Most of us probably know that anesthetists use the activated coagulation time (ACT) to monitor unfractionated heparin therapy during coronary bypass graft surgery, and that the heparin plasma concentration is raised to 2.5 units/mL during surgery. The ACT is a time-honored adaptation of the Lee-White clotting time in which fresh whole blood is pipetted to a 12 X 75 mm tube containing a negatively-charged particulate activator. The interval between blood collection and clot formation is typically 110–150 seconds. While it can be performed manually, most cardiac operating rooms are equipped with a Hemochron®, from ITC Inc, which provides a semiautomatic ACT.
Yesterday, Prof. Bernadette Rodak, Indiana University, asked me about the target therapeutic range, and checking around, I realized most laboratorians don’t know, because the test is invariably performed in the operating room. I contacted Laura Taylorat UAB Academic Health Sciences Center and Dave McGlasson at Wilford Hall USAF Medical Center. I learned anesthetists try to hit 1.5–2.0 X the baseline, for instance 165–300 seconds. The amount of heparin needed to reach and maintain a certain ACT varies as does the body’s clotting potential at that ACT. If there are clotting or bleeding problems, the dosages and ACT may need to be adjusted accordingly. After surgery, the ACT may be maintained within a narrow range (for instance, 175 – 225 seconds) until the patient has stabilized.
Thanks to “Bunny” for the question, and Laura and Dave for helping me with the answer, which I hope is useful information for everyone.