To enhance the practical applicability of the calibrated automated thrombogram (CAT) we investigated whether frozen-thawed platelet-rich plasma (ft-PRP) can be used to assess the function of the protein C inhibitory pathway, while preserving the natural phospholipid composition. Recalcified ft-PRP triggered with 0.5 pM recombinant human tissue factor shows a median thrombin potential of 1,779 nM·min, against 1,576 nM·min for fresh PRP. To obtain ∼70% inhibition, 6.7 nM activated protein C (APC) has to be added, instead of 25 nM in fresh PRP; so the relative APC resistance of PRP appears to depend upon the presence of intact platelets. Factor VIII, added to normal ft-PRP to obtain a concentration of 3.3 U/ml, increases the thrombin potential in the presence of APC 1.5-fold, from 524 to 808 nM·min, in keeping with previously published increases in thrombotic risk in patients with high factor VIII levels. We conclude that thrombography in ft-PRP, with and without added APC, can be used to assess known risk factors for thrombosis, which allows the design of large clinical studies aimed at proving the relationship between thrombin potential and clinical outcome.