From the Guidelines
MPO-ANCA is approximately 90% specific for microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis, while PR3-ANCA is about 95% specific for granulomatosis with polyangiitis. The specificity of myeloperoxidase (MPO) and proteinase 3 (PR3) is crucial in diagnosing certain vasculitis conditions.
- MPO-ANCA is highly specific for microscopic polyangiitis and eosinophilic granulomatosis with polyangiitis (formerly Churg-Strauss syndrome) 1.
- PR3-ANCA is highly specific for granulomatosis with polyangiitis (formerly Wegener's granulomatosis) 1. These autoantibodies target enzymes found in neutrophil granules and are essential for distinguishing between different types of ANCA-associated vasculitis. When testing for these markers, both immunofluorescence and enzyme immunoassays are typically used to maximize diagnostic accuracy, as recommended by recent guidelines 1. The high specificity of these markers makes them valuable diagnostic tools, though they should be interpreted alongside clinical presentation and other laboratory findings. False positives can occasionally occur in conditions like infections, malignancies, or with certain medications, so clinical correlation is essential for proper diagnosis and treatment planning 1. In clinical practice, the diagnosis of EGPA should be considered in patients with asthma, chronic rhinosinusitis, and eosinophilia who develop end-organ involvement, particularly peripheral neuropathy, lung infiltrates, cardiomyopathy, or other complications, and ANCA testing should be performed in all patients with suspected EGPA 1.
From the Research
Specificity of Myeloperoxidase (MPO) and Proteinase 3 (PR3)
- The specificity of MPO and PR3 antibodies is crucial in diagnosing ANCA-associated vasculitis (AAV) 2, 3.
- A study found that defining thresholds for antibody levels and assigning test result interval-specific likelihood ratios (LRs) can improve clinical interpretation of PR3- and MPO-ANCA results 3.
- The study determined thresholds for antibody levels based on predefined specificities (95,97.5,99, and 100%) and used them to delimit test result intervals 3.
- For all assays, the LR for AAV increased with increasing antibody level, and high antibody levels were associated with a higher likelihood of disease 3.
- Another study found that GPA is associated with anti-neutrophil cytoplasmic autoantibodies (ANCA) with specificity for proteinase 3 (PR3-ANCA), while MPA is associated with specificity for myeloperoxidase (MPO-ANCA) 4.
- The specificity of PR3-ANCA and MPO-ANCA can be used to differentiate between GPA and MPA, with PR3-ANCA being more closely related to GPA and MPO-ANCA being more closely related to MPA 5, 6.
- A study found that patients with GPA presented more commonly with ear, nose, and throat and eye/orbital manifestations, while patients with MPA presented more often with weight loss, myalgia, renal involvement, interstitial lung disease, cutaneous purpura, and peripheral nerve involvement 6.