Will Plavix (clopidogrel) affect laboratory test results, particularly those related to coagulation and platelet function?

From the Guidelines

Yes, Plavix (clopidogrel) can significantly affect laboratory test results related to coagulation and platelet function. Clopidogrel is an antiplatelet medication that works by irreversibly inhibiting the P2Y12 receptor on platelets, preventing them from aggregating. This mechanism directly impacts tests that measure platelet aggregation and function, such as light transmission aggregometry, VerifyNow P2Y12 assay, and platelet function analyzer (PFA) tests. Patients taking Plavix will typically show prolonged closure times on PFA tests and reduced platelet aggregation responses. Bleeding time may also be prolonged. However, Plavix generally does not affect standard coagulation tests like prothrombin time (PT), international normalized ratio (INR), or activated partial thromboplastin time (aPTT) since these tests measure the coagulation cascade rather than platelet function. If accurate platelet function assessment is needed, it's essential to inform laboratory personnel about Plavix use, as it should be discontinued 5 days before testing if clinically appropriate and safe to do so, as recommended by the French working group on perioperative haemostasis (GIHP) 1. This washout period allows for new, unaffected platelets to be produced, providing more accurate baseline platelet function results. The European Society of Cardiology (ESC) and the European Association for Cardio-Thoracic Surgery (EACTS) have also drafted a grade IIb recommendation on the interest of measuring the degree of platelet dysfunction to determine the time interval between last medication intake and cardiac surgery 1. It is crucial to consider the individual variability in response to P2Y12 inhibitors, including clopidogrel, when interpreting laboratory test results and making clinical decisions 1. In some cases, platelet function testing may be used to guide the management of antiplatelet therapy and reduce the risk of bleeding complications 1. Overall, the use of Plavix can significantly impact laboratory test results related to coagulation and platelet function, and healthcare providers should be aware of these effects when interpreting test results and making clinical decisions.

From the FDA Drug Label

Clopidogrel at recommended doses forms less of the active metabolite and so has a reduced effect on platelet activity in patients who are homozygous for nonfunctional alleles of the CYP2C19 gene, (termed “CYP2C19 poor metabolizers”). P2Y12 inhibitors (thienopyridines), including clopidogrel, increase the risk of bleeding. P2Y12 inhibitors (thienopyridines), inhibit platelet aggregation for the lifetime of the platelet (7 to 10 days)

Yes, Plavix (clopidogrel) will affect laboratory test results, particularly those related to:

• Coagulation: as it increases the risk of bleeding
• Platelet function: as it inhibits platelet aggregation for the lifetime of the platelet (7 to 10 days) 2

From the Research

Effect of Plavix on Laboratory Test Results

Plavix (clopidogrel) is known to affect laboratory test results related to coagulation and platelet function. The following points summarize the effects of Plavix on these tests:

• Plavix inhibits ADP-induced platelet aggregation, which can be measured by platelet aggregometry or whole blood aggregation tests 3, 4, 5.
• The use of Plavix can lead to an increased incidence of platelet transfusion therapy due to increased chest tube drainage in patients undergoing cardiopulmonary bypass 3.
• Thrombelastography (TEG) may not be a reliable monitor for clopidogrel-induced ADP receptor blockade, as normal preoperative TEG results do not preclude clopidogrel-induced ADP receptor blockade 3.
• Platelet function tests, such as P2Y12 platelet function testing, can identify patients who are resistant to clopidogrel, which may be associated with an increased risk of ischemic complications 4.
• Clopidogrel can also affect coagulation parameters, such as bleeding time, which can be prolonged in patients taking the drug 6, 5.
• However, coagulation parameters, such as platelet-dependent thrombin generation, antithrombin III, thrombin-antithrombin III complex, and prothrombin fragment 1 + 2, may not be significantly affected by clopidogrel 6.

Laboratory Tests Affected by Plavix

The following laboratory tests may be affected by Plavix:

• Platelet aggregometry
• Whole blood aggregation tests
• Thrombelastography (TEG)
• P2Y12 platelet function testing
• Bleeding time
• Fibrinogen plasma concentration
• Platelet count ratio

Clinical Implications

The effects of Plavix on laboratory test results have important clinical implications, including:

• Monitoring of platelet function and coagulation parameters in patients taking Plavix
• Identification of patients who are resistant to clopidogrel
• Adjustment of antiplatelet therapy to minimize the risk of ischemic and hemorrhagic complications 4, 6, 5