Skip to content
How It Works
Explore
Resources Ask a Question
Who It's For
Patients
Decode results, prep for appointments, weigh options.
Caregivers & advocates
Carry the research load for someone you love.
Providers & clinicians
Resources for patients in your practice.
Log In Ask a Question
PubMed This is a summary of 21 peer-reviewed journal articles Updated
Hematology · Factor VIII Inhibitor

Factor VIII Inhibitors: What They Are and How to Manage Them

At a Glance

Factor VIII inhibitors develop when the immune system mistakenly attacks standard Hemophilia A replacement therapies. They can be eliminated using Immune Tolerance Induction (ITI) or managed with bypassing agents and therapies like emicizumab to prevent and treat bleeding episodes.

The development of an inhibitor is the most significant challenge in the treatment of Hemophilia A. An inhibitor is not a new disease; rather, it is a complication where the body’s immune system mistakenly identifies the Factor VIII (8) replacement protein as a “foreign” threat and develops antibodies (inhibitors) to neutralize it [1][2].

Who is at Risk?

Inhibitors develop in approximately 25% to 35% of patients with severe Hemophilia A who have never been treated before [1].

  • Genetic Factors: The risk is highest in those with specific “high-risk” mutations in the F8 gene, such as large deletions or intron inversions [3][4].
  • Timing: They most often appear within the first 50 days of treatment with Factor VIII [1].
  • Classification: Doctors use the Bethesda Assay to measure how “strong” an inhibitor is. A “low-responder” (<5 BU) can sometimes be managed with higher doses of regular factor, while a “high-responder” (≥5 BU) requires more specialized strategies [5][6].

Eradicating the Inhibitor: ITI

The gold-standard treatment for getting rid of an inhibitor is Immune Tolerance Induction (ITI) [7][8].

  • The Goal: To “teach” the immune system to tolerate Factor VIII [9].
  • The Process: This involves giving the patient frequent (often daily) doses of Factor VIII over a long period—sometimes months or even years [8][10].
  • Success Rate: ITI is successful in about 70% of cases [11][12]. Once successful, the patient can return to standard Factor VIII prophylaxis.

Managing Bleeds: Bypassing Agents

Because inhibitors block regular Factor VIII from working, doctors must use bypassing agents to stop active bleeding [13][14]. These medicines “skip” the step in the clotting process that requires Factor VIII.

  • rFVIIa (NovoSeven): A recombinant protein that works at the site of injury to kickstart clotting.
  • aPCC (FEIBA): A concentrate of several different clotting factors that work together to bypass the block.

The Emicizumab (Hemlibra) Revolution

The introduction of emicizumab has transformed life for those with inhibitors [15]. Because emicizumab is a synthetic antibody—not Factor VIII—the inhibitor antibodies cannot “see” it or block it [15][16].

  • Continuous Protection: It provides a steady baseline of protection through a simple injection under the skin (subcutaneous) [17].
  • Safety Note: If you are on emicizumab and need a bypassing agent for a “breakthrough” bleed, doctors usually prefer rFVIIa [18]. Using aPCC (FEIBA) while on emicizumab requires extreme caution due to a risk of dangerous blood clots (thrombosis) [19][20].

Prophylaxis Comparison for Patients with Inhibitors

Feature Bypassing Agent Prophylaxis Emicizumab Prophylaxis
Delivery Intravenous (IV) Subcutaneous (Under skin)
Frequency Often daily or every other day Weekly, every 2 weeks, or monthly
Mechanism Bypasses FVIII step Mimics FVIII function [15]
Cost/Ease High burden/High cost Lower burden/Cost-saving [21]

If an inhibitor is detected, your care team will work with you to decide whether to start ITI immediately, switch to emicizumab, or use a combination of both to protect your joints while working to eradicate the inhibitor [7][12].

Common questions in this guide

What is a Factor VIII inhibitor?
An inhibitor is an antibody created by your immune system that mistakenly attacks standard Factor VIII replacement therapy. This stops the replacement factor from working effectively to prevent or treat bleeding.
How are Factor VIII inhibitors diagnosed and measured?
Doctors use a blood test called the Bethesda Assay to measure the strength of the inhibitor. Based on these results, you will be classified as a low-responder or high-responder, which helps determine your treatment plan.
What is Immune Tolerance Induction (ITI)?
Immune Tolerance Induction is a treatment designed to teach your body's immune system to accept Factor VIII. It involves receiving frequent doses of Factor VIII over months or years, and it successfully eliminates the inhibitor in about 70 percent of cases.
How do you treat a bleed if you have a high-responding inhibitor?
Because standard Factor VIII will not work, doctors use special medications called bypassing agents to stop active bleeding. These medicines skip the step in the blood clotting process that requires Factor VIII.
How does emicizumab work for patients with inhibitors?
Emicizumab is a synthetic antibody that mimics the function of Factor VIII. Because it is structurally different from standard Factor VIII, the inhibitor antibodies cannot block it, allowing it to provide steady bleed protection.

