Activation of the alternative pathway of the complement system by radiographic contrast media.

Nine patients who underwent radiographic contrast media (RCM) infusion for diagnostic purposes were studied. Simultaneously, venous blood samples were collected and complement was measured. None of the patients had a clinical history of allergy to previous infusion of RCM. A significant drop in plasma complement (CH50) levels was detected in five of nine patients. The drop in CH50 was observed within the first 20 min after the initiation of RCM infusion. All five patients in whom a drop in complement was found showed a decrease in hemolytic C3, properdin, and factor B. In addition, by radial immunodiffusion low levels of C3, C6, properdin, and factor B were observed and split products of C3 and factor B were identified by immunoelectrophoresis. No significant changes in C1q and C4 were detected by radial immunodiffusion during the study of these five patients. This study strongly suggested that in some patients undergoing RCM infusion, activation of the complement system occurred and this activation appears to be through the alternative pathway.