Antiplatelet Therapy Monitoring

Antiplatelet medications are widely used to treat or prevent thrombosis in a variety of cardiovascular disorders. Currently available antiplatelet drugs comprise four categories:

• 1.

  Acetylsalicylic acid (aspirin) inhibits cyclooxygenase (COX-1),

• 2.

  P2Y12 receptor antagonists (clopidogrel, ticlopidine, prasugrel, cangrelor, ticagrelor) block the platelet adenosine diphosphate (ADP) receptor,

• 3.

  Phosphodiesterase inhibitors (dipyridamole, anagrelide, cilostazol) inhibit the platelet phosphodiesterases, and

• 4.

  Glycoprotein (GP)IIb/IIIa inhibitors (abciximab, eptifibatide, tirofiban, lamifiban) block the platelet GPIIb/IIIa receptor.

The response to these medications varies among individuals, with some patients failing to achieve the expected decrease in platelet activation. This phenomenon, called high on-treatment residual platelet reactivity (HOPR), may be attributable to either the drug failing to inhibit the target (true resistance) or extrinsic factors that increase platelet reactivity despite the drug’s presence. Detection of true drug resistance requires highly specific tests that directly measure the effect of the drug on the target. On the other hand, evaluation of HOPR for any reason can be performed with nonspecific tests that measure the end result of platelet activation. Unfortunately, major problems arise from the weak correlation among the currently available methods in detecting poor drug response. Nonspecific tests tend to identify significantly more patients with HOPR than the highly specific ones. This problem makes it difficult to compare results utilizing different assessment methods. Furthermore, while initial studies generally showed favorable outcomes in patients whose treatments with antiplatelet medications were adjusted based on a given test modality, larger prospective studies tended to produce less impressive results. At present, the indications for testing outside of monitoring compliance with therapy and assessment of the presence of medication before invasive procedures remain controversial. The field is rapidly evolving, and further clinical studies may well provide an impetus to expand platelet therapy monitoring. Because HOPR is mostly described in clopidogrel and aspirin therapy, the chapter mostly focuses on monitoring the effect of these medications.