As in other types of vasculitis there is no single test which confirms the diagnosis. The diagnosis will depend on the doctor recognising the pattern of symptoms and examination findings. Blood tests usually show evidence of inflammation. The blood tests for ANCA are usually positive which help support the diagnosis but are not specific for GPA or vasculitis. If the kidneys or lungs are affected then a biopsy may be helpful in confirming the diagnosis.

GPA often presents in a similar way to other diseases and can be difficult to diagnose. If the lungs are primarily affected it is not uncommon for the disease to be mistaken for lung cancer or TB.