False-Positive Myeloperoxidase-Antineutrophil Cytoplasmic Antibody in a Patient with Rheumatoid Arthritis.

BACKGROUND Antineutrophil cytoplasmic antibody (ANCA)-associated vasculitis (AAV) is a form of vasculitis predominantly affecting small blood vessels and systemic organs, including the lungs and kidneys. The serum ANCA is an important diagnostic marker for AAV. However, ANCA levels can be nonspecifically elevated in autoimmune diseases like rheumatoid arthritis (RA) and some infectious diseases.