Comparison of Acquired Activated Protein C Resistance, Using the CAT and ST-Genesia(®) Analysers and Three Thrombin Generation Methods, in APS and SLE Patients

Background: Acquired activated protein C resistance (APCr) has been identified in antiphospholipid syndrome (APS) and systemic lupus erythematosus (SLE). Objective: To assess agreement between the ST-Genesia(®) and CAT analysers in identifying APCr prevalence in APS/SLE patients, using three thrombin generation (TG) methods. Methods: APCr was assessed with the ST-Genesia using STG-ThromboScreen and with the CAT using recombinant human activated protein C and Protac(®) in 105 APS, 53 SLE patients and 36 thrombotic controls. Agreement was expressed in % and by Cohen’s kappa coefficient. Results: APCr values were consistently lower with the ST-Genesia(®) compared to the CAT, using either method, in both APS and SLE patients. Agreement between the two analysers in identifying APS and SLE patients with APCr was poor (≤65.9%, ≤0.20) or fair (≤68.5%, ≥0.29), regardless of TG method, respectively; no agreement was observed in thrombotic controls. APCr with both the ST Genesia and the CAT using Protac(®), but not the CAT using rhAPC, was significantly greater in triple antiphospholipid antibody (aPL) APS patients compared to double/single aPL patients (p < 0.04) and in thrombotic SLE patients compared to non-thrombotic SLE patients (p < 0.05). Notably, the ST-Genesia(®), unlike the CAT, with either method, identified significantly greater APCr in pregnancy morbidity (median, confidence intervals; 36.9%, 21.9–49.0%) compared to thrombotic (45.7%, 39.6–55.5%) APS patients (p = 0.03). Conclusion: Despite the broadly similar methodology used by CAT and ST-Genesia(®), agreement in APCr was poor/fair, with results not being interchangeable. This may reflect differences in the TG method, use of different reagents, and analyser data handling.