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Anti-Neutrophil Cytoplasmic Antibodies

(Download  Anti-Neutrophil Cytoplasmic Antibodies.pdf)

The definitive diagnosis of systemic vasculitic disorders involves the demonstration of suggestive changes on biopsy tissue of involved organs, in the setting of a characteristic clinical syndrome [1]. However, serodiagnostic tests of high predictive value can provide valuable supportive evidence of vasculitis in many cases. Anti-neutrophil cytoplasmic antibodies (ANCA) were first described in 1982 by Davies et al [2] in patients with idiopathic crescentic necrotising glomerulonephritis, using an indirect immunofluorescent (IDIF) assay utilising ethanol-fixed slides of human neutrophils. Three years later, van der Woude et al [3] reported the association between ANCA and Wegener’s granulomatosis, and subsequently ANCA was reported in patients with other systemic vasculitides [4]. By 1990, two major IDIF patterns had been defined: C (classic)-ANCA and P (perinuclear)-ANCA. The major autoantigenic targets of these patterns were identified as proteinase-3 (Pr-3) in C-ANCA and myeloperoxidase (MPO) in P-ANCA [5,6]. While a number of other antigenic targets have since been found to be associated with these two IDIF patterns (Table 1), only antibodies to Pr-3 and MPO (measured by ELISA) are sufficiently specific for vasculitic disorders to be of any clinical value at present.

Clinical Associations

The clinical associations of the two major ANCA patterns are listed in Table 2. There is currently great controversy over whether these antibodies are directly pathogenic, or simply represent the epiphenomenon of neutrophil activation. The presence of ANCA in non-vasculitic disorders such as cystic fribrosis would argue in favour of the latter possibility. ANCA occurring through treatment with drugs such as propylthiouracil usually display multiple specificities, eg positive antibodies to both Pr-3 and MPO simultaneously.

Testing for ANCA is indicated in a number of clinical settings which are suggestive of systemic vasculitis (Table 3). Hagen [7] assessed the sensitivity and specificity of ANCA, by IDIF and ELISA, and the results are summarised in Table 4. The capture ELISA method may further improve sensitivity, but it is currently only available in research settings [8].

ANCA Serology

The optimal approach to serodiagnosis in ANCA-associated vasculitides is to assay IDIF and ELISA sequentially. The use of a "screening" ANCA IDIF test followed by a "confirmatory" ELISA increases the positive predictive value of ANCA serology to over 90% whenever the pre-test probability of systemic vasculitis exceeds 30% (Figure 1). The currently recommended testing protocol for patients with suspected ANCA-associated vasculitis is shown in Figure 2. An antinuclear antibody (ANA) test should be requested in association with an ANCA screen as a homogenous ANA staining pattern may obscure a true P-ANCA pattern and specific ELISA testing for MPO antibodies is then performed. While ANCA titres (by IDIF or ELISA) have been used as a guide to tailor the aggressiveness of immunosuppressive therapy in vasculitic disorders, this is currently a controversial practice [7,9], and ANCA levels should at best serve as a complementary piece of information to a thorough clinical assessment and the measurement of inflammatory markers such as C-reactive protein.

Daily Testing

ANCA assessment by IDIF and ELISA is available daily through the Hunter Immunology Unit. Urgent testing can be performed by special arrangement after discussion with the on-call immunopathologist.

References

1. Fauci AS, Haynes BF, Katz P, et al. Wegener’s granulomatosis: prospective clinical and therapeutic experience with 85 patients for 21 years. Ann. Intern. Med. 1983; 98: 76-85
2. Davies DJ, Moran JE, Niall JF, et al. Segmental necrotising glomerulonephritis with anti-neutrophil antibody: Possible arbovirus aetiology? Br. Med. J. 1982; 285: 606
3. Van der Woude FJ, Rasmussen N, Lobatto S, et al. Autoantibodies against neutrophils and monocytes: Tool for diagnosis and marker of disease activity in Wegener’s granulomatosis. Lancet 1985; 1: 425-429
4. Hall JB, Wadham BM, Wood CJ, et al. Vasculitis and glomerulonephritis: A subgroup with an anti-neutrophil cytoplasmic antibody. Aust. N.Z. J. Med. 1984; 14: 277-278
5. Ludemann J, Utrecht B, Gross WL. Anti-neutrophil cytoplasm antibodies in Wegener’s granulomatosis recognise an elastinolytic enzyme. J. Exp. Med. 1990; 171: 357-362
6. Falk RJ, Jennette JC. Anti-neutrophil cytoplasmic autoantibodies with specificity for myeloperoxidase in patients with systemic vasculitis and idiopathic necrotising and crescentic glomerulonephritis. N. Engl. J. Med. 1988; 318: 1651-1657
7. Hagen EC, Daha MR, Hermans J, et al. Diagnostic value of standardised assays for anti-neutrophil cytoplasmic antibodies in idiopathic systemic vasculitis. EC/BCR Project for ANCA Assay Standardisation. Kidney Int. 1998; 53(3): 743-753
8. Baslund B, Segelmark M, Wiik A, et al. Screening for ANCA: Is indirect immunofluorescence the method of choice? Clin. Exp. Immunol. 1995; 99: 486-492
9. Kallenberg CG, Brouwer E, Weening JJ, et al. Anti-neutrophil cytoplasmic antibodies: Current diagnostic and pathophysiological potential. Kidney Int. 1994; 46:1-15

Written by:             Dr Glenn Reeves, Immunology, HAPS
Written:                   6 October 1998

Last Reviewed:         10.5.2001