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Plasma C1 inhibitor (C1-INH) was determined in 64 patients with different malignant diseases and in 58 healthy persons. Cl-INH antigen concentration was measured by radial immunodiffusion (RID), whereas its functional activity was assayed with chromogenic substrate. Within-run and day-to-day precision of both methods were good, with CVs ranging from 3.6 to 5.4%. Plasma C1-INH antigen concentrations were significantly higher in the patients than in healthy controls (P=4.0 x 10^-3), as were their C1-INH functional activities (P=3.5 x 10^-3). C1-INH activities obtained in the patient plasma samples were in correlation with their antigen concentrations (r = 0.914), showing that C1-INH synthesized in malignant disease was functional. However, the specific activity of the C1-INH (functional activity/antigen concentration ratio) was significantly lower in the patient group as compared with the controls (P=2.1 x 10^-6), indicating partial inactivation of plasma C1-INH in malignant diseases. The C1-INH specific activity in patients was inversely proportional to its antigen concentration.