Summary
A simple assay for quantitative determination of factor VIII (AHF) inhibitors has
been developed. The presence of AHF inhibitor is confirmed by determining clotting
time in serial dilutions of patient’s plasma. The amount of inhibitor is then quantitat-ed
by adding known units of human AHF concentrate (1 to 100 units/ml) to plasma until
neutralization occurs. The neutralization point is defined as that level of AHF concentrate
which clots the patient’s plasma in the same time as the control plasma (inhibitor-free)
would clot in the presence of 1 unit of AHF concentrate. (This point, minus the initial
1 unit/ml, gives the titer of inhibitor present (units/ml). The dosage of AHF concentrate
required to overcome the inhibitor and raise the free circulating AHF to the desired
theoretical level is then calculated from the known titer of inhibitor. This method
has been evaluated by assays of 75 samples from 22 patients with acquired or induced
AHF inhibitors. In vivo plasma AHF activity correlated well with the calculated in vitro values. The assay provides reliable and accurate quantitation of factor VIII inhibitors
enabling precise calculation of effective AHF dosage and monitoring of therapeutic
response. The assay can be readily adapted to quantitation of other clotting factor
inhibitors.
