Cross-contamination of specimens with Mycobacterium tuberculosis: clinical significance, causes, and prevention

At the Veterans Affairs Lakeside Medical Center, two episodes of specimen cross-contamination with Mycobacterium tuberculosis were detected during a 54-month period by molecular strain typing using DNA restriction fragment length polymorphism for 3 patients without clinical or radiologic signs of tuberculosis (TB). A cross-contaminated specimen was the only culture-positive specimen for each of the 3 patients. Laboratory features of cross-contamination included acid-fast smear negativity, growth only in broth or solid medium, and growth in solid medium with 5 or fewer colonies. Retrospective analysis demonstrated identical features for occasional culture-positive specimens from 54 patients with TB during the same period. However, productive cough, pleural pain, weight loss, night sweats, chest radiograph results suggestive of TB, positive tuberculin skin testing, and/or multiple culture-positive specimens were invariably present in patients with TB with such specimens. Most patients with TB (50/54; 93%) had multiple specimens positive in culture for M. tuberculosis, and the few patients with TB with single culture-positive specimens were symptomatic. These results indicate that correlation with clinical manifestations is necessary to determine the significance of isolated, acid-fast smear negative, and/or low-yield culture-positive specimens. Although the prevalence of specimen cross-contamination is low (0.1%), possible sources (especially the use of single-reagent delivery systems for multiple specimens) should be eliminated by mycobacteriology laboratories.