Acquired activated protein C resistance (APCr) and anti-protein C antibodies (anti-PC) antibodies are common in #antiphospholipid antibodies (aPL)-positive individuals, especially those with thrombotic antiphospholipid syndrome (APS), as demonstrated by a recent APS ACTION Registry study 𝗦𝘁𝘂𝗱𝘆 𝗣𝗼𝗽𝘂𝗹𝗮𝘁𝗶𝗼𝗻 ◦ ​370 aPL-positive patients, categorized as: aPL-only (n=77), pregnancy morbidity (PM, n=43), venous #thromboembolism (VTE, n=132), arterial #thrombosis (AT, n=87), or combined VTE+AT (n=31), and 51 healthy controls. ◦ ​Prospective follow-up available for 283 patients (median 8.3 years). 𝗠𝗮𝗶𝗻 𝗥𝗲𝘀𝘂𝗹𝘁𝘀 ◦ ​APCr markedly higher in all aPL-positive patient subgroups compared to healthy controls (p < 0.001). Greatest APCr observed in patients with VTE + AT and in triple aPL-positive patients. ◦ ​Anti-PC antibodies were resent in 42% of patients overall. High-avidity anti-PC significantly more prevalent in the VTE+AT group (79%) compared to aPL-only patients (38%, p < 0.05). → Strong association between APCr and high-avidity anti-PC. ◦ ​New thrombosis occurred in 25/283 (8.8%) patients during follow-up. → No significant association between APCr or anti-PC and new thrombotic events overall (p > 0.60). ◦ ​In the non-anticoagulated subgroup, patients with APCr showed a higher rate of thrombotic events (HR 4.5, 95% CI 0.8–25.9, p=0.14), but this did not reach statistical significance. In this study APCr was highly prevalent across aPL-positive patients and is strongly linked to high-avidity anti-PC antibodies. However, no clear overall association with future thrombosis was found, likely due to confounding by widespread anticoagulation at baseline and relatively few events. *Tohidi-Esfahani I, Venturelli V, Tektonidou M, et al. Prevalence and prognostic value of activated protein C resistance and anti-protein C antibodies in patients with antiphospholipid antibodies: an APS ACTION Registry study. Rheumatology (Oxford). Published online June 13, 2026. 🔗doi.org/10.1093/rheumatology…