Experience with a Modified Nijmegen-Bethesda Method for Measurement of Inhibitors in Hemophilia Patients: The CDC Inhibitor Surveillance Pilot Project.

A pilot project of a prospective surveillance system for coagulation factors VIII and IX inhibitors has been initiated at 9 U.S. Hemophilia Treatment Centers (HTCs). More than 500 patients have been enrolled. Risk factor and product exposure data are recorded at each HTC. A blood specimen is collected upon entry, annually, at product switch, or for clinical indication and tested centrally at the CDC, using a modified Nijmegen-Bethesda method. Plasma from specimens collected in 4.5mL evacuated tubes containing 3.2% sodium citrate is shipped overnight on cold packs. Specimens are heated to 56^oC for 30 minutes to remove endogenous and infused FVIII and centrifuged. Specimens are initially screened for inhibitor using a single dilution of 3 parts patient plasma to 1 part normal pool plasma buffered with imidazole to pH 7.4 (BNPP). Specimens showing inhibition and those from previously positive patients are tested in multiple dilutions at 1 part patient dilution to 1 part BNPP. Dilution is in naturally FVIII-deficient plasma containing normal von Willebrand factor. After a 2-hour incubation at 37^oC, FVIII remaining in the patient mixture is divided by FVIII remaining in a 1:1 mixture of BNPP and FVIII-deficient plasma and expressed as % residual activity (RA). %RA is converted to Nijmegen-Bethesda units (NBU) using a curve with one NBU equal to 50% RA. An inhibitor plasma of known titer is run with each assay as positive control. Split specimens shipped frozen and on cold packs showed a correlation of 0.998. 3:1 and 1:1 mixtures showed a correlation of 0.97. Almost 50% of the first 200 specimens received had measurable FVIII activity. The heating step was introduced to remove FVIII without damaging the antibody. 65 specimens went from >100% RA to a titer of 0–0.2 NBU after heating. Among 538 specimens tested for FVIII inhibitors, 435 (81%) were from patients with no previous history of inhibitor (shown below). 429 (98.6%) were < 0.5 NBU. The 6 specimens (1.4%) with > 0.6 NBU are under investigation as possible seroconversions.

Among 121 specimens tested for factor IX (FIX) inhibitors, 113 were from patients with no previous history of inhibitor and all had NBU <0.2, using a comparable FIX inhibitor assay. Testing in a central laboratory by uniform methods will facilitate detection of new inhibitors and investigation of risk factors.