I received a question last Friday asking whether the activated clotting time assay (ACT) is affected by low molecular weight heparin such as Lovenox. Yes, but first…
…For readers under forty, the ACT has been around for many years, and is based on an even older test I learned as the Lee-White clotting time. This test was a misery, as you drew fresh venous blood, dispensed 1 mL each from the syringe to three 10×12 mm test tubes, and tilted the tubes, one by one, every 30 seconds, continuing until the third tube clotted. The Lee-White normal was 4 to 8 minutes if you kept the tubes in a 37 degree incubator, or 8 to 15 minutes if tested at room temperature. However the test was used to monitor unfractionated heparin. With heparin therapy it would run to 30-40 minutes of grinding boredom and uncertainty. Some would never clot. Further, it seems like the heparin was always started at midnight, so the assay was needed at 4:00 AM.
The ACT was a wonderful refinement, as a particulate negatively charged activator similar to what you now find in APTT reagent was added to the tube, reducing normals to less than 100 seconds, with therapeutic heparin running to 190 seconds. International Technidyne Company (ITC) developed the Hemochron instrument series, which automates the ACT, and today you will find a Hemochron in nearly every cardiac surgery suite in the USA. Their main application is to monitor extreme high heparin levels during coronary artery bypass graft surgery, when the PTT would exceed linearity.
According to Dr. Soumaya El-Rouby, senior manager of clinical affairs at ITC, the ACT may be employed to monitor intravenous low molecular weight heparin therapy. Dr. El-Rouby quotes Cavusoglu E, Lakhani M, Marmur JD. The activated clotting time can be used to monitor Enoxaparin and Dalteparin after intravenous administration. J Invasive Cardiol 2005;17:416-21. This seems valuable, as some surgeons are switching to low molecular weight heparin. Geo
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