Laboratory diagnosis of APC-resistance: a critical evaluation of the test and the development of diagnostic criteria.

The APC-resistance test consists of two APTT's, one in the presence and one in the absence of a fixed amount of Activated Protein C (APC), and is a simple and reliable method to detect a reduced sensitivity to the anticoagulant action of APC (APC-resistance). At a fixed concentration of APC the prolongation of the APTT is dependent on the activator, the CaCl 2 concentration, the citrate concentration in the sample, and on sample handling. The effect of sample handling can be reduced by calculating the APC-Sensitivity Ratio (APC-SR). The actual prolongation of the APTT is also influenced by low protein S levels (reduction of APC-SR) and by reduced levels of factors V, VIII and IX (increase of APC-SR)