False Positive C-ANCA Caused by Antinuclear Antibody: a Case Report and Literature Review

Antineutrophil cytoplasmic antibody (ANCA) is an autoantibody against the cytoplasmic components of neutrophils and monocytes. ANCA related vasculitis includes granulomatous polyangiitis (GPA), microscopic polyangiitis (MPA), and eosinophilic granulomatous polyangiitis (EGPA). The detection of ANCA has important clinical significance for the diagnosis, differential diagnosis, classification, condition monitoring, and prognosis of these diseases. The treatment and prognosis of ANCA associated vasculitis are closely related to the titer and activity of ANCA, so the detection of ANCA has important clinical application value. The interference factors of ANCA detection include antinuclear antibody (ANA), which may affect the accuracy of ANCA detection results.

Antineutrophil cytoplasmic antibody and antinuclear antibody were detected by indirect immunofluorescence, and the related antibodies in antinuclear antibody were detected by western blotting. In the diagnosis of GPA, C-ANCA test results should be interpreted in combination with the clinical manifestations of patients, and interference should be excluded. C-ANCA is positive and MPO is negative. It should be considered that antinu-clear antibody (ANA) may interfere with ANCA test results.

The patient was positive for C-ANCA (+++) and negative for MPO antibody. The patient showed no symptoms related to vasculitis. Further detection of antinuclear antibodies and indirect immunofluorescence results showed that the nuclear homogeneous type and specific ds-DNA antibody were positive, suggesting the presence of systemic lupus erythematosus or another autoimmune disease with a high specificity for these autoanti-bodies.

Antinuclear antibody test showed that ana positive and ds-DNA antibody positive indirect immunofluorescence results were nuclear homogeneous type. Therefore, the patient was considered as a false positive of C-ANCA caused by antinuclear antibody.