What are the methods used to measure levels of factor VIII (FVIII) inhibitors in hemophilia A?

Updated: Apr 08, 2020
  • Author: Douglass A Drelich, MD; Chief Editor: Srikanth Nagalla, MBBS, MS, FACP  more...
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In the United States, levels of FVIII inhibitors are most often measured by the Bethesda method. In this method, 1 Bethesda unit (BU) equals the amount of antibody that destroys one half of the FVIII in an equal mixture of normal plasma and patient plasma in 2 hours at 37°C. Inhibitor levels are described as low titer or high titer, depending on whether they are less than or more than 5 BU, respectively; high-titer inhibitor levels are typically far higher than 5 BU.

The Nijmegen modification uses immunodepleted FVIII–deficient plasma instead of an imidazole saline buffer to ensure pH control to prevent non–antibody-mediated loss of FVIII-C activity during the prolonged 2-hour incubation period. [14] The Bethesda assay tends to underestimate the titer of VIII autoantibody because of its characteristics, in contrast to a hemophilic antibody. [15] The Oxford assay is another modification of the Bethesda inhibitor test.

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