Assessing aspirin responsiveness in subjects with multiple risk factors for vascular disease with a rapid platelet function analyzer

Comparison of current platelet functional tests for the assessment of aspirin and clopidogrel response

The two most widely used antiplatelet drugs in the world are aspirin and clopidogrel. However, some patients on aspirin and/or clopidogrel therapy do not respond appropriately to either aspirin or clopidogrel. This phenomenon is usually called “aspirin/clopidogrel resistance”. Several platelet function tests have been used in various studies for the assessment of aspirin and clopidogrel resistance in healthy individuals and patients admitted in cardiology departments. An accurate assessment of platelet response to aspirin/clopidogrel could benefit patients by proposing tailored-antiplatelet therapy based on test results. However, there is a clear lack of standardisation of such techniques and their analytical variability may induce misinterpretation. After a quick report of the mechanisms responsible for aspirin/clopidogrel resistance, we describe the pre-analytical aspects and the analytical performances of current platelet function tests (Light-transmission aggregometry, whole-blood aggregometry, VerifyNow®, Platelet Function Analyzer®, thromboelastography, VASP assay) that are used for the assessment of aspirin/clopidogrel resistance in clinical studies. Considering the different variables that have to be taken into account with each of the platelet function tests, a particular attention should be paid when interpreting results.

Cardiology: Aspirin Resistance in Patients with Stable Coronary Artery Disease with and without a History of Myocardial Infarction

Background:

Aspirin therapy is a cornerstone in the prevention of atherothrombotic events, but recurrent vascular events are estimated to occur in 8-18% of patients taking aspirin for secondary prevention after 2 years. Estimates of biologic aspirin resistance vary from 5% to 60%, depending on the assay used. However, the relationship between biologic measurements of aspirin resistance and adverse clinical events remains unclear.

Objective:

To determine whether patients with documented myocardial infarction (Ml) while on aspirin therapy (cases) were more likely to be aspirin resistant than were patients with coronary artery disease (CAD) who had no history of Ml (controls) and to assess clinical predictors of aspirin resistance in patients with stable CAD.

Methods:

This case-control study examined aspirin responses using the VerifyNow Aspirin Assay system in 50 cases and 50 controls who had taken a dose of aspirin within 48 hours of presentation to the clinic visit. Odds ratios were estimated to determine the association between aspirin resistance and MI, Independent predictors of aspirin resistance were determined using univariate and multivariate analyses.

Results:

An increase in the prevalence of aspirin resistance among cases (16% vs 12% in controls) was not observed (OR 1.40; 95% CI 0.45 to 4.37; p = 0.566). In the overall CAD population, female sex was independently associated with aspirin resistance (OR 4.01; 95% CI 1.15 to 13.92; p = 0.029).

Conclusions:

Additional large studies are required to understand whether biologically defined aspirin resistance is associated with increased risk for cardiovascular events, with special attention paid to sex differences.

[Prevalence and prognosis of aspirin resistance in critical limb ischemia patients]

OBJECTIVES

To assess the prevalence and the association between aspirin resistance in critical limb ischemia patients using the VerifyNow^® bed-side platelet test, and occurrence of cardiovascular morbidity and/or death at one year.

MATERIALS AND METHODS

National multicenter prospective observational study related to COPART II centers. From 2010 through 2014, 64 subjects hospitalized for critical limb ischemia and already treated by aspirin before the VerifyNow^® test were included. A VerifyNow^® test>550 ARU was defined as aspirin resistance. Critical limb ischemia was defined according to the TASC I criteria. The primary outcome was a composite including death, acute coronary syndrome, stroke and major amputation during the one-year follow-up period.

RESULTS

In all, 9/64 patients were aspirin resistant, the status was confirmed in one case. The prevalence of aspirin resistance was 14.06%. There was no significant difference between aspirin resistant and aspirin non-resistant groups in terms of cardiovascular history and glycemia status. Neither was there significant difference between the two groups in terms of survival.

CONCLUSION

Aspirin resistance was not predictive of poorer survival in critical limb ischemia patients. However, our population was limited. Considering that a clear definition of aspirin resistance and standardized diagnostic tests are lacking, complementary studies might be useful.

