The diagnosis of vasculitis.

Early diagnosis of primary systemic vasculitis is important to allow the early commencement of therapy in order to avoid damage and poor outcomes. The heterogeneous nature of vasculitis presents a diagnostic challenge which may hinder early diagnosis. Anti-neutrophilic cytoplasmic antibody testing has been of particular benefit in defining a subgroup of small-vessel vasculitides and facilitating their earlier diagnosis. Suspicion of vasculitis is an important first step to begin a process of investigation to make or refute the diagnosis. No useful diagnostic criteria exist, but classification criteria have been developed to permit a vasculitis patient to be placed in a diagnostic subgroup. The exclusion of vasculitis 'mimics' and secondary causes of vasculitis are components of diagnosis, which otherwise relies on the recognition of a compatible clinical presentation supported by specific laboratory or imaging tests and confirmatory histology. When the diagnosis remains uncertain, observation over time, repeat investigation and a therapeutic trial may improve the probability of the diagnosis or identify an alternative disease.