Relationship of activated clotting time to heparin dose depends on the type of cuvette used with the hemochron signature elite

The activated clotting time (ACT) is a widely used point-of-care test for adjusting heparin dose during interventional procedures. In early 2010, clinicians from our cerebrovascular, vascular, cardiac, and cardiothoracic divisions noted that they were using substantially higher doses of heparin to achieve target ACT values. Potency standards for unfractionated heparin (UFH) manufactured in the United States had been modified a few months before this time. We investigated whether details of the ACT measurement might relate to the apparent difference between "new" versus "old" heparin. We measured ACT on blood spiked with old and new heparin using both the regular (ACT+) and low-range (ACT-LR) cuvettes for the Hemochron Signature Elite (ITC, Edison, NJ). We also compared concurrent ACT+ and ACT-LR values during 10 neurointerventional procedures. We found differences in the ACT responsiveness to new versus old heparin. Hemochron ACT-LR cuvettes were more sensitive than ACT+ cuvettes to any given dose of either heparin. Target ACT values were more easily achieved using ACT-LR cuvettes than ACT+ cuvettes, particularly when using the new heparin. After 50 to 80 units/kg of new heparin, most patients achieved ACT-LR values of 250 to 300 seconds, but ACT+ values remained at 150 to 200 seconds. Clinicians need to be aware that target values for ACT during interventional procedures must be tailored not just to the Hemochron and Medtronic devices but within the Hemochron system to the tube method as well as the two types of cuvettes.