Clopidogrel Resistance Testing

Primary Testing Methods

The European Society of Cardiology recommends light transmission aggregometry (LTA) using ADP as the agonist as the reference standard for assessing clopidogrel's antiplatelet effect 1, 2
VASP (vasodilator-stimulated phosphoprotein) phosphorylation assay directly measures the effect of clopidogrel on the platelet P2Y12 receptor, the drug's pharmacological target 1, 2

The European Society of Cardiology guidelines note that for optical aggregometry with ADP, cut-off values include >60% aggregation with 20 mM ADP 1, 2, >50% aggregation with 5 mM ADP 1, and >70% aggregation with 10 mM ADP (associated with stent thrombosis) 1
For VASP phosphorylation, a platelet reactivity index (PRI) >50% indicates inadequate response 1, 2
For VerifyNow P2Y12, device-specific cut-off values have been established through ROC analysis in post-PCI patients 1

The correlation between different testing methods is only modest, ranging from 66-78% agreement, and a patient may be classified as resistant by one method but responsive by another 3
The clinical utility of routine testing remains controversial because cut-off values are not standardized and vary widely between studies 1, 3
Test results can be affected by blood preservative choice (citrate vs. antithrombins) 3 and pre-analytical and analytical errors can significantly impact results 4

Clopidogrel resistance frequency ranges from 5-44% depending on the test used, with approximately 21% of patients undergoing PCI showing incomplete platelet inhibition 4, 5

Resistance rates are dose-dependent, with lower prevalence occurring with 600 mg loading doses versus 300 mg, and with 150 mg/day maintenance versus 75 mg/day 4