The impact of oral anticoagulant therapy, factor VIII level and quality of factor V-deficient plasma on three commercial methods for activated protein C resistance

Several methods are now available for the laboratory assessment of activated protein C resistance (APCR). In this study, we evaluated two activated partial thromboplastin time-based assays [Coatest(R) activated protein C (APC) and Diagen protein C activator (PCA)I, with and without predilution of test plasma in factor V-deficient plasma (FVdp) and an amidolytic assay (Immuno Ltd, Vienna, Austria). Testing plasmas from normal volunteers who had received 1-deamino-8-D-arginine vasopressin (DDAVP) also assessed the effect of elevated factor VIII on APCR. In the unmodified clotting tests, the Coatest kit gave overlapping results for normal and heterozygous FV:Q(506) samples; some FV:Q(506) samples on oral anticoagulant therapy (OAT) were misclassified as normal, and some normal samples with high factor VIII levels would be classified as APC resistant. The unmodified Diagen kit correctly classified these three types of sample, but had the disadvantage that prolonged PCA clotting times gave serious problems with instrument end-point detection. Both kits modified by diluting the samples in FVdp correctly classified all the samples, as well as samples from patients with lupus anticoagulant (LA) and patients receiving heparin. The Immunochrom kit correctly classified the normal and FV:Q506 samples, but would have misclassified most normal persons on OAT as well as some patients with LA or receiving heparin therapy as APC resistant. Blood Coagul Fibrinolysis 12:179-186 (C) 2001 Lippincott Williams & Wilkins.