Assessment of platelet function: Laboratory and point-of-care methods

In the event of blood vessel damage, human platelets are promptly recruited on the site of injury and, after their adhesion, activation and aggregation, prevent blood loss with the formation of a clot. The consequence of abnormal regulation can be either hemorrhage or the development of thrombosis. Qualitative and/or quantitative defects in platelets promote bleeding, whereas the residual reactivity of platelets, despite antiplatelet therapies, play an important role in promoting arterial thrombotic complications. Platelet function is traditionally assessed to investigate the origin of a bleeding syndrome, to predict the risk of bleeding prior surgery or during pregnancy or to monitor the efficacy of antiplatelet therapy in thrombotic syndromes that, now, can be considered a new discipline. “Old” platelet function laboratory tests such as the evaluation of bleeding time and the platelet aggregation analysis in platelet-rich plasma are traditionally utilized to aid in the diagnosis and management of patients with platelet and hemostatic disorders and used as diagnostic tools both in bleeding and thrombotic diathesis in specialized laboratories. Now, new and renewed automated systems have been introduced to provide a simple, rapid assessment of platelet function including point of care methods. These new methodologies are also suitable for being used in non-specialized laboratories and in critical area for assessing platelet function in whole blood without the requirement of sample processing. Some of these methods are also beginning to be incorporated into routine clinical use and can be utilized as not only as first panel for the diagnosis of platelet dysfunction, but also for monitoring anti-platelet therapy and to potentially assess risk of both bleeding and/or thrombosis.

Assessment methods for aspirin-mediated platelet antiaggregation in type 2 diabetic patients: degree of correlation between 2 point-of-care methods

AIMS

Impaired response to antiplatelet therapy in diabetic patients results in a higher incidence of drug-eluting stent thrombosis. This study determined the prevalence of high on-aspirin (AS) platelet reactivity in type 2 diabetic patients treated with percutaneous coronary intervention (PCI) using the VerifyNow Aspirin Assay (VN) and platelet function analyzer PFA-100 (PFA-100) and analyzed the correlation between both methods.

METHODS

Type 2 diabetic patients (100) with non-ST-elevation acute coronary syndrome who underwent PCI and Xience V drug-eluting stent implantation were included in this study. After PCI, platelet antiaggregation mediated by acetylsalicylic acid was assessed by VN and PFA-100. The degree of correlation and concordance was then determined.

RESULTS

When assayed with VN, 7% of the patients were nonresponders to aspirin (aspirin reaction units >550), and when assayed with PFA-10, 41% were nonresponders (closure time <193 seconds). Of the patients, 4% were nonresponders to aspirin according to VN but were sensitive to aspirin according to PFA-100, and 38% were sensitive to aspirin according to VN and nonresponders according to PFA-100. Overall, 55% of the patients were aspirin-sensitive in both methods. The Spearman's coefficient between VN and PFA-100 results was r = 0.09 (P = 0.35). The kappa index value was 0.0062 (P = 0.91).

CONCLUSIONS

There is no concordance or correlation between the VN and PFA-100 results. Therefore, the use of these analyses should be restricted to clinical research, which limits its application in clinical practice.

Clinical implications of aspirin resistance

Aspirin (acetylsalicylic acid) is one of the main therapeutic medications used in the prevention of thromboembolic vascular events. Aspirin exhibits its antiplatelet action by irreversibly inhibiting platelet cyclooxygenase-1 enzyme, thus preventing the production of thromboxane A2 (TXA2). Aspirin resistance, as measured in vitro, is the inability of aspirin to reduce platelet activation and aggregation by failure to suppress the platelet production of TXA2. Laboratory tests of platelet TXA2 production or platelet function dependent on TXA2 can detect aspirin resistance in vitro. The clinical implication of this laboratory definition has not yet been elucidated via prospective trials that have controlled for confounders, such as hypertension, diabetes and dyslipidemia. Large meta-analyses have found low-dose aspirin to be as effective as high-dose aspirin in preventing vascular events, making a dose-dependent improvement in laboratory response clinically irrelevant. Possible causes of aspirin resistance include poor compliance, inadequate dose, drug interactions, genetic polymorphisms of cyclooxygenase-1, increased platelet turnover and upregulation of non-platelet pathways of thromboxane production. However, there is currently no standardized approach to the diagnosis and no proven effective treatment for aspirin resistance. Further research exploring the mechanisms of aspirin resistance is needed in order to better define aspirin resistance, as well as to develop a standardized laboratory test that is specific and reliable, and can correlate with the clinical risk of vascular events. The intent of this paper is to review the literature discussing possible mechanisms, diagnostic testing and clinical trials of aspirin resistance and to discuss its clinical relevance as it pertains to cerebrovascular and cardiovascular disease.

Antiplatelet therapy: aspirin resistance and all that jazz!

Platelets play a crucial role in the pathogenesis of atherosclerosis, thrombosis, and stroke. Aspirin used alone or in combination with other antiplatelet drugs has been shown to offer significant benefit to patients at high risk of vascular events. Resistance to the action of aspirin may decrease this benefit. Aspirin resistance has been defined by clinical and/or laboratory criteria; however, detection by laboratory methods prior to experiencing a clinical event will likely provide the greatest opportunity for intervention. Numerous laboratory methods with different cutoff points have been used to evaluate the resistance. Noncompliance with aspirin treatment has also confounded studies. A single assay is currently insufficient to establish resistance. Combinations of results to confirm compliance and platelet inhibition may identify "at-risk" individuals who truly have aspirin resistance. The most effective strategy for managing patients with aspirin resistance is unknown; however, studies are currently underway to address this issue.

