Interpretation

Positive MPO-ANCA (P-ANCA):
- Suggests ANCA-associated vasculitis, especially MPA and EGPA.
- Correlates with disease activity, but not always perfectly.
Negative MPO-ANCA:
- Does not completely rule out vasculitis.
- Other antibodies (e.g., PR3-ANCA) may be positive in GPA.
False positives may occur in:
- Ulcerative colitis
- Rheumatoid arthritis
- Systemic lupus erythematosus
- Chronic infections
- Certain drugs (hydralazine, propylthiouracil, minocycline)