Antibodies to streptokinase (SK) are widespread in the population, but
reports of their effect on the action of SK are conflicting. Specific
anti-SK IgG was purified from the sera of 10 patients, five with low
titres of anti-SK IgG and five with high titres. The effect of increas
ing specific anti-SK IgG antibodies on the action of SK was evaluated
in vitro using a fluorimetric assay for plasmin and by a fibrin plate
lysis assay. The inhibition of SK by whole plasma from a further group
of patients was also assessed by the fibrin plate assay. There was a
positive correlation between the serum antibody concentration and the
quantity of specific anti-SK eluted (r = 0.797; P < 0.005). The additi
on of specific anti-SK IgG caused a dose-related decrease in SK activi
ty (fluorimetric assay r = -0.93; P = 0.02; fibrin plate assay r = -0.
98; P < 0.001). The addition of patient plasma to the fibrin plate ass
ay also resulted in decreased lysis, which was dependent upon antibody
titre (r = -0.95; P < 0.0001). Significant in vitro reduction of the
activity of SK by specific antibody was demonstrated, and this was sim
ilar with plasma containing comparable amounts of antibody. The findin
gs suggest that treatment with SK would be unlikely to induce an effec
tive thrombolytic state when antibody titres are high (such as those s
een within 2 years of an initial dose of SK).