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The page below is a sample from the LabCE course Antiplatelet Medication Response Testing: Aspirin and Clopidogrel. Access the complete course and earn ASCLS P.A.C.E.-approved continuing education credits by subscribing online.

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Aspirin Response Analysis: Optical Light Transmission Aggregometry

Patient platelet-rich plasma (PRP) is reacted with a series of platelet-activating reagents (agonists) such as arachidonic acid, collagen, or ADP, to induce platelet aggregation. The reactions take place sequentially in an aggregometer, a spectrophotometer with a strip recorder. As platelets aggregate and a thrombus forms, the turbidity of the platelet-rich plasma in the cuvette declines and the amount of light transmitted to the photometer increases. These changes are indicated by characteristic tracings on the strip recorder. Platelet dysfunction is indicated by lack of platelet aggregation.
A test result, using this method, of ≥20% aggregation in response to arachidonic acid (0.5mmol/L - 1.6 mmol/L) and/or ≥70% aggregation in response to 10 µmol/L ADP is generally interpreted as inadequate response to aspirin therapy. This means that the patient may still be at increased risk for a cardiovascular event.