Factor VIII inhibitors demonstrate type I or type II kinetics in high‐titre alloantibodies and autoantibodies

Background In the past, most references have indicated that inhibitors in inherited haemophilia A (HA) patients had type I reaction kinetics, while inhibitors in acquired haemophilia A (AHA) had type II reaction kinetics. However, researches have also shown that samples from AHA patients exhibit type I kinetics, while inhibitor samples from HA patients may also exhibit type II kinetics. Aim To characterise inhibitor kinetics in HA patients and AHA patients in our centre. Methods Inhibitor measurement was performed using the Bethesda assay, and inhibitor type was classified by kinetic study with dilutions. The recovery of four HA patients with type II inhibitors was tested after FVIII replacement therapy. Results In the 156 HA patients with high‐titre inhibitors, 131 patients exhibited type I kinetics and 15 patients exhibited type II kinetics. In the 79 AHA patients with high‐titre inhibitors, 21 patients exhibited type I kinetics and 38 patients exhibited type II kinetics. Pharmacokinetic showed that FVIII replacement was effective in four patients with type II kinetic inhibitors. Conclusion The inhibitor kinetic type cannot be used by itself determine auto‐ versus allo‐anti‐FVIII antibody type. Inhibitors for both HA alloantibodies and AHA autoantibodies may display either type I or type II kinetics. Pharmacokinetic study suggested that patients with type II kinetics may still be treated with higher doses of FVIII replacement therapy.