Activated clotting time (ACT) testing: analysis of reproducibility

Activated Clotting Time (ACT) has been the standard for monitoring heparin anticoagulation in cardiac surgery for three decades. Although a 10% coefficient of variation (CV) is the referenced standard for the test, no recent reports of precision are available. The precision of Hemochron FTCA510 (celite) and KACT (kaolin) ACT test tubes was evaluated using a retrospective analysis of results from both laboratory studies and routine clinical usage. Laboratory studies of reproducibility included analysis of the CV from repetitive testing using multiple lots of ACTs. Substrates used included 40 consecutive lots of control plasma and freshly heparinized donor blood. Across the lots of control plasma, the celite ACT yielded an average CV of 5.4% for the normal control level and 4.0% in the abnormal control level (range 3.6-9.7% and 2.7-6.3%, respectively). The KACT showed similar performance for the normal (mean = 4.5%, range 2.2-7.8%) and abnormal (mean = 3.8%, range 2.0-10.0%). These values, significantly less than 10%, reflect the combined variability of both the ACT tests and the lyophilized, single use vial, control material. Fresh whole blood samples exhibited improved ACT precision when compared to this artificial substrate. CVs for the celite ACT range from 0.6-6.0% at one unit heparin/ml blood to 2.4-11.6% at 5 units/ml where clotting times exceed 650 sec. The KACT showed even lower CVs at all heparin levels, with values of 2.4-7.0%. Clinical evaluations included samples (N = 56) collected from cardiac surgery patients with celite ACT values ranging to 744 sec. Duplicate values differed by an average of 7.5 sec or 1.8%. There was only one clinically significant difference in paired values; a 376 sec paired with a 406 sec, 400 sec being the clinical target time. This retrospective data analysis demonstrates that Hemochron ACT variability is significantly less than 10%.