We investigated the effect of clot activators carried over from the serum tube on major coagulation tests during phlebotomy. First, blood specimens from 30 normal subjects were mixed with small amounts of fluid containing clot activators, and their effects on various coagulation tests were determined. Only the value of fibrin monomer complex displayed a remarkable change when thrombin-containing fluid was added to the blood specimens. Subsequently, 100 paired blood specimens (taken from 75 healthy volunteers and 25 patients taking warfarin) were collected in coagulation tubes before and after the serum tube using standard phlebotomy procedures. Various coagulation tests were performed to determine the effect of contamination of thrombin-containing blood on coagulation parameters. Differences between the 2 tubes were minimal but significant for some of the coagulation tests. Therefore, we conclude that the effect of clot activators in the serum tube on coagulation tests is minimal when standard phlebotomy procedures are used.