Questions to Ask Your Doctor

Curated prompts to bring to your next appointment.

  1. 1.What is my (or my child's) current inhibitor titer in Bethesda Units (BU), and are we 'low' or 'high' responders?
  2. 2.Based on the specific F8 mutation, what is our statistical likelihood of success with Immune Tolerance Induction (ITI)?
  3. 3.If we start ITI, what is the protocol for daily factor dosing, and how will we monitor progress?
  4. 4.If a breakthrough bleed occurs while on emicizumab, which bypassing agent should we use first, and in what dose?
  5. 5.Should we consider adding immunosuppressant medications (like rituximab) if the inhibitor doesn't respond to ITI alone?

Questions For You

Tap a prompt to share your answer — we'll use it plus this page's context to start a tailored conversation.

References

References (21)
  1. 1

    Timing of inhibitor development in more than 1000 previously untreated patients with severe hemophilia A.

    van den Berg HM, Fischer K, Carcao M, et al.

    Blood 2019; (134(3)):317-320 doi:10.1182/blood.2019000658.

    PMID: 31186271
  2. 2

    First prospective report on immune tolerance in poor risk haemophilia A inhibitor patients with a single factor VIII/von Willebrand factor concentrate in an observational immune tolerance induction study.

    Kreuz W, Escuriola Ettingshausen C, Vdovin V, et al.

    Haemophilia : the official journal of the World Federation of Hemophilia 2016; (22(1)):87-95 doi:10.1111/hae.12774.

    PMID: 26202305
  3. 3

    F8 gene mutation spectrum in severe hemophilia A with inhibitors: A large cohort data analysis from a single center in China.

    Sun J, Li Z, Huang K, et al.

    Research and practice in thrombosis and haemostasis 2022; (6(4)):e12723 doi:10.1002/rth2.12723.

    PMID: 35702590
  4. 4

    Large Intron Inversions in Romanian Patients with Hemophilia A-First Report.

    Brinza M, Grigore A, Dragomir M, et al.

    Medicina (Kaunas, Lithuania) 2023; (59(10)) doi:10.3390/medicina59101821.

    PMID: 37893540
  5. 5

    Recombinant porcine factor VIII corrects thrombin generation in vitro in plasma from patients with congenital hemophilia A and inhibitors.

    Négrier C, Oldenburg J, Kenet G, et al.

    Research and practice in thrombosis and haemostasis 2022; (6(4)):e12731 doi:10.1002/rth2.12731.

    PMID: 35765670
  6. 6

    ITI Treatment is not First-Choice Treatment in Children with Hemophilia A and Low-Responding Inhibitors: Evidence from a PedNet Study.

    van den Berg HM, Mancuso ME, Königs C, et al.

    Thrombosis and haemostasis 2020; (120(8)):1166-1172 doi:10.1055/s-0040-1713097.

    PMID: 32572865
  7. 7

    Key questions in the new hemophilia era: update on concomitant use of FVIII and emicizumab in hemophilia A patients with inhibitors.

    Carcao M, Mancuso ME, Young G, Jiménez-Yuste V

    Expert review of hematology 2021; (14(2)):143-148 doi:10.1080/17474086.2021.1875817.

    PMID: 33499681
  8. 8

    Low-dose immune tolerance induction therapy in children of Arab descent with severe haemophilia A, high inhibitor titres and poor prognostic factors for immune tolerance induction treatment success.

    Elalfy M, Elghamry I, Hassab H, et al.

    Haemophilia : the official journal of the World Federation of Hemophilia 2022; (28(1)):65-72 doi:10.1111/hae.14456.

    PMID: 34797008
  9. 9

    Hemophilia A with inhibitor: Immune tolerance induction (ITI) in the mirror of time.

    Nakar C, Shapiro A

    Transfusion and apheresis science : official journal of the World Apheresis Association : official journal of the European Society for Haemapheresis 2019; (58(5)):578-589 doi:10.1016/j.transci.2019.08.008.

