PRTN3 and rheumatoid arthritis: ACPA often co‐occur with RF and are present in approximately 50%‐60% of early RA patients and 60%‐90% of patients with established disease.12, 13, 14 Only 1%‐3% of healthy individuals are positive for ACPA, and mostly at low levels.15 ACPA are substantially more specific for RA than RF (95% for ACPA and 85% for RF16), which makes them a prominent RA biomarker.