BACKGROUND: The antiplatelet effect of clopidogrel has been linked to cytochrome 
P450 2C19 (CYP2C19) carrier status. The presence of loss of function and gain of 
function variants were found to have a gene-dose effect on clopidogrel 
metabolism. However, genotyping is only one aspect of predicting response to 
clopidogrel and several platelet function tests are available to measure 
platelet response. Patients and methods We studied the influence of CYP2C19 
allelic variants on on-treatment platelet reactivity as assessed by light 
transmission aggregometry (LTA), the VerifyNow P2Y12 assay, the VASP assay, 
multiple electrode aggregometry (MEA), and the Impact-R in 288 patients after 
stenting for cardiovascular disease. Allelic variants of CYP2C19 were determined 
with the Infiniti® CYP450 2C19+ assay and categorized into four metabolizer 
states (ultrarapid, extensive, intermediate, poor).
RESULTS: Platelet reactivity increased linearly from ultrarapid to poor 
metabolizers using the VerifyNow P2Y12 assay (P = 0.04), the VASP assay (P = 
0.02) and the Impact-R (P = 0.04). The proportion of patients with high 
on-treatment residual platelet reactivity (HRPR) identified by LTA, the 
VerifyNow P2Y12 assay and the VASP assay increased when the metabolizer status 
decreased, while no such relationship could be identified for results of MEA and 
Impact-R. The presence of loss of function variants (*2/*2, *2-8*/wt, *2/*17) 
was an independent predictor of HRPR in LTA and the VASP assay while it did not 
reach statistical significance in the VerifyNow P2Y12 assay, MEA, and the 
Impact-R.
CONCLUSION: Depending on the type of platelet function test differences in the 
association of on-treatment platelet reactivity with CYP2C19 carrier status are 
observed.