The Efficient Diagnosis of Tuberculosis

Abstract & Commentary

By Stan Deresinski, MD, FACP, Clinical Professor of Medicine, Stanford University; Associate Chief of Infectious Diseases, Santa Clara Valley Medical Center.
Dr. Deresinski serves on the speaker's bureau for Merck, Pharmacia, GlaxoSmithKline, Pfizer, Bayer, and Wyeth, and does research for Merck. This article originally appeared in the July 2007 issue of Infectious Disease Alert. It was peer reviewed by Connie Price, MD. Dr. Price is Assistant Professor, University of Colorado School of Medicine. She reports no financial relationships relevant to this field of study.

Synopsis: In patients with suspected pulmonary tuberculosis who are unable to expectorate sputum, culture of 3 induced sputum samples, all collected on the same day, is an effective and efficient means of diagnosis.

Source: Brown M, et al. Prospective study of sputum induction, gastric washing, and bronchoalveolar lavage for the diagnosis of pulmonary tuberculosis in patients who are unable to expectorate. Clin Infect Dis 2007; 44:1415-1420.

Brown and colleagues evaluated the most efficient means of microbiological diagnosis of tuberculosis in patients. Adults in whom the presence of pulmonary tuberculosis was suspected, based on the presence of compatible abnormalities on chest X-ray, who were unable to produce an expectorated sputum sample, were evaluated in order to determine the relative value of specimens obtained by sputum induction, gastric washing, and bronchoalveolar lavage (BAL). Also studied was the relative value of sputum induction on 3 consecutive days as opposed to obtaining all 3 samples on the same day, approximately 4 hours apart.

Three induced sputum specimens were obtained on a single day, followed by additional morning samples on 2 subsequent days. Analysis of the 79 patients from whom all 5 samples were obtained found that at least one of the 3 specimens collected on a single day were positive in 27 (34%) patients compared to a positive result in 29 (37%; P = 0.63) patients, in at least one of the 3 daily induced sputum specimens. There was no correlation between the volume of sputum obtained for testing and the results.

Twenty-one patients whose smears were negative underwent bronchoscopy, and BAL cultures proved to be positive in 5 (24%) — but all 5 had positive day-one induced sputum cultures. In addition, 2 individuals with positive day-one induced sputum samples had negative BAL cultures.

At least 3 induced sputum specimens and 3 gastric washing specimens were available for 107 of the 140 patients enrolled; Mycobacterium tuberculosis was recovered in cultures from one or more cultures obtained from 46 (43%) of the 107. At least one of the first 3 induced sputum samples obtained were culture positive in 42 (39%) patients compared to gastric washings from 32 (30%; P = 0.03) patients.

Commentary

This is a valuable study from which a number of conclusions regarding the diagnosis of pulmonary tuberculosis in patients unable to provide an expectorated sputum sample can be drawn:

  • Culture of 3 induced sputum samples was more sensitive than culture of 3 gastric washings;
  • Culture of BAL specimens did not contribute to the diagnosis obtained from the induced sputum samples;
  • Collecting induced sputum samples on consecutive days was not superior to obtaining all 3 specimens on a single day.

These results have practical implications for the diagnostic management of patients with suspected pulmonary tuberculosis. The referral center in the United Kingdom where this study was performed has, in fact, altered their procedures as a consequence of these results. In patients unable to expectorate sputum, they no longer perform gastric washings, and instead, obtained 3 induced sputum specimens — all on the same day. Bronchoscopy is performed only in limited circumstances. Thus, in most cases, their entire evaluation is performed in one day.