Aspirin (acetyl‑salicylic acid) is a classic antiplatelet agent. Its main pharmacologic action is the irreversible inhibition of cyclo‑oxygenase‑1 (COX‑1) in platelets. By blocking COX‑1, aspirin prevents the formation of thromboxane A₂, a potent stimulator of platelet aggregation and vasoconstriction. Because platelets cannot synthesize new COX‑1, the effect lasts for the lifetime of the platelet (about 7–10 days).
What this means for clotting tests
| Test | What it measures | How aspirin affects it |
|------|------------------|------------------------|
| Bleeding time | Time for bleeding to stop from a standardized skin incision | Prolonged – aspirin weakens platelet plug formation, so bleeding takes longer. |
| Platelet aggregation assays (e.g., ADP or collagen‑induced) | Degree of platelet clumping in vitro | Reduced – aspirin markedly blunts aggregation responses. |
| Prothrombin time (PT) | Coagulation cascade activity via the extrinsic pathway | Unchanged – aspirin does not interfere with clotting factors. |
| Activated partial thromboplastin time (aPTT) | Intrinsic coagulation pathway | Unchanged – aspirin has no significant effect. |
| Platelet function tests (e.g., VerifyNow, LTA) | Functional platelet response | Prolonged/clotting time increased – these tests are specifically designed to detect aspirin’s effect. |
Key points to remember
- Aspirin’s effect is platelet‑specific; it does not alter the coagulation cascade, so routine coagulation tests (PT, aPTT) remain normal.
- The bleeding time and other platelet‑function tests will be lengthened. In a typical “bleeding time” test, a 3 mm incision is made and the time until bleeding stops is recorded; aspirin can extend this time from the normal 1–2 minutes to several minutes or more, depending on dose and duration of therapy.
- Because the effect on platelets is irreversible, the platelet inhibition persists for the lifespan of the platelet (≈ 7–10 days). Restarting aspirin after a period off the drug will re‑establish platelet inhibition within a day or two.
- Clinical relevance: Aspirin’s antiplatelet action is why it is used to reduce the risk of arterial thrombotic events (heart attack, stroke). However, it also increases bleeding risk, especially in patients undergoing surgery or with other bleeding disorders.
If you’re preparing for a surgical procedure, the surgeon will usually recommend stopping aspirin a few days in advance (or using a low‑dose “holiday” strategy) to reduce bleeding risk. Always discuss any medication changes with a healthcare professional.