Although the activated clotting time (ACT) is commonly used to assess adequacy of anticoagulation during percutaneous transluminal coronary angioplasty (PTCA), there is uncertainty whether measurements on samples from the arterial and venous circulations are directly comparable. We performed ACT determinations on 115 patients undergoing PTCA at our institution. Blood samples were drawn in a sequential fashion from the arterial and femoral venous sheaths at the conclusion of each case, and ACT determination were performed in the catheterization laboratory immediately thereafter. The venous ACT exceeded the arterial value in 63 patients (55%), and was identical in only 2 instances. The arterial and venous ACT differed by more than 100 s in 10 patients. In 23 patients (20%) one ACT determination was > or = 300 s, while the value from the other circulation was < 300 s. We conclude that there is substantial variability between arterial and venous ACT determinations in heparinized patients undergoing PTCA.