To some degree it does, but it is by far not as effective as warfarin or other blood thinners (anticoagulants). However, a very noteworthy study was presented today (Dec 12, 2012) at the annual American Society of Hematology meeting. It showed that aspirin was much more effective than no treatment (or placebo) in preventing further clots in patients who had a history of DVT or PE [reference 1]. Aspirin was given after the patients had been treated with warfarin for 6-18 months. This is thrilling news. However, the study needs to be replicated and the results confirmed in a second study.
Difference between clots in veins and in arteries
Clots in veins causing DVT and PE are mostly made up of the clotting protein called “fibrin”. Blood thinners (anticoagulants) work by preventing fibrin from forming and are, therefore, quite effective in preventing DVT and PE. Blood thinners are: warfarin (coumadin, Jantoven), heparin, so-called low-molecular-weight-heparins (Lovenox = Enoxaparin, Fragmin = Dalteparin, Innohep = Tinzaparin), Arixtra (Fondaparinux), and the new oral drugs Pradaxa (Dabigatran) and Xarelto (Rivaroxaban).
Aspirin is a drug that makes blood platelets slick and prevents them from sticking together and forming a clot. Aspirin is called an anti-platelet drug. As blood platelets play only a minor role in the formation of clots in veins, Aspirin typically appears to be not very effective in preventing DVT and PE. On the other hand, blood clots in arteries, i.e. those causing strokes and heart attacks, are mostly made up of platelets; therefore, aspirin is quite effective in preventing those clots and is the mainstay of treatment to prevent stroke and heart attacks.
Aspirin and DVT and PE: Previous data
There has been some evidence over the years that aspirin protects some against DVT and PE [ref 2,3]. However, the effect observed has only been mild. Therefore, the conclusion has been for years that aspirin does not, in any major way, protect from DVT and PE.
The new study
The study presented today at the annual meeting of the American Society of Hematology (WARFASA study) investigated patients who had had one unprovoked DVT or PE and had been treated with warfarin for 6-18 months. At the end of the warfarin treatment the patients were switched to aspirin or placebo. They were followed for an average of 2 years to see how many developed a new DVT or PE. The aspirin dose used was 100 mg a day, i.e. a little more than an 81 mg baby aspirin tablet and less than a 325 mg adult aspirin tablet size. 403 patients were enrolled in the study, 205 of whom received aspirin, 198 placebo. The study found that aspirin was effective in preventing some clots: while 1 out of 9 patients (i.e. 11.0 %) on placebo developed a new clot, only 1 out of 17 patients (i.e. 5.9 %) on aspirin developed a new DVT or PE. (In comparison, on warfarin typically less than 1 in 50 patients develop a new clot). Aspirin did not lead to an increased risk for bleeding. Thus, the appropriate conclusion that aspirin was safe and effective in preventing some clots. A few limitations of this new study need to be mentioned. It has only been presented and published as a short abstract, not yet in a peer-reviewed journal. And, as always, such a study needs a second, follow-up study, to see whether the results can be confirmed.
The study findings are remarkable: aspirin can prevent some DVTs and PEs, without an increase in bleeding risk.
- Does this study change my clinical practice? Yes. I used to tell patients who came off warfarin after they had been treated for an appropriate length of time with warfarin (often 3-6 months) that there was no strong reason to take aspirin. I will now tell them that it is worthwhile to take aspirin long-term.
- What dose will I recommend? I will tell patients that, here in the U.S. where we do not have 100 mg tablet sizes, either a baby aspirin (81 mg) or an adult aspirin (325 mg) would be appropriate.
- Would I recommend that patients who are on long-term warfarin now stop warfarin and switch to aspirin instead? No. Clearly not. Warfarin is much more effective than aspirin. Aspirin is not a replacement for warfarin.
- Becattini C et al. Aspirin for the prevention of recurrence of venous thromboembolism. The WARFASA study.
- Antiplatelet Trialists’ Collaboration: Collaborative overview of randomized trials of antiplatelet therapy – III. Reduction in venous thrombosis and pulmonary embolism by antiplatelet prophylaxis among surgical and medical patients. BMJ 1994;308:235-246.
- Prevention of pulmonary embolism and deep vein thrombosis with low dose aspirin: Pulmonary Embolism Prevention (PEP) trial. Lancet 2000,Apr 15;355(9212):1295-302.
Disclosure: I have no financial conflict of interest relevant to this post.
Last updated: Dec 12th, 2011