Platelet Reactivity Testing
In addition to its role in disorders associated with platelet dysfunction,27 platelet reactivity testing is a crucial component of the management of cardiovascular disease 
in order to identify patients at higher risk for poor clinical outcome related to their response to antiplatelet drugs. The measurement of platelet aggregation at baseline following administration of DAPT is subject to considerable variability, therefore on-treatment platelet reactivity is the preferred means of platelet reactivity testing.28 However, it is a poorly standardised process. Several studies have shown large inter-laboratory variation in platelet reactivity testing practice.29
Platelet reactivity has been historically measured with light transmittance aggregometry (LTA), but this method is technically complex, time-consuming, requires sample manipulation, and is subject to operator-induced error.30 Point-of-care tests have been developed that are more practical and facilitate the clinical management of patients. The VerifyNow P2Y12 assay was designed to overcome the limitations of conventional optical platelet aggregation assays. It produces results in five minutes and requires no pipetting or sample preparation, reducing the risk of error.
The VerifyNow System (Accumetrics Inc, San Diego, CA, US) is a pointof- care assay that measures agonist-induced platelet aggregation by turbidimetric based optical detection, using a four-channel disposable cartridge (see Figure 1). In the channels, platelets are activated by the presence of agonists and bind to fibrinogen-coated beads, causing agglutinates to drop out of solution. The change in optical density is then measured.31 Results are reported as P2Y12 reaction units (PRU), a BASE (base PRU) value, and a percentage of inhibition calculated from the BASE and PRU.
A lower PRU value corresponds to a higher degree of P2Y12 receptor inhibition and thus a decreased possibility of platelet activation and aggregation. Advantages of the VerifyNow system include simplicity, sensitivity, speed and user-friendliness.32,33 The VerifyNow assay correlates well with LTA and with other point-of-care assay devices,34,35 and has been validated in a head-to-head comparison of platelet function tests.30