Which laboratory marker is sufficient to establish a diagnosis of granulomatosis with polyangiitis?

Granulomatosis with polyangiitis (GPA), formerly known as Wegener's granulomatosis, is an autoimmune condition characterized by vasculitis affecting small to medium-sized blood vessels. It is commonly associated with specific autoantibodies, particularly antiproteinase 3 antibodies (c-ANCA/anti-PR3).

The presence of these antibodies is essential for diagnosing GPA because they indicate an autoimmune process involving granulomatous inflammation and vasculitis. Detecting c-ANCA/anti-PR3 in the laboratory provides strong evidence of the disease, supporting the clinical findings and serving as a reliable biomarker for GPA.

Other laboratory markers such as antinuclear antibodies (ANA) and rheumatoid factor are not specific to granulomatosis with polyangiitis. While they may be positive in various autoimmune diseases, they do not confirm GPA. Similarly, evaluating complement levels is useful in other settings, such as assessing certain types of vasculitis, but is not definitive for GPA. Thus, the presence of antiproteinase 3 antibodies is a clear and sufficient marker for establishing a diagnosis of granulomatosis with polyangiitis.