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Aspirin Resistance High Among Elderly, Women, Others
March 6, 2005 - While aspirin remains a crucial and
cost-effective therapy for the prevention and management of
cardiovascular diseases, research suggests that a significant percentage
of the 25 million Americans on a chronic aspirin regimen are "aspirin
resistant," or do not achieve sufficient antiplatelet effects from
aspirin.
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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.
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, and further indicates that
women, the elderly and those taking lower doses of aspirin most likely
to be aspirin resistant.
These and other characteristics are now said to be
"predictors" of aspirin resistance. Additionally, research indicates
that resistance to aspirin and clopidogrel (or Plavixฎ) may occur
together, as studied in patients undergoing coronary procedures, such as
percutaneous coronary intervention (PCI).
These are among the numerous findings released at
the ACC, emphasizing the scope of the issue and the role of testing for
aspirin resistance, adding to the growing body of research and interest
in this topic from the cardiology community.
The VerifyNow Aspirin Test (Ultegra)
maker says their
product is emerging as an important tool 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 %).
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," said 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."
Another study demonstrated that resistance to
aspirin and clopidogrel (or Plavixฎ), 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, are the current standard of
care for preventing complications associated with coronary stenting, yet
may not be providing adequate antiplatelet protection in all patients."
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 the news release from Accumetrics,
the developer. 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, the company says.
Information Source: Accumetrics has developed
the VerifyNow System to test response to anti-platelet drugs used to
prevent and treat heart attack and stroke. The company markets a
first-of-its-kind test to evaluate aspirin resistance in patients taking
daily aspirin. Accumetrics also markets a test for assessment of
intravenous therapy with the class of agents known as GP IIb/IIIa
inhibitors, and the company intends to launch a test for clopidogrel (Plavixฎ)
response. Each of these tests is designed to provide a new level of
ease, rapidity and specificity to determine if patients are achieving
adequate response to antiplatelet therapy, an important risk factor in
protecting against major cardiovascular events. More information is
available at
http://www.accumetrics.com.
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