From the Research
The false positive rate for C-ANCA testing is approximately 60.1% in clinical practice, as reported in a study published in 2019 1. This means that in about 60.1% of cases, the test may indicate a positive result when the patient does not actually have the associated disease, typically granulomatosis with polyangiitis (formerly Wegener's granulomatosis). False positives can occur in various conditions including infections, inflammatory bowel disease, rheumatoid arthritis, and certain malignancies. The specificity of C-ANCA testing improves significantly when combined with anti-proteinase 3 (PR3) antibody testing, which is more specific for granulomatosis with polyangiitis. For this reason, current practice typically includes testing for both C-ANCA pattern by immunofluorescence and specific PR3 antibodies by ELISA methods. Some of the key factors that can lead to false positive C-ANCA results include:
- Antinuclear antibody (ANA) interference, as reported in a case study published in 2025 2
- Low-medium ELISA antibody titers, as reported in a study published in 2019 1
- Presence of other autoimmune diseases, such as systemic lupus erythematosus When interpreting C-ANCA results, it's essential to consider the clinical context and not rely solely on serology for diagnosis. A positive C-ANCA result should be confirmed with anti-PR3 testing and correlated with clinical symptoms, other laboratory findings, and potentially tissue biopsy for definitive diagnosis of ANCA-associated vasculitis. It's also important to note that the false positive rate can vary depending on the population being tested and the specific testing methods used. Therefore, it's crucial to interpret C-ANCA results in the context of the individual patient's clinical presentation and medical history.