Nevertheless, more and more physicians in the US have begun to routinely test their patients. Whether evidence is conclusive or not at this time, they say, it’s reasonable to alter therapy rather than to continue to prescribe a drug that testing shows may well be ineffective for the given patient. Add to that the unnecessary complications of possible gastrointestinal bleeding, tinnitus, a worsening of asthma and the host of other, well-known complaints with chronic aspirin ingestion, and the benefits of considering another method of treatment do add up.
What is clear - at least, what clinicians appear to agree on - is that aspirin resistance is a poorly defined term. Doctors question the clinical data on which tests recently marketed in the US are based, and await the development of a sensitive and specific assay that can reliably predict treatment failure (Pharmacotherapy, 2005; 25: 942-53; BMJ, 2004; 328: 477-9). Proposed causes of failure include anything from poor patient compliance and the use of contraindicated medications to an increased turnover of platelets and poor aspirin uptake.
Whether resistance means the inability of aspirin to protect against arterial thrombosis, the failure of aspirin to affect platelets and reduce clotting in a test-tube, or a specific urinary concentration of a metabolite called thromboxane is anyone’s guess. What we do know is that it’s a documented problem - which is more than many physicians (not to mention the aspirin suppliers, protecting their cash cow) want us to know.