Long-term aspirin and clopidogrel response evaluated by light transmission aggregometry, VerifyNow, and thrombelastography in patients undergoing percutaneous coronary intervention.

Background: A reduced response to aspirin and clopidogrel predicts ischemic events, but reliable tests are needed to identify low responders. We compared 3 platelet-function tests during long-term dual treatment with aspirin and clopidogrel.

Methods: Patients who underwent a percutaneous coronary intervention and were receiving a combination of 325 mg/day aspirin and 75 mg/day clopidogrel were followed for 1 year. Blood was sampled 5 times during this period for 3 tests: light transmission aggregometry (LTA) assay, with 5.0 micromol/L ADP or 1.0 mmol/L arachidonic acid (AA) used as an agonist; VerifyNow assay, with the P2Y(12) or aspirin cartridge (Accumetrics); and thrombelastography (TEG), stimulated by 2.0 micromol/L ADP or 1.0 mmol/L AA.

Results: Twenty-six of 33 patients completed all scheduled visits. A low response to clopidogrel was found in a few patients at variable frequencies and at different visits, depending on the method and criteria used. We found a moderate correlation between the LTA (ADP) and VerifyNow (P2Y(12) cartridge) results, but the TEG (ADP) results correlated poorly with the LTA and VerifyNow results. A low response to aspirin was found with the VerifyNow (aspirin cartridge) and TEG (AA) methods on 6 and 2 occasions, respectively, but not with the LTA (AA) method, except for 1 occasion caused by probable noncompliance.

Conclusions: Detecting a low response to clopidogrel depends largely on the method used. Which method best predicts ischemic events remains uncertain. A low response to aspirin is rare with AA-dependent methods used at the chosen cutoffs. In some patients, the response to clopidogrel or aspirin may be classified differently at different times, even with the same method.