Use of Platelet Function Testing — Does it really aid patient management?
A major issue with clopidogrel is that one-third of patients are nonresponders because they lack the enzymes to metabolize it. While nonresponders can be identified with testing, this adds cost. So with the introduction of the newer antiplatelet agents, which are metabolized differently, some hospitals and physicians are opting for ticagrelor or prasugrel. However, with generic versions of clopidogrel now available at a much lower cost than the newer agents, the question remains if patients should be tested first to see if they will respond to Plavix.
Patients with 2C19 that is less active due to loss of gene copies are poor metabolizers of clopidogrel, opening patients to the possibility of a higher rate of mayocardial infacrtion (MI). There are assays for platelet function response (VerifyNow) and genetic tests to measure patient response, which could help determine if generic clopdogrel or more expensive agents should be used.
However, Malcolm R. Bell, MBBS, FRACP, FACC, Mayo Clinic, Rochester, Minn., said platelet function tests have only a modest predictive value. He said these tests do not have optimal methods defined and timing is undefined.
Most cardiologists do not use routine platelet testing because the patient is at high risk for poor clinical outcomes. This is because testing only counts for less than 20 percent of hypo responders due to variations in CYP2C19 enzymes. Bell and other experts presenting on antiplatelet response at the American College of Cardiology (ACC) 2012 said there is not enough information to base practice on the results of these tests. Part of the nonresponse issue is due to poor compliance by the patient. Guidelines state a IIb level of evidence to test for a high risk of poor clinical outcomes or to switch agents, but not for routine testing of platelet aggregation.
In an ACC 2012 cardiac pharmacology panel discussion, there was discussion of making platelet response testing a prior authorization requirement before newer, more expensive agents would be covered. But, speakers emphasized there is no data to say that a patient who has sticking platelets on clopidogrel will respond better with another agent. For now, it is not known what to do with this information.
There is already clinical data to support use of prasugrel and ticagrelor over clopidogrel without testing. Therefore, platelet testing may be a moot point due to the increased cost.