The investigation of pleural effusion of unknown cause may include analysis of pleural aspirate for presence of acid-fast bacilli (AFB) by smear and culture. We reviewed data on all pleural aspirates sent for AFB analysis over 11 years (January 2000 to December 2010) to identify the diagnostic yield of pleural aspirate AFB smear and culture in our hospital where there is a low incidence of tuberculosis (TB). Data were crosschecked with the TB notification list obtained from the Consultant in Communicable Disease Control (CCDC) to ensure identification of all tuberculous effusions. A list of all AFB positive specimens (including smears, cultures and histology) was also obtained from the pathology laboratory. The medical records of patients with AFB positive aspirates were reviewed. We also reviewed the medical records of patients with AFB negative pleural effusion who were diagnosed to have TB by other means. In total, 960 pleural aspirate samples were sent for AFB analysis. None of these were smear positive and only 13 (1.4%) were found to be positive on cultures. The ethnic breakdown of this figure was one, five, and seven cases for Asian, Caucasian, and Afro-Caribbean patients, respectively. Five of these patients were known or found to be HIV positive, all of whom were Afro-Caribbean. Eight of the 13 patients with positive pleural aspirate cultures underwent pleural biopsy (three by thoracoscopy), all of which confirmed a diagnosis of TB. One patient with positive cultures of pleural aspirate also had TB confirmed on culture of bronchial washings. A further ten patients (1%) whose pleural aspirates were AFB negative on smear and culture were diagnosed with tuberculosis by other means. The yield of AFB analysis on pleural aspirate is very low. Its role in commencing treatment in those who ultimately are diagnosed to have tuberculosis is limited. Risk factors for tuberculosis need to be considered before sending aspirate for AFB analysis. Where risk of TB is considered to be significant, pleural fluid aspiration should be combined with simultaneous pleural biopsy, as the latter may provide crucial diagnostic information at an earlier stage.
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