Does anyone utilize P2Y12 assays to determine if clopidogrel may be ineffective when used for DAPT?   

Yes, if the patient has undergone genetic testing indicating nonresponsiveness to clopidogrel, I would consider substituting ticagrelor. In most cases, DAPT is short-term, typically lasting for one month, after which aspirin monotherapy would be sufficient.

I do check the P2Y12 assay in patients experiencing stroke on clopidogrel, especially before asking neurosurgery to see for "failed best medical therapy " in ICAD.

Yes. I do especially with another ischemic event in those on Plavix monotherapy (for small or large vessel disease) or DAPT (even for short period with an event recurring due to atherosclerotic disease or small vessel disease) during that time of DAPT therapy. Also if genetic testing is done, I do c...