Clopidogrel is an antithrombic compound whose active metabolite is a selective, irreversible antagonist of the platelet purinergic P2Y12 receptor (IC50 = 100 nM). Clopidogrel inhibits ADP-induced platelet aggregation ex vivo and functions as a prodrug whereupon biotransformation to its active thiol metabolite via CYP2C19 in the liver enables its anti-aggregating activity. An estimated 15% of administered clopidogrel is metabolized by CYP2C19 to the thiol metabolite. Clopidogrel carboxylic acid is a major inactive metabolite of clopidogrel. Most (85%) of circulating clopidogrel is hydrolyzed by esterases to this carboxylic acid form. This inactive metabolite can be used as a reference standard for quantitative analysis of clopidogrel metabolism.