AspirinWorks Test:  Millions of Americans are taking aspirin to help prevent heart attacks and strokes. But what is not generally known is that up to 25% of patients on aspirin therapy may not receive the benefit from its anti-clotting effects. The suboptimal response to aspirin is commonly known as “aspirin resistance”.  And because aspirin can cause stomach upset, ulcers, and even gastrointestinal bleeding, this question has become a significant medical concern. The AspirinWorks Test measures a urine metabolite of a substance, thromboxane, which is associated with the formation of blood clots. Aspirin is supposed to reduce the formation of this substance in the body and results in low levels of the metabolite detected in the urine.  High levels may mean that aspirin is not effective for decreasing your risk of a heart attack or stroke. See FDA recommendation and brochure.