The study involved patients who are prescribed aspirin long-term as a way of preventing clots from forming in the blood.

Researchers at Toronto General Hospital said patients labeled "aspirin resistant" have blood cells, or platelets, which aren't affected in the same way as those of patients who are responsive to the drug, or people labeled "aspirin sensitive."

The researchers identified 20 studies, involving 2,930 patients with cardiovascular disease, all of whom had been prescribed aspirin as a way of preventing clots from forming in the blood -- 28 percent were classified as aspirin resistant.

The study, published in the British Medical Journal, concluded that there needs to be further studies on aspirin resistance to identify the most useful test to determine the condition.

The researchers also said aspirin resistance "is a biological entity that should be considered when recommending aspirin as antiplatelet therapy."