If ischemic stroke recurs or if a coronary artery stent becomes blocked while patients are taking clopidogrel, clinicians should suspect impaired metabolism of clopidogrel (ineffective conversion of clopidogrel to its active form because cytochrome P-450 2C19 [CYP2C19] activity is reduced); a test to determine CYP2C19 status (eg, genetic testing for CYP450 polymorphisms) is recommended.