Is the ACT two minutes after heparin injection reliable?

Blood for activated clotting time (ACT) measurement to verify the effect of the initial dose of heparin before cannulation in heart surgery has traditionally been drawn 5 minutes (min) after injection of the heparin. However, there has been an increasing demand to reduce the waiting time. The aim of this study was to investigate if ACT measured 1, 2, 3 and 4 min after heparin injection is as reliable as ACT measured 5 min after heparin injection.

Fifty adult patients undergoing routine cardiac surgery with a heart-lung machine. Heparinization was obtained with unfractioned porcine heparin. The ACT was measured with 5 Hemochron® Jr. machines 1, 2, 3, 4 and 5 min after the heparin injection. Full heparinization was defined as an ACT >400 seconds.

At 1 and 2 min, 94% (n=47) of the ACTs were > 400. All ACTs >400 seconds after 2 min remained >400 seconds at 3, 4 and 5 min. Mean values declined from 533 to 498. ANOVA analysis showed statistically significantly higher values at 1, 2 and 3 min, compared to 5 min, but not at 4 min. However, the estimated differences were small: 3.7-36 seconds. There was no significant difference between variances for the five sample times. Standard deviation declined from 123 to 100. Values at 2 min correlated as well as those at 5 min with mean 1-5 min values.

The range of the ACT values tends to diminish over time and, consequently, the reliability of the results increases. However, the difference is small and has little or no clinical relevance. Giving time for the circulation to distribute the heparin in the bloodstream, we recommend measuring the ACT two min after heparin administration.