Which laboratory tests are essential for confirming a diagnosis of EGPA, and how should I interpret their results?

Essential laboratory tests for confirming eosinophilic granulomatosis with polyangiitis (EGPA) include:

• Full blood count with differential: to detect peripheral eosinophilia, a hallmark of EGPA, typically showing elevated eosinophil counts above normal ranges.
• Antineutrophil cytoplasmic antibodies (ANCA) testing: particularly perinuclear ANCA (p-ANCA) with specificity for myeloperoxidase (MPO). Approximately 40-60% of EGPA patients are ANCA-positive, and a positive MPO-ANCA supports the diagnosis but is not universally present.
• Inflammatory markers: such as erythrocyte sedimentation rate (ESR) and C-reactive protein (CRP), which are often elevated reflecting systemic inflammation.
• Additional tests: may include serum IgE levels and total IgE, which can be elevated due to the allergic and eosinophilic nature of the disease.

Interpretation of results: Peripheral eosinophilia is a key diagnostic clue; a high eosinophil count strongly supports EGPA in the appropriate clinical context. A positive MPO-ANCA test further substantiates the diagnosis but its absence does not exclude EGPA. Elevated inflammatory markers indicate active vasculitis but are nonspecific. Elevated IgE levels may support the allergic component of EGPA but are not diagnostic alone.

This integrated approach aligns with UK clinical practice guidelines emphasizing eosinophilia and ANCA testing as central to diagnosis, while recent literature (Özdemir et al., 2021) highlights the variability of ANCA positivity and the importance of correlating laboratory findings with clinical features to avoid misdiagnosis, especially in contexts such as COVID-19 where overlapping features may occur.

Therefore, the diagnosis of EGPA relies on a combination of laboratory evidence of eosinophilia and ANCA status, interpreted alongside clinical presentation and imaging, rather than any single test result alone ¹ (Özdemir et al., 2021).