Myeloperoxidase Antibody Test

Myeloperoxidase (MPO) antibodies are a type of autoantibody—antibodies the immune system mistakenly produces against the body’s own proteins. In this case, they target MPO, an enzyme normally stored inside neutrophils, which are white blood cells that help fight infections. When MPO antibodies are present, they can disrupt the normal function of neutrophils, triggering inflammation in blood vessels, a condition known as vasculitis.

These antibodies are most commonly linked to a group of autoimmune diseases called ANCA-associated vasculitis (AAV). ANCA stands for “anti-neutrophil cytoplasmic antibody.” In AAV, the immune system attacks small and medium-sized blood vessels throughout the body, often affecting the kidneys, lungs, nerves, and skin. MPO antibodies are one of two main types of ANCA (the other targets a protein called proteinase 3, or PR3). In patients with MPO-ANCA vasculitis, the presence and levels of these antibodies can help diagnose the condition, predict flares, and monitor disease activity over time.

What makes MPO antibodies clinically important is that their levels often reflect how active the disease is. For example, rising levels may precede a renal flare—a worsening of inflammation in the kidneys—especially in patients who previously had kidney involvement. This makes them a valuable tool not just for diagnosis but for anticipating and potentially preventing disease relapse.

Beyond vasculitis, MPO antibodies have been detected in a range of other conditions:

• Rheumatoid arthritis (RA): MPO antibodies appear more often in RA patients with lung involvement or nodules, indicating a link with more severe disease forms.
• Idiopathic pulmonary fibrosis (IPF): A small percentage of patients with IPF go on to develop microscopic polyangiitis (MPA), a form of AAV. MPO antibody positivity in these cases may signal a risk of progression to systemic autoimmune disease.
• Diabetic retinopathy: Elevated MPO antibody levels have been linked to damage in the retina, suggesting a role in worsening vision in diabetes.
• Acute leukemia: In blood cancers, monoclonal antibodies against MPO help pathologists distinguish between different leukemia subtypes, such as acute myeloid versus lymphoid leukemia.

In rare cases, infections can temporarily trigger the production of MPO antibodies and cause a vasculitis-like illness. These cases are tricky because using strong immunosuppressive drugs (which are normally used to treat vasculitis) could worsen an underlying infection. Therefore, correct diagnosis is crucial.