title: Thrombin generation and bleeding in haemophilia inhibitor patients during immune tolerance induction.
abstract: BACKGROUND
  Inhibitor formation complicates haemophilia treatment and requires immune tolerance induction to rid inhibitors over 5 BU. In the prospective, randomized International Immune Tolerance Study, immune tolerance induction was equally effective with high-dose (HD) (200 IU kg-1day-1) and low-dose (LD) (50 IU kg-13× per week) factor VIII, but haemorrhages were twofold higher in the LD arm. This finding was unexpected as inhibitors neutralize FVIII activity. We hypothesized that the thrombin generation assay (TGA), a global measure of clot formation, might predict bleeding better than FVIII levels.
  
  
  METHODS
  We evaluated TGA using relipidated tissue factor (TF) on 83 thawed, recalcified corn trypsin inhibitor/citrate plasma samples from 31 subjects (17 HD, 14 LD) who participated on the ITI study, and who had sufficient sample available and appropriate informed consent.
  
  
  RESULTS
  There were no significant differences in peak thrombin, estimated thrombin potential, maximum rate or lag time between HD and LD arms; between pre-, during and post-ITI time points, or after FVIII spiking. In 19 subjects (12 HD, 7 LD) with anti-FVIII<1.0 BU, the prevalence of non-neutralizing antibody (NNA) and neutralizing antibody (NA) was 89.5% (17/19), and the latter strongly correlated with anti-VIII titer, r = 0.73 [95% CI: 0.55, 0.88].
  
  
  CONCLUSIONS
  In haemophilia inhibitor patients, thrombin generation is present, but does not predict bleeding risk. Following tolerance induction, NNA remains detectable in the majority.
authors.affiliation: Division of Hematology/Oncology, Department of Medicine, University of Pittsburgh, Pittsburgh, PA, USA.|Hemophilia Center of Western Pennsylvania, Pittsburgh, PA, USA.
authors.forename: M V
authors.initials: MV
authors.lastname: Ragni
authors.affiliation: Division of Blood Diseases and Resources, National Heart Blood Lung Institute, Bethesda, MD, USA.
authors.forename: D M
authors.initials: DM
authors.lastname: DiMichele
authors.affiliation: Department of Hematology, Manchester University Royal Infirmary, Manchester, UK.
authors.forename: C M
authors.initials: CM
authors.lastname: Hay
authors.affiliation: Division of Hematology/Oncology, Department of Medicine, University of Pittsburgh, Pittsburgh, PA, USA.|Department of Pediatrics, Children's Hospital of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: L M
authors.initials: LM
authors.lastname: Malec
authors.affiliation: Division of Hematology/Oncology, Department of Medicine, University of Pittsburgh, Pittsburgh, PA, USA.|Hemophilia Center of Western Pennsylvania, Pittsburgh, PA, USA.
authors.forename: C D
authors.initials: CD
authors.lastname: Seaman
authors.affiliation: Department of Health Policy and Management, Graduate School of Public Health, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: J
authors.initials: J
authors.lastname: Li
authors.affiliation: Center for Research on Health Care Data Center, University of Pittsburgh, Pittsburgh, PA, USA.
authors.forename: J G
authors.initials: JG
authors.lastname: Yabes
authors.affiliation: Department of Biochemistry, University of Vermont, Colchester, VT, USA.
authors.forename: S
authors.initials: S
authors.lastname: Butenas
authors.affiliation: Department of Biochemistry, University of Vermont, Colchester, VT, USA.
authors.forename: K
authors.initials: K
authors.lastname: Brummel-Ziedins
keywords: factor VIII; hemophilia A; hemostasis; immune tolerance; inhibitors