Reproducibility and variability of activated clotting time measurements in the cardiac catheterization laboratory

The objective of this study was to characterize the reproducibility and variability in the measurement to the activated clotting time (ACT) when performed on two different types of instruments, the HemoTec ACT (Medtronic) and the Hemochron 801 (International Technidyne). The ACT has evolved into the most common point-of-care test used in the cardiac catheterization lab to manage patient heparinization. Since the test has not been standardized, different systems frequently produce different results under the same clinical conditions. Duplicate paired ACT tests (n = 885) from 359 patients were performed on both instruments. Prothrombin times (PT) and activated partial thromboplastin times (aPTT) were also determined on subsets of these same samples (PT = 533; aPTT = 487). The performance and relationships between the two tests were determined using a variety of statistical analytical techniques. The average difference between the ACT devices was only 8 sec, yet more than 60% of the measurements varied by more than 10%. Over one-fourth of measurements varied by more than 20%. The reproducibility to the HemoTec instrument was superior to the Hemochron instrument across the entire range of ACTs measured (mean coefficient of variation 2.4% +/- 3.1% vs. 7.2% +/- 6.1% for HemoTec and Hemochron, respectively; P < 0.00001; range = 65-555 sec). The relationship between the two ACTs was nonlinear. In therapeutic ranges used for interventional procedures (200-350 sec), HemoTec and Hemochron ACTs are not comparable to one another. Statistical comparative analysis indicated that the HemoTec ACT has better overall performance.