Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (717 page)

Decreased In

   NA

   Limitations

   High blood levels are also seen in people with inflammation of the lining of the stomach, stomach ulcers, and stomach cancer.

   APC antibodies may occur with increased frequency in unaffected family members, a small percentage of healthy individuals, and patients with other autoimmune diseases, such as autoimmune thyroiditis.

ANTINEUTROPHIL CYTOPLASMIC ANTIBODY (ANCA)

   Definition

   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).