Monitoring of Antiplatelet Therapy in Clinical Practice: Is it Necessary or Not?

Abstract

Patient response to antiplatelet agents varies, and poor response may be associated with increased risk of ischaemic events. Monitoring antiplatelet therapy with platelet function tests may identify poor responders who would benefit from a change in therapy, such as a dosage increase. The relevant tests for monitoring aspirin therapy are light transmission aggregometry (LTA) using arachidonic acid as the agonist or the VerifyNow ASA cartridge. The response is all-or-none, and no pre-treatment measurement is required. However, the very low number of aspirin non-responders makes mass monitoring unnecessary. In contrast, there is a wide variability in response to clopidogrel treatment. Clopidogrel monitoring may be justified if it can be demonstrated that poor responders benefit from dose increases. The gold standard to monitor clopidogrel is the vasodilator-stimulated phosphoprotein (VASP) assay due to its P2Y 12 selectivity and the fact that it does not require pre-treatment measurement. However, the relevant cut-off to distinguish poor responders from good responders has not been defined. In contrast to the VASP assay, LTA requires pre-treatment measurement, while the question of threshold is no better defined. The VerifyNow P2Y12 assay is a good alternative; however, as with the other tests, the relevant thresholds are not clearly defined. The platelet function analyzer PFA-100 can be used to identify patients with severe platelet defects or von Willebrand disease. However, due to its lack of sensitivity, it is not suitable for monitoring antiplatelet therapy. In the context of antiplatelet therapy monitoring, some evidence suggests that the results of platelet function testing can be used to identify patients at risk of cardiac events, but no studies have shown that platelet function testing can be used to guide therapy. It is thus unclear whether the results of platelet function tests can be used in clinical decision-making. © The Author 2007.