Page 180 - The Vasculitides, Volume 1: General Considerations and Systemic Vasculitis

156 Elena Csernok

ANCA detection strategies. The alternative to conventional IFT is image analysis, which
quantifies fluorescence in a single dilution of a patient sample in comparison with the
intensity of standardized calibrators.

Evaluation of the image analysis for the potential to detect ANCA in a cohort a
consecutive PR3-ANCA positive GPA patients [9] revealed detectable ANCA levels in 75%
of patients at the time of renal relapse, indicating a lower diagnostic sensitivity as compared
with IFT and capture ELISA (100%). Quantitative image analysis predicted disease relapse
somewhat better than IFT, but was comparable to direct ELISA [8]. Multicenter comparative
studies have not yet been performed.

A multiplex technology offers the unique opportunity to detect the presence of multiple
autoantibodies at the same time and in the same sample. This new automated immunoassay
builds on a synthesis of bead-based multiplex assays and flow cytometry. Trevisin and
colleagues [10] compared the performance of a flow cytometric immunoassay for PR3 and
MPO-ANCA with IFT and ELISAs in active and treated vasculitis and inflammatory bowel
disease and found a specificity of 88% compared with 96% and 94% respectively for IFT and
both ELISA. The PR3- and MPO-ANCA immunoassay was almost as sensitive as IFT, and
more sensitive than, but just as specific as most ELISA in detecting ANCA in active and
treated vasculitis [10]. A major advantage of this assay is the ability to screen simultaneously
for a panel of autoantibodies relevant to vasculitis, however, prospective studies are needed
this automated method in the initial screening of patients with suspected vasculitis.

Currently, the most appropriate method for the detection of vasculitis-associated ANCA
is by IFT and PR3- and MPO-ANCA immunometric assay. A future goal is the proper use of
IFT and antigen-specific assays to improve commercial testing reagents, and in turn to
increase laboratory expertise in ANCA testing and interpretation.

Diagnostic Significance of ANCA

Whereas PR3- and MPO-ANCA have been shown to be highly specific for AAV, the
diagnostic value of such testing in other nonvasculitic conditions is very limited. An
overestimation of the diagnostic relevance of a positive ANCA test may erroneously
misdirect clinicians and delay adequate treatment. ANCA-positivity has been noted in
connective tissue diseases notably, systemic lupus erythematous, RA, and Felty syndrome;
among diverse gastrointestinal disorders namely ulcerative colitis, Crohn disease, and
autoimmune hepatitis; the infectious disorders including tuberculosis, leprosy, and subacute
bacterial endocarditis; as well as in malignancy, drug-induced syndromes related to the
administration of propylthiouracil, hydralazine, and illicit cocaine use; and other disorders
unassociated with vascular inflammation. Since the diagnostic accuracy of ANCA testing in
vasculitis should be improved by an increased pretest probability, it stands to reason that such
testing should be guided by the likelihood of AAV. According to the consensus statement [4],
adherence to clinical ordering guidelines for ANCA testing can reduce the number of false-
positive tests by 27% without missing a single patient with AAV [11].

A symptom related gating policy for ANCA testing could reduce the total number of
ANCA tests performed by more than 20% with appreciably increased efficiency and cost
saving. Arnold and colleagues [12] investigated the impact of a gating policy at a single

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