"This study has important implications, as aspirin, along with Plavix, [is] the current standard of care for preventing complications associated with coronary stenting, yet may not be providing adequate antiplatelet protection in all patients.

New studies reported at the 2005 Annual Scientific Session of the American College of Cardiology (ACC) confirm that up to 27 percent of aspirin users with coronary artery disease (CAD) are resistant to aspirin's antiplatelet effects.

Resistance Predictors

Women, the elderly and those taking lower doses of aspirin are most likely to be aspirin resistant, the studies show. These and other characteristics are now said to be "predictors" of aspirin resistance.

Previous studies have shown that aspirin resistance is associated with triple the risk of heart attack, stroke and death. Once patients are tested and identified as aspirin resistant, physicians may opt for an alternative approach to therapy, which may include increasing the dosage of aspirin or placing the patient on another antiplatelet medication.

Dosage Level Correlates with Resistance

The VerifyNow Aspirin Test (Ultegra) developed by Accumetrics is designed to aid clinicians in quantifying this risk and taking appropriate action.

Up to 27 percent of patients with well-documented CAD on an aspirin regimen for at least four weeks were found to be resistant to aspirin's antiplatelet effects, determined by the VerifyNow Aspirin test, according to research reported by Dr. W.H. Chen and colleagues at the University of Hong Kong.

The study of 468 patients provided insight into the nature of aspirin resistance by identifying patient characteristics significantly associated with increased risk for the condition. Women, the elderly, those with renal insufficiency, low hemoglobin levels and those taking low-dose aspirin were found to have the highest likelihood of being aspirin resistant.

In particular, the authors identified that dosage of aspirin directly correlates with the prevalence of aspirin resistance. The highest prevalence of aspirin resistance was associated with doses of less than 100 mg (30.2%), was less prevalent in higher aspirin doses of 150 mg (16.7%), and non-existent with 300 mg (0%).

One Dose Does Not Fit All

Separately, a meta-analysis of 200,000 people also presented at the ACC meeting showed that risk of bleeding increases with increasing doses of aspirin (especially 325 mg).

In combination, these two studies suggest that a balance between safety and effectiveness needs to be achieved so that the lowest, effective dose is prescribed and patients are confirmed responsive to their aspirin regimen.

"Knowledge of particular patient groups susceptible to aspirin resistance underscores the need to confirm aspirin response so that alternative or additional antiplatelet therapy can be considered in this population at higher risk," says Daniel Simon, associate director, interventional cardiology at Brigham and Women's Hospital and professor of medicine at Harvard University.

"Physicians don't use coumadin or heparin without monitoring their anti-thrombotic effects. Antiplatelet therapy is no different. It is clear that the individual patient responds differently and one dose does not fit all," Dr. Simon emphasizes.

Plavix Resistance Found

Another study demonstrated that resistance to aspirin and clopidogrel (or Plavix), a widely used antiplatelet prescription drug, may often occur together, according to research presented by Dr. Eli Lev and colleagues at Baylor College of Medicine in Houston.

In fact, 50% of those patients resistant to aspirin were also resistant to clopidogrel. Using the VerifyNow Aspirin test, the study evaluated 80 patients who had received aspirin for at least a week, finding 15% to be aspirin resistant.

Separately, 24% of patients were found to be resistant to clopidogrel. A total of 7.5% of patients in the study were resistant to both aspirin and clopidogrel.

"This research suggests that a positive test for aspirin resistance raises the possibility that the patient may be clopidogrel resistant as well," said Dr. Lev. "This study has important implications, as aspirin, along with Plavix, [is] the current standard of care for preventing complications associated with coronary stenting, yet may not be providing adequate antiplatelet protection in all patients."

Blood Test for Aspirin Resistance

The VerifyNow Aspirin test is a rapid, easy, point-of-care blood test that measures the degree to which a patient's platelets aggregate, according to Accumetrics. The test provides a result in 30 minutes and gives one easy-to-understand number indicating whether aspirin is effectively inhibiting platelet function or if a patient is resistant.

The VerifyNow Aspirin test can be performed in the physician office. The test is also available at hospitals and laboratories throughout the country and is covered by Medicare and by most insurance providers, says Accumetrics.