European Heart Journal Advance Access originally published online on June 22, 2007
European Heart Journal 2007 28(14):1673-1675; doi:10.1093/eurheartj/ehm232
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Laboratory detection of ‘aspirin resistance’: what test should we use (if any)?
Unità di Ematologia e Trombosi, Ospedale San Paolo, Dipartimento di Medicina, Chirurgia e Odontoiatria, Università di Milano, Via di Rudinì 8, Milano 20142, Italy
Corresponding author. Tel: +39 (0) 250323095; fax: +39 (0) 250323095. E-mail address: marco.cattaneo{at}unimi.it
This editorial refers to ‘A comparison of six major platelet function tests to determine the prevalence of aspirin resistance in patients with stable coronary artery disease’ by M. Lordkipanidzé et al., on page 1702
Aspirin is widely used to decrease the risk of occlusive arterial events in patients at risk. It irreversibly inhibits the cyclooxygenase-1 (COX-1)-dependent synthesis of thromboxane A2 (TxA2), which is essential for the full aggregation response of platelets.
In the last years, the issue of ‘aspirin resistance’ has been emphasized in the medical literature.1–4 Despite several studies published on this subject, its definition, diagnosis, prevalence, causes, and clinical consequences are still uncertain.1
Definition of ‘aspirin resistance’
The term ‘resistance’ to a drug should be used when a drug is unable to hit its pharmacological target, due to inability to reach it (as a consequence of reduced bioavailability, in vivo inactivation, negative interaction with other substances) or to alterations of the target.1 Based on this definition, the term ‘resistance’ to aspirin should be limited to situations in which aspirin is unable to inhibit COX-1-dependent TxA2 production (and, consequently, TxA2-dependent platelet functions).
‘Aspirin resistance’ vs. ‘treatment failure’
If the anti-thrombotic effects of aspirin are dependent on inhibition of COX-1, one would expect that patients in whom aspirin does not inhibit COX-1 do not benefit from the protection that this drug provides and are exposed to high risk of atherothrombotic events. However, it is incorrect to consider ‘resistant’ to aspirin all those patients who experience atherothrombotic events while on treatment. This phenomenon has been named ‘clinical resistance’, but it should be more properly termed ‘treatment failure’.1,2 It can be observed with any kind of treatment and is expected to be particularly frequent for drugs, like aspirin and all other anti-thrombotic agents, that are used to prevent multi-factorial diseases, such as those associated with vascular occlusion, independently of their efficacy to hit their pharmacological targets.
‘Aspirin resistance’ vs. ‘high on-treatment residual platelet reactivity’
In consideration of the importance of TxA2 pathway in platelet activation, one would expect that its inhibition negatively affects not only thrombus formation in vivo, but also platelet activation in vitro. Therefore, many studies used various techniques to measure platelet function in vitro in order to evaluate the degree of its inhibition by aspirin and, in some instances, to predict the risk of atherothrombotic events. Although this approach is justified and rational, one should be aware that the relative importance of the TxA2 pathway in platelet activation varies considerably among different subjects and with the type of laboratory test used.1 Therefore, the finding of high, residual platelet reactivity in vitro in patients on aspirin treatment may not necessarily imply that these patients are resistant to treatment, especially when platelet function is measured by laboratory tests that are unspecific for the effect of aspirin on its pharmacological target. Despite this, unspecific tests may still prove useful to identify patients with high residual platelet reactivity. However, only the use of specific tests that measure the pharmacological effect of aspirin will clarify whether their platelet hyper-reactivity is due to insufficient pharmacological effect of the drug or to other causes.
In conclusion, the term ‘aspirin resistance’ should be limited to situations in which failure of the drug to hit its pharmacological target has been documented with specific laboratory tests. The term ‘clinical resistance’ to aspirin should not be used to identify situations in which aspirin is unable to prevent atherothrombotic events. Although global tests measuring platelet activation in vitro may identify patients with high residual platelet reactivity, they do not necessarily identify patients who are resistant to aspirin.
