Aspirin Response Analysis: Platelet Function Testing for Aspirin Resistance

Many laboratories and clinicians regard optical light transmission aggregometry as the gold standard for assessing platelet function, as it is one of the few tests having predicted outcomes for patients on aspirin therapy. This method requires a citrated blood sample that has been centrifuged to create platelet-rich plasma, with a platelet count of 200 - 300 x 10⁹/L. Agonists are added to induce platelet aggregation. The time it takes for the aggregation to occur is measured and correlated with aspirin resistance values. Aspirin almost completely inhibits platelet aggregation induced by arachidonic acid and collagen, but aggregation in response to ADP and epinephrine is only partially inhibited. Arachidonic acid is the platelet agonist chosen most often to determine the effect of aspirin, since it detects the inhibition of thromboxane formation (it is the precursor of thromboxane).

There are also whole-blood methods available for testing aspirin response. These methods do not require the use of platelet-rich plasma, so they are much less labor- and time-intensive. The PFA-100 analyzer (Siemens) and the VerifyNow instrument (Accumetrics) are two instruments that measure aspirin resistance using whole blood samples. These instruments will be discussed further later in the course.