Answer
They are antibodies that neutralise factor IX and reduce treatment effectiveness. They are detected using specialist blood tests.
Sources for this answer
- RIXUBIS 1000 IU and 2000 IU powder and solvent for solution for injection — 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.