Diagnostic pitfall: Mycobacterium avium complex pulmonary infection and positive ANCA.

We report the case of a 58-year-old female who presented with productive cough, weight loss, pulmonary nodular infiltrates and cavitations. She had a positive anti-neutrophil cytoplasmic antibodies (ANCA) test. A diagnosis of vasculitis was considered and a video-assisted thoracoscopic biopsy of the lung nodules was organised. However, prior to the biopsy, the sputum results revealed the presence of acid-fast bacilli, which were identified as Mycobacterium avium complex. A repeat ANCA assay was positive for atypical ANCA with negative proteinase-3 and myeloperoxidase titres. The patient was treated with rifampicin, ethambutol and clarithromycin with clinical and radiological improvement. The objective of this report is to highlight a rare association between positive ANCA titres and a non-tuberculous mycobacterial infection as a misdiagnosis and treatment of this patient with immunosuppressive therapy might have led to serious consequences.