Pathology of Tuberculosis

Abstract

Tuberculous lesions are microscopically heterogeneous and vary depending on the stage of the disease, host immunity response, and phenotypic characteristics of the tuberculous bacillus. The hallmark of tuberculosis infection is necrotizing granulomatous inflammation, composed of epithelioid cells surrounding a central necrotic zone, and accompanied by a variable number of multinucleated giant cells and lymphocytes. Necrosis is typically caseous. Suppurative forms of tuberculosis without granulomas are rare, and mimic pyogenic infection, grossly and microscopically. In immunocompromised persons, tuberculosis may not elicit granulomatous inflammation. The diagnosis of extrapulmonary tuberculosis often requires invasive procedures to obtain cytological and tissue specimens for microbiological, cytological, and histological investigations. Necrosis and granulomas are frequently seen in fine-needle aspiration cytology, but are rarely observed in body fluids. Histological and cytological examinations, notably in endemic areas where molecular tests are unavailable, provide a rapid diagnosis allowing the initiation of the treatment. Ancillary techniques on formalin-fixed paraffin-embedded tissue such as special stains, immunochemistry and molecular tests help to establish the diagnosis. Acid-fast bacilli are identified in specimens containing necrotizing granulomas, especially in areas with suppuration and cavitation. Acid-fast bacilli are less frequently seen in nonnecrotizing granulomas and very rarely seen in nongranulomatous lesions.