Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (660 page)

   Activated clotting time (ACT) is a rapid point-of-care standardized clotting time, performed by automated well-calibrated instruments, such as the Medtronic automated coagulation timer (ACT). A baseline ACT has to be established in each POCT area after induction of anesthesia and opening the chest for cardiopulmonary bypass surgery, because surgery and anesthesia shorten it. The ACT may also vary slightly with the lot number of the control cartridge.

   
Normal range in the absence of heparin (with Medtronic coagulometer):
74–125 seconds.

   Use

   ACT is the most widely used measure of anticoagulation with heparin (and neutralization of heparin with protamine) during extracorporeal circulation. After the initial dose of heparin, the ACT is maintained at >275 seconds for off-pump coronary procedures and >350 seconds for on-pump procedures by periodic administration of heparin.

   Interpretation

   There is some controversy concerning whether monitoring heparinization by ACT alone ensures optimal heparin and protamine doses. A poor correlation was found between ACT and heparin measurements using anti-Xa assays. Nevertheless, experience has shown that institution of anticoagulation and monitoring under ACT guidance and reversal improves hemostasis, limits blood loss, and reduces the need for transfusions.

   Limitations

   The response of ACT to heparin varies from individual to individual and with heparin potency.

   Underlying coagulopathies (antithrombin III deficiency, clotting factor deficiencies, DIC) must be excluded.

   Medications that inhibit platelet function (aspirin, NSAIDs) may affect ACT.

   Preanalytical errors (sample dilution or contamination with heparin, blood activation) must be avoided. It is particularly important to avoid the use of blood samples contaminated by heparin flushes.

ACTIVATED PROTEIN C RESISTANCE (APCR)
*

   Definition