Oral anticoagulation with warfarin is a widely used form of treatment for an increasing number of medical conditions. Nevertheless, appropriate therapeutic monitoring and dosage readjustments should be carried out in order to ensure its safety and efficacy. Although prothrombin time (usually expressed as International normalized ratio [INR]) is the most common warfarin response marker, clotting factors (namely factors II and X) are also indicated as alternative anticoagulant effect markers. In this paper, we examine the relationship between these warfarin response markers using information obtained from eighty 80 patients undergoing long-term warfarin therapy. Within the usual INR therapeutic range (2.0–3.5), a moderate inverse correlation between INR and both clotting factors II and X was observed. However, for INR values above 3.5, a non-proportional relationship were found between INR and both response markers. Therefore, it can be concluded that below critical clotting factor concentrations (20.6% and 15.6% of factors II and X activity, respectively), time required for clot formation becomes non-proportional and haemostasis will be jeopardised.