How does the age of a blood sample affect it's activated clotting time? Comparison of eight different devices

Monitoring activated clotting time (ACT) during extracorporeal procedures is virtually universal. The ACT test is usually performed immediately following blood collection. However, certain situations may occur that delay rapid measurement. It is unknown how an aged blood sample affects the ACT measurement. It is hypothesized that the ACT will be affected as a blood sample ages. Multiple blood samples were taken from six patients undergoing cardiopulmonary bypass (CPB). Samples were divided into two groups, heparinized (H) and unheparinized (UH). ACT/HMT tests were performed with each sample on eight devices (Array Actalyke, Gem PCL, Hemochron Jr. Signature, Hemochron Response, Hemochron 801, Hemotec HMS, Rapidpoint Coag, and Sonoclot II) at three different sample ages [< 60 s (fresh blood), 10 min, and 15 min after sample collection. ACT/HMT results of aged samples (10 min and 15 min after sample collection] were compared to ACT/HMT results for fresh blood using a repeated measures analyses of variance (ANOVA) with Student's-Newman-Keuls post hoc test. In the unheparinized group, no device produced an ACT significantly different from the fresh sample counterpart at the 10 min time point. At the 15 min time point, the Hemochron 801 produced a significantly lower average ACT when compared to the fresh sample. (120 +/- 25 vs. 135 +/- 5 s). In the heparinized group, the Actalyke device produced results with 10 and 15 min aged blood that were significantly longer than fresh blood sample results (ACT < 60 s = 426 +/- 66, 10 min = 457 +/- 82, 15 min = 450 +/- 68 s, p < .05). No other device produced significant differences for either time period. Based on this limited sample population, it seems that accurate ACT may be performed on blood samples up to 15 min old in many devices.