Antineutrophil Cytoplasmic Antibody (ANCA)
ANCA testing plays a critical role in the diagnosis and classification of vasculitides. It is associated with a number of vasculitides, including Wegener granulomatosis (WG), Churg-Strauss syndrome (CSS), microscopic polyangiitis (MPA), and idiopathic necrotizing and crescentic glomerulonephritis.
Two types of ANCA assays are currently in wide use: IFA and ELISA. Of these two techniques, IFA is more sensitive, and ELISA is more specific. The optimal approach to clinical testing for ANCA is, therefore, to screen with IFA and confirm all positive results with ELISAs directed against the vasculitis-specific target antigens proteinase 3 (PR3) and myeloperoxidase antibodies (MPOs).
When the sera of patients with ANCA-associated vasculitis are incubated with ethanol-fixed human neutrophils, two major IFA patterns are observed: the cytoplasmic neutrophil antibody (cANCA) and perinuclear antineutrophil cytoplasmic antibody (pANCA) patterns. Other staining patterns have been described and are generally noted as "atypical. "
Specific immunochemical assays demonstrate that cANCAs comprise mainly antibodies to PR3 and pANCA antibodies to MPO.
The PR3-ANCA pattern has been predominantly associated with cases of active WG and CSS, but many also be seen in MPA.
MPO-ANCA has been primarily in MPA, CSS, and rarely in WG.
pANCA pattern variations not associated with MPO (atypical) patterns may be observed on IFA testing in patients with immune-mediated conditions other than systemic vasculitis (e.g., connective tissue disorders, inflammatory bowel disease, infections, and autoimmune hepatitis).
Normal value: negative.
Use
Evaluation of patients suspected of having WG or systemic vasculitis, especially patients with renal disease, pulmonary disease, or unexplained multiorgan disease possibly due to vasculitis
Interpretation
Increased In
c-ANCA (PR3 positive):
Systemic necrotizing vasculitis
Common: WG
CSS
May also be seen in systemic necrotizing vasculitis of polyarteritis group, pauci-immune type of idiopathic crescentic glomerulonephritis.
Propylthiouracil drug
pANCA (MPO positive):
Systemic necrotizing vasculitis
Common: microscopic polyarteritis
CSS
Uncommon in WG
Hydralazine, minocycline, propylthiouracil
pANCA (to various antigens, MPO negative):
Connective tissue disease
Antiphospholipid antibody syndrome
Juvenile chronic arthritis
Polymyositis/dermatomyositis
Relapsing polychondritis
RA
Sj Άgren syndrome
SLE
Inflammatory bowel disease
Ulcerative colitis (60 " 85%)
Crohn disease (10 " 40%)
Bacterial enteritis (rare)
Autoimmune liver diseases
Primary sclerosing cholangitis
Autoimmune hepatitis
Infections
Chromomycosis
HIV-1
Acute malaria
Five percent healthy controls
Limitations
There is a subjective component to the interpretation of IFA, because the tests are based on visual interpretation of the IF pattern, which is not straightforward. It depends on the experience of the individual who performs the assay.
ANCA testing is not standardized; the sensitivity and specificity will vary with the laboratory. The cANCA pattern has a greater specificity than the pANCA pattern for vasculitis. However, even positive cANCA IFA results were associated with vasculitis in only 50% of patients.
Antibodies to a host of azurophilic granule proteins can cause a pANCA staining pattern; these include antibodies directed against lactoferrin, elastase, cathepsin G, bactericidal permeability inhibitor, catalase, lysozyme, ²-glucuronidase, and others. A positive pANCA IFA staining pattern may also be detected in a wide variety of inflammatory illnesses and has a low specificity for vasculitis.
Individuals with ANA frequently have "false-positive " results on ANCA testing by IFA.
Certain medications may induce forms of vasculitis associated with ANCA. The strongest links between medications and ANCA-associated vasculitis are with drugs employed in the treatment of hyperthyroidism: propylthiouracil, methimazole, and carbimazole. Hydralazine and minocycline are less commonly associated with the induction of ANCA-associated vasculitis. Other implicated drugs include penicillamine, allopurinol, procainamide, thiamazole, clozapine, phenytoin, rifampicin, cefotaxime, isoniazid, and indomethacin.
Using IFA and ELISA testing in a sequential fashion substantially increases the positive predictive value of an ANCA assay.
Elevations in the titers of ANCA do not predict disease flares in a timely manner. If a patient was ANCA positive during a period of active disease, a persistently ANCA-negative status is consistent with, but not absolutely a proof of, remission.
ANCA testing should not be used to screen nonselected patient groups where the prevalence of vasculitis is low. These tests are most valuable when selectively ordered in clinical situations where some forms of ANCA-associated vasculitis are seriously considered.
A negative ANCA result should not be used to exclude disease.