Destruction of ristocetin cofactor by coagulation at 4 degrees C

Ristocetin cofactor (VIIIR:RCo) and factor VIII-related antigen (VIIIR:Ag) were measured in anticoagulated and non-anticoagulated blood incubated at 4 degrees C, room temperature (RT) or 37 degrees C for 24 hours. A marked decrease in VIIIR:RCo, to almost undetectable levels, and a smaller decrease in VIIIR:Ag occurred when whole blood clotted at 4 degrees C. These changes were slight or absent when blood clotted at RT or 37 degrees C. VIIIR:RCo lost at 4 degrees C was not recoverable by further incubation at 37 degrees C but the less-marked loss of VIIIR:Ag was partially recovered. In blood which had clotted at 4 degrees C there was a change in the electrophoretic profile of VIIIR:Ag on crossed immunoelectrophoresis: there was more anodal migration of the VIIIR:Ag peak, consistent with a decrease in the mean molecular size. Further experiments showed that the decrease in VIIIR:RCo during coagulation at 4 degrees C preceded the decrease in fibrinogen levels. In cell-free plasma VIIIR:RCo also decreased markedly when coagulation occurred at 4 degrees C. The results show that loss of VIIIR:RCo occurs when blood is allowed to clot at 4 degrees C: this is not due to cryoprecipitation and does not require the presence of blood cells. The data suggest that it is probably caused by plasma proteases activated early in the coagulation pathway.