Vasculitis can be classified by the cause, the location, the type of vessel or the size of vessel.
Underlying cause. For example, the cause of syphilitic aortitis is infectious (aortitis simply refers to inflammation of the aorta, which is an artery.) However, the causes of many forms of vasculitis are poorly understood. There is usually an immune component, but the trigger is often not identified. In these cases, the antibody found is sometimes used in classification, as in ANCA-associated vasculitides.
Location of the affected vessels. For example, ICD-10 classifies "vasculitis limited to skin" with skin conditions (under "L"), and "necrotizing vasculopathies" (corresponding to systemic vasculitis) with musculoskeletal system and connective tissue conditions (under "M"). Arteritis/phlebitis on their own are classified with circulatory conditions (under "I").
Type or size of the blood vessels that they predominantly affect. Apart from the arteritis/phlebitis distinction mentioned above, vasculitis is often classified by the caliber of the vessel affected. However, there can be some variation in the size of the vessels affected.
According to the size of the vessel affected, vasculitis can be classified into:
In pediatric patients varicella infection may be followed by vasculitis of intracranial vessles. This condition is called post varicella angiopathy amd this may be responsible for arterial ischaemic strokes in children. 
Several of these vasculitides are associated with antineutrophil cytoplasmic antibodies. These are
granulomatosis with polyangiitis (formerly known as Wegener's granulomatosis)
eosinophilic granulomatosis with polyangiitis (Churg-Strauss syndrome).
Other organ functional tests may be abnormal. Specific abnormalities depend on the degree of various organs involvement. A Brainspect can show decreased blood flow to the brain and brain damage.
The definite diagnosis of vasculitis is established after a biopsy of involved organ or tissue, such as skin, sinuses, lung, nerve, brain and kidney. The biopsy elucidates the pattern of blood vessel inflammation.
An alternative to biopsy can be an angiogram (x-ray test of the blood vessels). It can demonstrate characteristic patterns of inflammation in affected blood vessels.
18F-fluorodeoxyglucose positron emission tomography/computed tomography (FDG-PET/CT)has become a widely used imaging tool in patients with suspected Large Vessel Vasculitis, due to the enhanced glucose metabolism of inflamed vessel walls. The combined evaluation of the intensity and the extension of FDG vessel uptake at diagnosis can predict the clinical course of the disease, separating patients with favourable or complicated progress.
Acute onset of vasculitis-like symptoms in small children or babies may instead be the life-threatening purpura fulminans, usually associated with severe infection.
Laboratory Investigation of Vasculitic Syndromes
Associated laboratory features
Systemic lupus erythematosus
ANA including antibodies to dsDNA and ENA [including SM, Ro (SSA), La (SSB), and RNP]
Leukopenia, thrombocytopenia, Coombs' test, complement activation: low serum concentrations of C3 and C4, positive immunofluorescence using Crithidia luciliae as substrate, antiphospholipid antibodies (i.e., anticardiolipin, lupus anticoagulant, false-positive VDRL)
Anti-glomerular basement membrane antibody
Epitope on noncollagen domain of type IV collagen
Small vessel vasculitis
Perinuclear antineutrophil cytoplasmic antibody
Granulomatosis with polyangiiitis
Cytoplasmic antineutrophil cytoplasmic antibody
Proteinase 3 (PR3)
perinuclear antineutrophil cytoplasmic antibody in some cases
Elevated CRP and eosinophilia
Cryoglobulins, rheumatoid factor, complement components, hepatitis C
Medium vessel vasculitis
Classical polyarteritis nodosa
Elevated CRP and eosinophilia
In this table: ANA = Antinuclear antibodies, CRP = C-reactive protein, dsDNA = double-stranded DNA, ENA = extractable nuclear antigens, RNP = ribonucleoproteins; VDRL = Venereal Disease Research Laboratory
Treatments are generally directed toward stopping the inflammation and suppressing the immune system. Typically, corticosteroids such as prednisone are used. Additionally, other immune suppression drugs, such as cyclophosphamide and others, are considered. In case of an infection, antimicrobial agents including cephalexin may be prescribed. Affected organs (such as the heart or lungs) may require specific medical treatment intended to improve their function during the active phase of the disease.
^Jennette JC, Falk RJ, Andrassy K et al. (1994). "Nomenclature of systemic vasculitides. Proposal of an international consensus conference". Arthritis Rheum.37 (2): 187–92. doi:10.1002/art.1780370206. PMID8129773.CS1 maint: Explicit use of et al. (link)