Pulmonary granulomas: differential diagnosis, histologic features and algorithmic approach

Pulmonary granulomas are a common finding in routine diagnostic pathology. However, these lesions are often problematic because of the broad differential diagnosis, the time required for identifying organisms and the need for familiarity with subtle variations in morphology and distribution of granulomas. This review aims to discuss the differential diagnosis, to compare and contrast the histologic features of granulomatous lung diseases, and to provide a basic algorithmic approach to the histologic findings. Infectious causes of granulomas are most commonly diagnosed by identifying mycobacteria or fungi on histologic sections. While some non-infectious diseases can be diagnosed on the basis of specific histologic features, others require additional clinical, radiologic and/or microbiologic information for definitive diagnosis. Finally, we discuss our approach to granulomas of unknown aetiology.