(RxWiki News) Taking low-dose aspirin daily has been touted for years to lower the risk of heart attack and stroke. Some people show resistance to this therapy in blood tests, but why?
A recent study used blood tests to gauge the effectiveness of uncoated, immediate release aspirin versus coated aspirin in the platelets of 400 healthy adults.
Researchers found the coated aspirin created a fake resistance to the therapy that did not exist with the uncoated aspirin.
"Talk to your doctor about aspirin therapy."
Tilo Grosser, MD, research assistant professor of Pharmacology, and Garret A. FitzGerald, MD, professor in translational medicine and therapeutics, at the Perelman School of Medicine at the University of Pennsylvania, led researchers in an investigating resistance to low-dose aspirin therapy.
Low-dose aspirin therapy, 81 mg of immediate release or coated aspirin, has been known to help prevent the risk of heart attack or stroke.
It works by lowering the “stickiness” or thickness of platelets in the blood stream, which can prevent heart attacks and strokes.
Aspirin can also cause stomach problems including ulcers and stomach bleeding. The coated aspirin was designed to protect the stomach by delaying the breakdown of the aspirin pill. It is unclear whether the coating actually helps protect the stomach.
There is a blood test that can detect whether a person can benefit from aspirin therapy. There is also a urine test, but it is less accurate.
For the study, researchers screened 400 healthy volunteers by giving them each 325 mg of immediate release or coated aspirin. Participants were given blood tests to determine whether or not they were resistant to aspirin therapy.
Those who tested “resistant,” where the blood test indicated the aspirin did not reduce the platelet stickiness in the blood stream, were tested again.
The absorption of the coated aspirin appeared to cause a resistance in up to 49 percent of cases initially, but none were resistant with uncoated aspirin.
If they tested “resistant” again, they were given 81 mg of coated aspirin with 75 mg of clopidogrel for one week. Clopidogrel (brand name Plavix) is a prescription medication to reduce platelet stickiness.
Results of the study showed that eventually all participants tested for less platelet stickiness either after repeated exposure to non-coated aspirin, or after exposing the platelets to aspirin in a blood sample after it was taken from the participant.
The authors said, “Pharmacological resistance to aspirin is rare; this study failed to identify a single case of true drug resistance.”
These researchers concluded that a fake resistance could occur due to the lack of absorption of the aspirin from the coating on the aspirin, which did not happen with the uncoated aspirin.
Dr. Grosser said, “When we looked for aspirin resistance using the platelet test, it detected it in about one-third of our volunteers.”
“But, when we looked a second time at the incidence of aspirin resistance in the volunteers, the one-third that we measured which was now resistant was mostly different people. Nobody had a stable pattern of resistance that was specific to coated aspirin.”
Dr. FitzGerald said, “These studies question the value of coated, low-dose aspirin. This product adds cost to treatment, without any clear benefit. Indeed it may lead to the false diagnosis of aspirin resistance and the failure to provide patients with an effective therapy. Our results also call into question the value of using office tests to look for such resistance.”
This study was published in December in Circulation. Funding for this trial was supported by the National Institutes of Health, the University of Pennsylvania and Bayer. No conflicts of interest were reported.