From the Guidelines
The specificity of a positive Myeloperoxidase (MPO) antibody test is approximately 95%, as indicated by the high level of evidence and expert consensus in the field of rheumatology 1. This means that when the test is positive, there is a high probability that the patient truly has an MPO-ANCA associated vasculitis, such as microscopic polyangiitis or renal-limited vasculitis. Some key points to consider when interpreting a positive MPO antibody test include:
- The test should not be used in isolation for diagnosis, but rather as part of a comprehensive diagnostic workup that includes clinical correlation and other laboratory findings 1.
- False positives can occur in other conditions, including infections, malignancies, and other autoimmune diseases.
- The test is most valuable when interpreted alongside clinical presentation, other laboratory findings, and sometimes tissue biopsy.
- For optimal diagnostic accuracy, both MPO and proteinase 3 (PR3) antibody tests are often ordered together as part of a comprehensive ANCA panel, as recommended by the EULAR guidelines for the management of ANCA-associated vasculitis 1. It's also important to note that the specificity of the test can vary depending on the laboratory method used, with enzyme-linked immunosorbent assay (ELISA) generally providing higher specificity than indirect immunofluorescence. However, according to the most recent and highest quality study, the specificity of a positive MPO antibody test is approximately 95% 1.
From the Research
Specificity of a Positive MPO Antibody Test
- The specificity of a positive Myeloperoxidase (MPO) antibody test is relatively high, with a specificity of 96.3% reported in one study 2.
- When the perinuclear ANCA (pANCA) pattern by immunofluorescence (IIF) is combined with MPO-ANCA testing, the specificity improves to 99.4% 2.
- Another study reported a specificity of 98% for MPO-ANCA testing using enzyme-linked immunosorbent assay (ELISA) 3.
- The specificity of MPO-ANCA testing can vary depending on the assay used and the population being tested, with reported specificities ranging from 88% to 99.4% 3, 2.
- It is worth noting that a significant proportion of patients with a positive MPO-ANCA test may not have evidence of vasculitis, particularly those with low-medium ELISA antibody titers 4.
- Using a higher threshold of ANCA titers may be required to improve specificity 4.