    PMID: 31447396
  10. 10

    Recombinant factor VIII Fc fusion protein for immune tolerance induction in patients with severe haemophilia A with inhibitors-A retrospective analysis.

    Carcao M, Shapiro A, Staber JM, et al.

    Haemophilia : the official journal of the World Federation of Hemophilia 2018; (24(2)):245-252 doi:10.1111/hae.13413.

    PMID: 29436077
  11. 11

    Inhibitor recurrence after immune tolerance induction: a multicenter retrospective cohort study.

    Antun A, Monahan PE, Manco-Johnson MJ, et al.

    Journal of thrombosis and haemostasis : JTH 2015; (13(11)):1980-8 doi:10.1111/jth.13143.

    PMID: 26382916
  12. 12

    Emicizumab in tolerized patients with hemophilia A with inhibitors: A single-institution pediatric cohort assessing inhibitor status.

    Batsuli G, Greene A, Meeks SL, Sidonio RF

    Research and practice in thrombosis and haemostasis 2021; (5(2)):342-348 doi:10.1002/rth2.12475.

    PMID: 33733033
  13. 13

    International recommendations on the diagnosis and treatment of acquired hemophilia A.

    Tiede A, Collins P, Knoebl P, et al.

    Haematologica 2020; (105(7)):1791-1801 doi:10.3324/haematol.2019.230771.

    PMID: 32381574
  14. 14

    PERSEPT 3: A phase 3 clinical trial to evaluate the haemostatic efficacy of eptacog beta (recombinant human FVIIa) in perioperative care in subjects with haemophilia A or B with inhibitors.

    Escobar M, Luck J, Averianov Y, et al.

    Haemophilia : the official journal of the World Federation of Hemophilia 2021; (27(6)):911-920 doi:10.1111/hae.14418.

    PMID: 34614267
  15. 15

    Disruptive technology and hemophilia care: The multiple impacts of emicizumab.

    Hermans C, Makris M

    Research and practice in thrombosis and haemostasis 2021; (5(4)):e12508 doi:10.1002/rth2.12508.

    PMID: 34027289
  16. 16

    Emicizumab, a bispecific antibody recognizing coagulation factors IX and X: how does it actually compare to factor VIII?

    Lenting PJ, Denis CV, Christophe OD

    Blood 2017; (130(23)):2463-2468 doi:10.1182/blood-2017-08-801662.

    PMID: 29042366
  17. 17

    Prophylactic emicizumab for hemophilia A in the Asia-Pacific region: A randomized study (HAVEN 5).

    Yang R, Wang S, Wang X, et al.

    Research and practice in thrombosis and haemostasis 2022; (6(2)):e12670 doi:10.1002/rth2.12670.

    PMID: 35284778
  18. 18

    Concomitant Use of rFVIIa and Emicizumab in People with Hemophilia A with Inhibitors: Current Perspectives and Emerging Clinical Evidence.

    Linari S, Castaman G

    Therapeutics and clinical risk management 2020; (16()):461-469 doi:10.2147/TCRM.S205310.

    PMID: 32547043
  19. 19

    Evaluating the safety of emicizumab in patients with hemophilia A.

    Langer AL, Etra A, Aledort L

    Expert opinion on drug safety 2018; (17(12)):1233-1237 doi:10.1080/14740338.2019.1551356.

    PMID: 30462521
  20. 20

    Synergistic Effect of Bypassing Agents and Sequence Identical Analogue of Emicizumab and Fibrin Clot Structure in the In Vitro Model of Hemophilia A.

    Zong Y, Antovic A, Soutari NMH, et al.

    TH open : companion journal to thrombosis and haemostasis 2020; (4(2)):e94-e103 doi:10.1055/s-0040-1710032.

    PMID: 32704613
  21. 21

    Budget impact and cost-utility analysis of prophylactic emicizumab versus on-demand bypassing agents for adolescent severe haemophilia A patients with inhibitors in India.

    Krishnamoorthy Y, Govindan D, Kannan N, et al.

    Heliyon 2024; (10(5)):e27089 doi:10.1016/j.heliyon.2024.e27089.

    PMID: 38468938

This page provides educational information about managing Factor VIII inhibitors in Hemophilia A. Always consult your hematologist before changing treatment plans, starting ITI, or managing breakthrough bleeds.

Get notified when new evidence is published on Hemophilia A.

We monitor PubMed for new peer-reviewed studies on this topic and email a short summary when something meaningful changes.