Methodological Considerations on the Determination of the APC Response in Plasma

Abstract

The performance of COATEST® APC<sup>TM</sup> Resistance on different coagulation instruments has been further evaluated through analysis of plasma from 100 blood donors on KC 10, ST 4, ACL 300R, Electra 900 and Thrombolyzer. The electromechanical instruments KC-10 and ST 4 showed a lower response to APC than the turbidime-tric or photometric instruments with median APC ratios of 2.7 and 2.8 for the two former versus 3.1 3.4 and 3.5, respectively, for the other three, which is in agreement with earlier initial findings. Similarly, the cut-off value varied between 2.1 and 2.6 for these instruments. The correlation of APC ratios between instruments was strong with rvalues ranging between 0.71 and 0.93 and, furthermore, none of the six plasmas with the lowest APC ratios on the Thrombolyzer ranked higher than 8 on any of the other instruments. Analysis of control plasmas with six consecutive kit batches on ACL and ST 4 resulted in APC ratio ranges of 3.3-3.7 and 2.7-3.0 for a normal control and 1.8-2.0 and 1.9-2.0 for an abnormal control on ACL and ST 4, respectively, illustrating a high reproducibility between batches. Repeated freezing and thawing of samples is disrecommended since this often resulted in increased APC ratios. In contrast, in spite of up to 40% decrease in FVIII activity upon storage of 10 different plasma samples for 5h, the effect on the APC ratio was only minor as was also the effect of addition of 1.0 IU/ml of FVIII. In neither case was any sample misclassified. Altogether, the results support the applicability of this kit for measuring the response of plasma to APC.