Answer
They are antibodies that neutralise factor VIII, making replacement treatment less effective. They are detected through clinical assessment and blood tests.
Sources for this answer
- ADVATE 250 IU, 500 IU, 1000 IU & 1500 IU powder and solvent for solution for injection 2 ml solvent — Current selected SmPCemc · Summary of Product Characteristics
- NHSNHS · NHS medicine guidance
General medicine information. Follow the instructions for your own medicine and speak to a pharmacist or clinician about your individual circumstances.