The role of induced sputum in the diagnosis of pulmonary tuberculosis

OBJECTIVES: To determine the sensitivity of referral guidelines for induced sputum in pulmonary tuberculosis (PTB) case detection and determine the diagnostic yield of the test. METHODS: A retrospective analysis of case notes of adult patients referred for induced sputum was carried out at the North Trent Regional Department of Infection and Tropical Medicine, the Royal Hallamshire Hospital, Sheffield, UK. Clinical characteristics were noted on patients referred during a 12-month period in 2001-2002, and of all patients with positive induced sputum results from April 1998 to March 2002. RESULTS: Over a 12-month period 114 patients were referred in our unit for induced sputum sampling. Twelve of the 114 patients had positive sputum cultures giving a positive yield for the test of 10.5%. With strict adherence to our referral guidelines, 10 of the 12 positive cases (83.3%) would have been detected. With modification to the referral guidelines, all 12 microbiologically positive cases would have been detected making the guidelines 100% sensitive in detecting microbiologically positive cases among patients referred with a clinical suspicion of PTB who are unable to self-expectorate. The positive yield for the test would have been increased to 13.2%. In the past 4 years 29 patients had positive induced sputum results. Of these, 11 patients (37.9%) had concurrent microbiological proven extra-pulmonary TB (EPTB) and 10 (34.5%) had past histories of TB infection. Seven patients had chest X-rays reported as normal. CONCLUSIONS: Induced sputum offers a relatively non-invasive investigation for patients with suspected PTB who are unable to bring up sputum. Induced sputum also offers a means of establishing a microbiological diagnosis in some cases of EPTB. An investigative algorithm is suggested.Summary. In an era of increasing drug resistant disease, establishing a microbiological diagnosis of TB is paramount. Sputum induction can be used as an alternative investigation to bronchoscopy in suspected PTB patients who are able to self-expectorate. Induced sputum is a sensitive investigation for suspected PTB patients but with a low diagnostic yield. Referral guidelines are suggested.