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Pharmacology · single question

A patient takes both aspirin and clopidogrel. Which laboratory parameter is affected by these drugs?

  1. Platelet count
  2. Prothrombin time (INR)
  3. Platelet function
  4. Partial thromboplastin time
Show answer and explanation

Correct answer: C. Platelet function

  • A. Platelet COUNT is unaffected. The number of circulating platelets is normal — they simply do not aggregate properly.
  • B. PT/INR measures the extrinsic and common coagulation pathways and is the monitoring test for WARFARIN. Antiplatelet drugs do not prolong it.
  • C. CORRECT. Both drugs are ANTIPLATELET agents and impair platelet FUNCTION without altering platelet number. Aspirin irreversibly acetylates COX-1, abolishing thromboxane A2 synthesis; clopidogrel irreversibly blocks the P2Y12 ADP receptor. Both effects last the lifetime of the affected platelet, roughly 7–10 days.
  • D. aPTT measures the intrinsic and common pathways and monitors unfractionated HEPARIN. It is unaffected by antiplatelet agents.

Note: There is no routine monitoring assay for antiplatelet therapy in general practice — bleeding time is obsolete and unreliable. Assessment is CLINICAL: history of bruising or bleeding, and local haemostatic control. Specialised platelet function assays exist but are not used for dental decision-making.

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