microscopic polyangiitis and focal necrotizing and crescentic glomerulonephritis is also high (Table 3.10). A third distinct ANCA, x-ANCA, has also been described. It may be seen in many disease processes, but it is most commonly found in chronic inßammatory bowel disease.

The discovery of ANCAs provided an invaluable tool in the evaluation of patients with ocular or orbital inßammation suggestive of systemic vasculitis [42Ð45]. Either type of ANCA can be found in patients with limited or generalized granulomatosis with polyangiitis (Wegener) with ocular or orbital inßammation [44, 45].

Because of its high speciÞcity, a positive ANCA is very suggestive of granulomatosis with polyangiitis (Wegener); however, because ANCA is positive only in 67% of patients with active limited disease and in 32% of patients in full remission after limited disease, one or even repeatedly negative ANCA testing does not exclude the diagnosis, especially in patients with limited clinical features and characteristic histologic Þndings.

3.2.1.5 Circulating Immune Complexes

The formation of circulating immune complexes (CICs) by binding of antigens to their corresponding antibodies is a physiologic process usually of beneÞt to the host because it allows the neutralization or the elimination of exogenous antigens. CICs primarily involve antigens from exogenous sources (food, drugs, or microbes) and endogenous sources (autoantigens and tumor antigens). Deposition of CICs seems to be dependent on a variety of factors, including size of complex, nature of antigen, immunoglobulin class, antibody afÞnity, ability to Þx complement, interaction with RFs, and clearance capacity of the reticuloendothelial system [46]. CICs usually are eliminated efÞciently by the mononuclear phagocytic system, particularly by Kupffer cells in the liver; [47] therefore, CICs can be detected for only a short period of time after a speciÞc antigenic challenge. However, CICs may persist in the blood circulation of patients with autoimmune disorders, such as polyarteritis nodosa, allergic granulomatous angiitis (ChurgÐStrauss syndrome), granulomatosis with polyangiitis (Wegener), and some connective tissue diseases; CICs may

deposit in renal glomeruli, synovial tissue, or vessel walls, participating in the development or persistence of major inßammatory lesions.

Although CICsÕ detection is not essential for the diagnosis of any condition, their presence helps support a speciÞc disorder under the following conditions: (1) CICs may be detected in early arthritis several months prior to the deÞnitive diagnosis of RA; [48] (2) CICs may help to distinguish seronegative RA from other arthropathies, such as ankylosing spondylitis or reactive arthritis, because CICs are found in 70% of patients with seronegative RA and they are found only rarely in patients with ankylosing spondylitis or reactive arthritis [49, 50]. CIC detection may also be useful to monitor disease activity; [51, 52] patients with rheumatoid vasculitis have high levels of CICs [53, 54]. SigniÞcant decreases in CIC level may be interpreted as a favorable response to therapy; conversely, signiÞcant increases of CIC level may signify the need for more aggressive therapy.

Because of the enormous diversity of antigens involved in CICs, it is doubtful that antigen-spe- ciÞc assays will ever Þnd widespread use in clinical immunological studies. Therefore, the techniques available for detection of CICs are antigen nonspeciÞc. The sensitivity of each method for detecting CIC varies according to the nature of the CIC involved and the inßuence of various interfering factors. Only a limited number of procedures are suitable for routine laboratory investigation. Some of the most widely used methods are described in the following sections.

Fluid-Phase Binding Assays

Fluid-phase binding assays are based on the fact that precipitation of radiolabeled C1q receptor (a subcomponent of the Þrst component of complement that binds CIC) differs from precipitation of radiolabeled C1q receptor bound to CIC.

C1q-Binding Assay

In the C1q-binding assay, CICs are allowed to bind C1q in liquid phase. Radiolabeled C1q is added to ethylenediaminetetraacetic acid (EDTA)- treated serum in the presence of polyethylene glycol, which precipitates CICs bound to C1q [55].