Screening for a resistance against activated
protein C (aPCR), which is in most cases caused by FV:Q506 mutation, is performed by functional tests measuring the effect of aPC on activated partial thromboplastin time (aPTT). Because of an insufficient discrimination between FV:Q506 mutation negative and positive individuals with the first generation of the functional test Coatest
aPC Resistance (Chromogenix AB, Mölndal, Sweden), the definition of an arbitrary cut-off level was only possible using the results of
DNA analysis. The use of an arbitrary cut-off level still resulted in unsatisfactory low sensitivity and specificity for the functional test. Thus, time- and cost-consuming
DNA analyses had to be performed frequently to establish the diagnosis. The objective of this study was to evaluate an improved version of this assay that uses predilution of samples with
factor V deficient plasma containing a
heparin neutralizer. Using the data from 32 FV:Q506 mutation positive and 55 mutation negative individuals, the authors calculated a cut-off value resulting in an enhanced sensitivity (0.91 versus 1.0) and specificity (0.77 versus 1.0) compared to the old one. Imprecision was lowered from 5.36% (first generation) to 2.43%, in particular in samples with longer clotting times. In patients with prolonged aPTT, either caused by
therapy with oral
anticoagulants or
heparin, correct results were obtained with the second generation assay, in contrast to the first generation assay. With this second generation assay the number of
DNA analyses can be substantially reduced.