An APTT-based assay for the detection of the APC resistance phenotype, i.e. the poor anticoagulant response to activated protein C (APC). The test result (APC ratio) gives an estimation of the anticoagulant function in vivo and provides information on the thrombotic risk associated with inherited and acquired APC resistance. Detects the APC resistance phenotype regardless of its cause. Detects both inherited and acquired APC resistance.