18th August, 1975 Dear Madam, In November, 1974, a meeting was held, sponsored by the Division of Blood Diseases and Resources, National Heart and Lung Institute, United States of America, to discuss Factor VIII inhibitors. We found that communication with each other about the potency of inhibitors was arduous, for each person had his or her own method of measuring inhibitors and of describing their potency in arbitrary “units,” which could only be defined by describing the method of measurement. The “units” defined by one method could not be translated by some conversion factor into the “units” of another method because the methods had too many variables, the effects of which are not understood. Moreover, the potency implied by “one unit” in one method might be very much greater than that implied by “one unit” in another method.

Each of us felt willing to alter his or her own method of measurement of inhibitors to the extent necessary to permit a uniform, although still arbitrary, description of inhibitor units among ourselves. We realized that any definition of an inhibitor in terms of units is only a rough, operational description, because the kinetics of different inhibitors vary.