P2Y12 receptor inhibitors like clopidogrel (Plavix), prasugrel (Effient), and ticagrelor (Brilinta) are powerful antiplatelet agents to promote anticoagulation in patients with acute coronary syndrome, peripheral artery disease, etc. Therefore, we often stop P2Y12 inhibitors for ~5-7 days before elective surgery and use a “Plavix effect” assay (more specifically, a VerifyNow P2Y12 assay) to determine P2Y12 mediated platelet activation. But how does this work?
A sample of citrated blood is deposited into the assay consisting of fibrinogen-coated beads and ADP. Prostaglandin E1 is present to limit P2Y1’s activation making the assay more specific for P2Y12. The goal is to activate platelets with ADP which will, in turn, bind to the fibrinogen beads decreasing the turbidity (cloudiness) of the fluid and allowing for more light transmittance. The test result is expressed as P2Y12 reaction units (PRU) with a normal reference range of 194 – 418. Values < 194 PRUs suggest the presence of P2Y12 platelet inhibition, and ideally, elective surgery should be postponed.
The assay can also be affected by platelet counts < 120K or > 502K, hematocrit values < 33% or > 52%, and patients taking glycoprotein IIb/IIIa inhibitors like abciximab (ReoPro), eptifibatide (Integrilin), and tirofiban (Aggrastat).
Emergent surgeries (type A dissections, coronary dissection, etc.) must go regardless of platelet inhibition. Because of the anticipated coagulopathy, I’ll often need to transfuse large volumes of platelets in addition to powerful procoagulants like NovoSeven to manage the bleeding.