Our results suggest that some patients may need more protection than aspirin alone can offer

John Eikelboom, study co-ordinator

For most patients, aspirin therapy can reduce heart attacks by 25%, say researchers at the University of Western Australia, Royal Perth Hospital, who carried out the research.

High levels of thromboxane in urine can identify patients who are resistant to aspirin.

The study found that patients taking aspirin who had a high level of thromboxane in their urine had a 3.5 times higher risk of cardiovascular death than patients who had the lowest level.

John Eikelboom, who co-ordinated the study, said: "Our results suggest that some patients may need more protection than aspirin alone can offer."

They suggest people with aspirin resistance may benefit from alternative antiplatelet therapies or treatments that more effectively block thromboxane production.

The research was based on data collected by the Heart Outcomes Prevention Evaluation (Hope) study.

Drug suitability

This looked at the effect of a blood pressure drug, ramipril, combined with vitamin E, to prevent heart attacks or stroke in patients with heart disease.

It was measured against the effects of a placebo and included more than 5,500 patients.

The research team identified patients who were taking aspirin for at least six months before study entry and kept taking aspirin throughout the study.

During a five-year follow-up, 488 of these patients had a heart attack, stroke or fatal event.

They were compared with 488 patients who were age and gender matched and had also taken aspirin, but had not suffered any complications.

When all the results were collated, the study concluded that those with the highest thromboxane levels - those with aspirin resistance - had twice the risk for heart attack as those with the lowest levels.

Mr Eikelboom stressed the increased risk was independent of other risk factors such as high blood pressure, high cholesterol, obesity, diabetes and smoking.

The British Heart Foundation (BHF) has warned that this research is not conclusive.

Belinda Linden from the BHF said: "Aspirin has a vital role in helping to prevent many people with coronary heart disease from suffering further heart attacks.

"Further research is needed to better understand the type of action that leads to production of the chemical thromboxane, which seems to create some resistance to aspirin's anti-platelet effect.

"This is a large-scale study, which in the long term may be able to help clarify who benefits the most from aspirin and identify who needs alternative anti-platelet therapies."