Lysed platelets shorten the activated coagulation time (ACT) of heparinized blood.

Aliquots of heparinized whole blood were incubated at 37 degrees C with preparations of lysed platelets or buffer. In the presence of lysed platelets, the ACT decreased to 127 seconds (+/- 41, n = 9) versus 248 seconds (+/- 43, n = 9) in the controls (P = 0.0002). The shortening of the ACT by lysed platelets followed a typical dose-response relationship. Fractionation of lysed platelet preparations revealed that the procoagulant membrane fraction, not the cytosol fraction containing PF4, was responsible for this effect of lysed platelets on the ACT. These findings demonstrate that the ACT can be artifactually low in the presence of heparin, indicating a potential problem in the use of the ACT for monitoring of heparin neutralization.