Challenge of a false-positive acid-fast bacilli: a diagnostic conundrum

Infectious and autoimmune diseases are distinct entities that require opposite therapeutic approaches. However, differentiating between the two can be a challenge, especially when the histopathology misguides the clinician. We highlight the case of a 66-year-old female who presented with fever, shortness of breath and cough for three months duration. She had raised inflammatory markers, and imaging revealed bilateral cavitating nodules. Histopathology revealed necrotising granulomatous inflammation along with acid-fast bacilli on Ziehl-Neelsen staining. The patient was diagnosed with tuberculosis and started on anti-tubercular therapy. When no response was seen, a rheumatologist was consulted, and a suspected diagnosis of granulomatosis with polyangiitis was made on the basis of clinical and laboratory features. Dramatic response to steroids and a negative mycobacteria culture confirmed the diagnosis. The patient responded to a combination of steroids and cyclophosphamide. This case highlights the importance of recognising the possibility of a false-positive acid-fast bacilli report.