Clinical significance ofmycobacterium avium-complex cultures: A retrospective review 2006-15

BACKGROUND: Mycobacterium avium -complex (MAC) is considered the most frequent cause of non-tuberculous mycobacterial disease (NTMD). However, contaminants in the laboratory are common. Diagnostic criteria to define NTMD are: abnormal chest x-ray with positive cultures from 2 separate sputum samples or one bronchoalveolar lavage and exclusion of tuberculosis and other diagnoses. Our aim was to evaluate the clinical significance of a positive MAC culture. METHODS: All MAC cultures were identified from 2006-15. Chest radiographs were reviewed. Number and source of the cultures identified. NTMD was established by ATS/IDSA 2007 criteria. Treatment and outcome were audited against these recommendations. RESULTS: 97 samples from 95 individuals had a positive MAC culture; 47 had a normal chest x-ray. 16 had >2 positive sputum cultures and one positive BAL; there were also two positive blood cultures, two positive lymph node aspirates and one positive bone marrow. Even within these 17, 5 had a normal chest radiograph, 3 grew another mycobacterial species and had clinical features which excluded NTMD, one had PCP, one had heart failure, one pneumonia which cleared with standard antibiotics and one had mediastinal lymphadenitis which cleared spontaneously. 5 patients with pulmonary NTMD, according to ATS/IDSA 2007 criteria were offered treatment and 3 accepted. Sputum conversion was achieved at 6 months in one and at 12 months in another; the third began treatment with anti-retroviral therapy which resolved his MAC infection. CONCLUSIONS: Most MAC positive cultures were contaminants, and only a small percentage had proven MAC disease. Reference laboratories are likely to overestimate MAC disease.