ALBERT

Beschreibung: 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 56oC 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 37oC, 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. Nijmegen-Bethesda Units in Patients with No Previous Inhibitor NBU No. Pts. (%) NBU No. Pts. (%) NBU No. Pts. (%) 0 220 (50.6%) 0.4 3 (0.7%) 0.8 1 (0.2%) 0.1 127 (29.2%) 0.5 0 0.9 0 0.2 59 (13.6%) 0.6 0 1.0–4.0 2 (0.5%) 0.3 20 (4.6%) 0.7 2 (0.5%) 5.0–19.0 1 (0.2%) Among 121 specimens tested for factor IX (FIX) inhibitors, 113 were from patients with no previous history of inhibitor and all had NBU