review results of acid-fast bacillus (AFB) smears and cultures of patients diagnosed with culture-proven pulmonary TB at the Institute of Respiratory Diseases of Policlinico San Matteo, University of Pavia, Italy, in the period from 1989 to 1998, in order to assess the diagnostic benefit of analyzing three or more sputum or other respiratory specimens for the diagnosis of pulmonary TB.Table 1.Frequency distribution of the first positive specimen for patients for whom three or more specimens were collected for AFB smear and cultureFirst culture-positive specimenNo. of patients (%) that were:Smear positiveaSmear negativeEither†First46 (82)18 (64)64 (76)Second5 (9)8 (29)13 (16)Third or later5 (9)2 (7)7 (8)aAt least one specimen was smear positive.bBoth smear-positive and smear-negative specimens.