SummaryForty-eight healthy pregnant women were studied prospectively and longitudinally. Blood sampling was performed at 10-15, 23-25, 32-34 and 38-40 weeks of gestation, within one week and at eight weeks postpartum. Classic and modified activated protein C ratio decreased as pregnancy progressed. In the third trimester 92% of the ratios measured with the classic test were above the lower reference level whereas all modified test ratios were normal. Slight activation of blood coagulation was shown with increased levels of prothrombin fragment 1+2, soluble fibrin and D-dimer. Fibrinogen, factor VIII and plasminogen activator inhibitor type 1 and type 2 increased. Protein S and tissue plasminogen activator activity decreased. Protein C remained unchanged. No correlation was found between the decrease in classic APC ratio and changes in factor VIII, fibrinogen, protein S, prothrombin fragment 1+2 or soluble fibrin, nor between the increase in soluble fibrin and changes in prothrombin fragment 1+2, fibrinogen and D-dimer.

This study was aimed at evaluating the performance of a chromogenic factor VIII assay on STA, an automated coagulation analyzer. Additionally, a correlation study was conducted with an aPTT-based one-stage factor VIII clotting assay. Throughout the study the performance of the chromogenic assay was tested in two ranges of factor VIII activity: a high range with activity between 20% and 150% and a low range with activity below 20%. Inter-assay coefficient of variation ranged from 1.9% to 8.9% and intra-assay coefficient of variation from 0.5% to 11.4%, depending on factor VIII concentration. Day-to-day reproducibility was tested over a 5-day period; between-day imprecision ranged from 7.1% to 9.4%. The chromogenic factor VIII assay showed a good correlation with the clotting assay in both ranges. The correlation coefficients were 0.924 and 0.792 for the high and low range, respectively. A statistically significant difference in mean values was observed in the high range (p < 0.0001). Comparison between the chromogenic assay on STA versus on microplates showed a high correlation (0.991), which was highly significant (p < 0.0001). In conclusion, the chromogenic assay for factor VIII on STA shows good analytical performance. It correlates well with the one-stage factor VIII clotting assay, although significant differences between individual samples occur. Probably these are partly related to differences in measurement principle and standardization. Altogether, this precise and rapid assay is suitable for determination of factor VIII by an automated procedure.

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The performance of COATEST® APC^TM 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.