Some drugs are more efficiently monitored by measuring their effects rather than their serum concentration. Warfarin dosing, for example, is better monitored by measuring the prothrombin time (PT) and International Normalized Ratio (INR). We monitor warfarin by looking at its physiological effects since the effects of warfarin will outlast its presence in the blood. Most anticoagulant drugs and products (Factors, heparin, etc.) are not measured directly. Instead, functional clot testing assesses how much drug is present.