Methods used to measure platelet function during aspirin treatment
Platelet function in vitro has been measured in patients on aspirin treatment by several functional assays, many of which have been criticized because they do not reproduce the physiological conditions that characterize the development of platelet aggregates in vivo. However, this potential drawback does not necessarily decrease their accuracy in measuring the efficacy of an anti-platelet drug to hit its pharmacological target. None of the functional assays that have been used so far displays sufficient specificity for measuring the effects of aspirin on platelet function. Even when arachidonic acid (AA), the precursor of TxA2, is used as platelet agonist in light-transmission aggregometry (LTA), the results obtained with this technique may over-estimate the prevalence of aspirin resistance.1 Methods that measure directly the capacity of platelets to synthesize TxA2 are certainly preferable. Of these, the urinary levels of the TxB2metabolite, 11-dehydrothromboxane B2, represent a time-integrated index of TxA2 biosynthesis in vivo.5 Because it is not formed in the kidney, detection of this metabolite in the urine reflects systemic TxA2 formation, which largely, albeit not exclusively occurs in the platelets. It has been calculated that about 30% of the urinary metabolite derives from extra-platelet sources6 and that in pathological conditions, such as in inflammatory diseases, the contribution of extra-platelet sources may increase. Therefore, this method is not highly specific for monitoring the effects of aspirin on platelet COX-1. In contrast, serum thromboxane B2 (TxB2) reflects the total capacity of platelets to synthesize TxA2, of which it is a stable metabolite. Because the contribution of other blood cells to its synthesis is marginal, serum TxB2 is the most specific test to measure the pharmacological effect of aspirin on platelets.11
Comparison of different laboratory methods
Direct comparison of different laboratory methods to detect aspirin resistance usually showed very weak or no correlation.7–12 indicating that they are sensitive to different parameters. In addition, the calculated prevalence of aspirin resistance in 11 studies that used different functional assays of platelet function, varied between 5.5 and 61%.3 It is clear that this wide variability cannot be simply explained based on differences in the patient populations studied, but is likely due to different accuracy of the different assays to detect the pharmacological effect of aspirin.
Lordkipanidzé et al.13 report the results of a comprehensive study, in which they compared the performance of six major tests (AA-induced aggregation with LTA; ADP-induced aggregation with LTA; whole blood aggregometry; PFA-100, Ultegra VerifyNow ASA, urinary 11-dehydrothromboxane B2) in 201 patients with stable coronary artery disease receiving 
80 mg aspirin daily. Overall, the correlation among the various platelet function tests was very poor. The prevalence of aspirin resistance was relatively low with the more specific tests, AA-induced platelet aggregation (4%) and VerifyNow Aspirin (6.7%), while it was > 50% when unspecific tests, such as platelet aggregation induced by 20 µM ADP (51.7%) and PFA-100 (59.5%), were used. Unfortunately, Lordkipanidzé et al. did not measure serum TxB2 levels, which, as mentioned before, is the most specific test to evaluate the inhibitory effect of aspirin on platelets. Studies that measured TxB2 levels in aspirin-treated patients reported a prevalence of ‘aspirin resistance’ that ranged between 1 and 1.7%.7,9,14 Therefore, aspirin resistance, when tested with appropriate, specific tests, appears to be extremely rare, and, in most instances, due to under-dosing or non-compliance.1,14
Anti-platelet treatment: to monitor or not to monitor?
The demonstration that some patients may be ‘resistant’ or ‘poor responders’ to the pharmacological effect of aspirin and other anti-platelet agents, such as ticlopidine and clopidogrel,1 prompted the need of laboratory monitoring of anti-platelet therapy, which, traditionally, at variance with anticoagulant therapy, has never been monitored with laboratory tests. However, some confusion was generated in the scientific community about the real aim of monitoring anti-platelet therapy. Is it useful to identify patients who are resistant to anti-platelet agents or to identify patients with high residual platelet reactivity, independently of their response to anti-platelet agents, who may be at risk of atherothrombotic events? Identification of ‘poor responders’ can only be accomplished using laboratory tests that are specific for the pharmacological target of the anti-platelet drug: ‘poor responders’ must be considered at heightened risk of atherothrombotic events de facto, unless the anti-platelet drug exerts its major anti-thrombotic effect through inhibition of other targets: a concept that is intriguing, yet unlikely.4 In contrast, the identification of patients with high residual platelet reactivity, who may be at risk of atherothrombotic events should be accomplished using those tests that will prove useful in large, ad hoc clinical studies. Although many promising results have been obtained with some laboratory tests, they must still be considered too preliminary. The ideal test should be inexpensive, easy to perform, quick, reproducible, accurate, and most importantly, well standardized, so that patients on anti-platelet treatment could be monitored in any laboratory, obtaining comparable results. In addition, it should be validated in clinical studies addressing two main issues:
- Do we know how to manage safely and effectively patients on anti-platelet treatment who are identified ‘at risk’ by the laboratory test? Today, the answer to this question is certainly: No.
- Is laboratory monitoring of anti-platelet therapy cost-effective? Needless to say, we do not know the answer yet.
In conclusion, a clear distinction should be made between monitoring patients on anti-platelet treatment to identify ‘poor responders’ and to identify patients with high residual platelet reactivity. Many studies need to be done to answer basic questions on its clinical utility and cost-effectiveness, before anti-platelet monitoring can be recommended in the clinical practice. Until then, monitoring of anti-platelet therapy should be done for investigational purposes only.
Conflict of interest: none declared.
Footnotes
The opinions expressed in this article are necessarily not those of the Editors of European Heart Journal or of the European Society of Cardiology.
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