Inhibitors in Hemophilia. Pathophysiology. Characterization and Properties of Inhibitors, страница 2

The antigenic regions on the factor VIII molecule to which neutralizing factor VIII inhibitors bind have been identified. MLID89001198 MLID87185841 MLID92376757  40–43 Interestingly, the epitopes to which allo- or autoantibodies are directed are limited to certain areas of the factor VIII light or heavy chains, or both MLID89001198 MLID87185841 MLID92376757  40–43 (Fig. 109-2). Immunoblotting and binding studies with fragments of factor VIII have shown that plasma from approximately 50% of inhibitor patients contain at least two different neutralizing antibodies directed at both the light and heavy chain, whereas the other 50% bind to only the light chain. MLID92376757  43 In addition, it has been noted that the inhibitor produced by a given patient may change over the course of time and a few non-neutralizing antibodies may occur as well. MLID89001198 MLID87185841 MLID92376757 MLID90105722  40–44

Laboratory Evaluation and Quantification of Inhibitor Titer

The presence of an inhibitor to factor VIII should be suspected in a hemophiliac if transfused factor VIII appears either to have a short half-life or is not efficacious in achieving hemostasis, or both. This can be suspected in the laboratory whenever the partial thromboplastin time of a mixture of patient's plasma and normal plasma, after incubation for 2 hours at 37єC, is longer than that of a mixture of patient plasma and hemophilic plasma known not to contain an inhibitor. The sensitivity of this assay can be markedly increased by using a 4:1 patient/normal plasma mixture and incubating with kaolin/cephalin suspension for 2 hours at 37єC. MLID77158810 MLID82227542  45,46 As these tests lack specificity, in order to confirm that an inhibitor acts specifically with factor VIII, a dilution of the patient's plasma must be incubated with an equal volume of normal plasma and factor VIII levels measured in subsamples removed immediately and after 60 and 120 minutes. If the inhibitor is specific for factor VIII, factor VIII will decrease over time in the incubation mixture. Other methods, using nephelometry, MLID81084762  47 inhibition of coagulation in agarose gel, MLID76062134 MLID85280367  48,49 and immunoradiometry, MLID82000379 MLID82278057 MLID83123080  50–52 have been described but have not been adopted for widespread use.

Most centers in the United States use the Bethesda assay for quantification of factor VIII inhibitors. 53 Factor VIII assays are done on 2-hour incubation mixtures of various dilutions of patient's plasma with normal plasma. A test sample producing a residual factor VIII activity of 50% of normal is considered to contain 1 Bethesda unit of inhibitor per milliliter, and the inhibitor titer equals the reciprocal of the dilution of inhibitor plasma that neutralizes 50% of normal factor VIII. In England, the New Oxford Method is used to quantitate factor VIII inhibitors. MLID82176818  54 One Bethesda unit equals 1.21 times 1 Oxford unit. MLID82176818  54 An inhibitor unit does not imply that any specific number of factor VIII units infused into the patient will neutralize any specific number of inhibitor units.

Therapy

Factors to be Considered in Selecting a  Blood Product for a Bleeding Episode

1.  Patients known to be high responders should not  receive blood products containing factor VIII to treat  minor hemorrhages, so as to avoid an anamnestic  response (unless they are undergoing induction of  immune tolerance). Conservative measures combined  with the administration of factor IX complex  concentrate is frequently adequate, since only a few inhibitor  titers rise on exposure to the small amount of factor  VIII coagulant antigen that may contaminate such  concentrates.

2.  If a hemorrhage is critical, an attempt should be made  to raise the plasma factor VIII level into the hemostatic  range of 30 to 50 U/ml.

3.  In those patients with a serious hemorrhage who are  either low responders or high responders with a low  inhibitor level (<5 Bethesda units), high-purity human  factor VIII can be given initially in an initial large bolus  of 5,000–10,000 U, followed by a continuous infusion  of 1,000 U/hr. Alternatively porcine factor VIII can be  used. By contrast, the same type of patient (low  inhibitor level of <5 Bethesda units) with a minor  hemorrhage can be most easily managed with factor IX  complex concentrate in doses of 75–100 U/kg repeated  once or twice at 8 - 12-hour intervals as necessary.  Factor IX complex concentrate has only trace amounts of  factor VIII antigen and only rarely causes an  anamnestic response. MLID80066000 55