EUH Resident Report: Gastric AFB

The Bottom Line: AFB smear of GA is a relatively insensitive but highly specific indicator of pulmonary tuberculosis warranting institution of antituberculosis treatment. Gastric AFB smear positivity appears to reflect a high bacillary burden within the respiratory tract.

Reference: Bahammam A., Choudhri S., Long R. “The validity of acid-fast smears of gastric aspirates as an indicator of pulmonary tuberculosis.” The International Journal of Tuberculosis and Lung Disease 3.1 (1999): 62-67.

Summary:  To establish a definitive diagnosis of pulmonary tuberculosis, Mycobacterium tuberculosis must be isolated from the respiratory tract. This usually involves examination of a sputum sample, but when the patient cannot produce sputum, one must resort to alternative diagnostic procedures such as gastric aspiration (GA) or induced sputum.

From 1994 to 1996 inclusive, 1155 GA were performed in 889 patients at a TB referral hospital in Canada. Mycobacteria were cultured from 109 (9%) GA. Thirteen of these were positive on smear (sensitivity 19%). All GA that were positive on smear were culture positive for Mycobacterium tuberculosis. There were no false positive smears (specificity 100%). The sensitivity and specificity of the sputum smear were 45% and 99%, respectively. Of the 96 culture positive, smear negative GA, 54 grew M. tuberculosis and 42 grew an NTM. Of 13 patients who had sputum and GA studied coincidentally, and in whom the sputum was both smear and culture positive, the GA culture was positive in 13 and the smear was positive in eight (66%).

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