ANCA testing in real-world clinical practice: diagnostic performance and predictive value in a Spanish cohort

OBJECTIVES: ANCA are a key biomarker for ANCA-associated vasculitis (AAV), particularly microscopic polyangiitis (MPA) and granulomatosis with polyangiitis (GPA). Although indirect immunofluorescence (IIF) has traditionally been the reference technique, its diagnostic value in contemporary real-world practice remains uncertain. The aim of this study was to evaluate the diagnostic performance of IIF ANCA testing in routine clinical practice. METHODS: We conducted a retrospective study of all patients with an ANCA request at a tertiary hospital over a 4-year period. All IIF-positive sera were subsequently tested for anti-PR3 and anti-MPO antibodies by chemiluminescent immunoassay (CLIA). Clinical data, test indication and final diagnoses were retrieved from electronic medical records. RESULTS: We included 5157 patients and IIF was positive in 653 (12.7%): perinuclear ANCA (P-ANCA) in 17.9%, cytoplasmic ANCA (C-ANCA) in 13.3% and atypical ANCA (A-ANCA) in 68.8%. CLIA was negative in 97.3% of A-ANCA. AAV was diagnosed in 47 patients, and 42 (89.4%) had positive IIF. For GPA and MPA, IIF showed a sensitivity of 93% and specificity of 88%, with a very high negative predictive value (NPV) (99.9%) but low positive predictive value (PPV) (6.1%). Specificity improved to 96.8% when restricted to typical patterns (C-ANCA or P-ANCA) and to 99.6% when combined with positive CLIA results >20 IU/ml. Almost all AAV cases were diagnosed in patients with high pre-test probability (such as renal disease, lung infiltrates or peripheral neuropathies) and interstitial lung disease was the most frequent non-AAV diagnosis in IIF-positive patients. CONCLUSIONS: ANCA IIF retains good diagnostic efficiency and a very high NPV for GPA and MPA, but has low PPV, particularly when tested for nonspecific symptoms.