Summary
The clinical and laboratory data on the 12 patients with an acquired inhibitor to
factor V have been reviewed. The degree of clinical bleeding in these patients varied
from none to severe, and in most patients the inhibitor was transient. The combination
of a markedly prolonged partial thromboplastin time and Quick prothrombin time and
failure of normal plasma to correct these tests, in the presence of a normal thrombin
and prothrombin and proconvertin time, seems to be pathognomonic for a factor V inhibitor.
The inhibitors have physicochemical properties of immunoglobulins and a few have been
characterized as polyclonal IgG antibodies or a mixture of IgM and IgG antibodies.
The etiology and pathophysiologic mechanism of their development is unknown, but there
seems to be a close relationship to major surgery. When tested with inhibitor plasma,
the plasmas from 9 patients with hereditary factor V deficiency from 7 unrelated families
did not contain factor V antibody-neutralizing material.