Aspirin prophylaxis for the prevention of thrombosis: expectations and limitations

Platelets play a very important role in the pathogenesis of acute vascular events leading to thrombosis of the coronary and cerebral arteries. Blockage of these arteries leading to regional ischemia of heart and brain tissues precipitate heart attacks and stroke. Acetyl salicylic acid (Aspirin) has been the drug of choice for over half a century for the primary and secondary prophylaxis of thrombotic events. In spite of its extensive use as an antiplatelet drug for the prevention of vascular thrombosis, there is considerable concern about the degree of protection it offers, to patients under aspirin therapy. In this paper, we explain the phenomenon of aspirin resistance, discuss the limitations of aspirin therapy, and suggest methods to monitor "at-risk" individuals. Ability to monitor and determine at risk patients will provide opportunities for the clinicians to customize antiplatelet therapies.

Platelet reactivity in type 2 diabetes mellitus: a comparative analysis with survivors of myocardial infarction and the role of glycaemic control

Patients with type 2 diabetes mellitus exhibit considerable platelet dysfunction, though this is poorly characterized in patients with diabetes taking aspirin for the primary prevention of cardiovascular events. We sought to compare platelet function in this patient population with that of a high-risk group of non-diabetic subjects with a history of previous myocardial infarction (MI), and to assess whether glycaemic control impacts on platelet function.

METHODS

Platelet aggregation was measured in response to incremental concentrations of five platelet agonists using light transmission aggregometry. All patients were taking aspirin, and aspirin insensitivity was defined as ≥ 20% arachidonic acid (AA) mediated aggregation. Patients with diabetes were divided according to glycaemic control (HbA(1c)): optimal ≤ 6.5, good 6.6-7.4 and suboptimal ≥ 7.5%.

RESULTS

In total, 85 patients with type 2 diabetes and 35 non-diabetic patients with previous MI were recruited. Compared to MI patients, diabetes patients had increased aggregation in response to multiple concentrations of epinephrine, collagen, adenosine diphosphate and AA. Aspirin insensitivity was more common in type 2 diabetes (15% vs. 0%, p=0.037). Platelet aggregation was increased in response to several agonists patients with suboptimal glycaemic control compared to patients with optimal control. Aspirin insensitivity was also more common in patients with suboptimal glycaemic control compared to those with good or optimal control (26.0% vs. 8.3% vs. 4%, p=0.04).

CONCLUSION

Patients with type 2 diabetes mellitus, without proven vascular disease, exhibit platelet dysfunction and have increased platelet aggregation and aspirin insensitivity compared to non-diabetic patients with previous MI. Platelet dysfunction in diabetes is more severe in patients with suboptimal glycaemic control.

Determination of the prevalence of aspirin and clopidogrel resistances in patients with coronary artery disease by using various platelet-function tests

BACKGROUND

Dual therapy with aspirin and clopidogrel has emerged as the gold standard therapy for patients treated with drug-eluting stents (DES). However, there is variability in patients' responses to this antiplatelet therapy, and some patients continue to show ischemic recurrences after therapy. The purpose of the study was to compare the simultaneously obtained results of various platelet-function tests for assessing the prevalence of antiplatelet resistance in coronary artery disease patients undergoing DES therapy.

METHODS

A total of 66 patients were administered a loading dose of aspirin, clopidogrel, and cilostazol at least 12 hr before stenting. The results of VerifyNow (Accumetrics, USA), multiplate analyzer (Dynabyte Medical, Germany), and vasodilator-stimulated phosphoprotein/P2Y12 (Biocytex, France) assays were compared with those of light transmission aggregometry (LTA) analysis.

RESULTS

The P2Y12 reaction units and P2Y12% inhibition values obtained using the VerifyNow assay showed strong correlation (r) with the results of the LTA analysis. All tests results showed low concordance in defining the antiplatelet resistance in patients, and the degrees of agreement were as follows: 0 for aspirin reaction units; 0.25, P2Y12% inhibition; 0, aspirin-sensitive patients' identification test; 0.21, ADPtest; and 0.14, platelet reactivity index, expressed as the κ statistics. The prevalence of aspirin and clopidogrel resistances in patients resulted in remarkable variations, from 0% to 22.7% and from 9.1% to 48.5%, respectively.

CONCLUSIONS

The clinical usefulness of the different assays for the correct classification of patients in terms of antiplatelet resistance remains unclear. Further studies are required to determine the best method for correlating the occurrences of adverse ischemic events.

Prevention of the renarrowing of coronary arteries using drug-eluting stents in the perioperative period: an update

The management of patients scheduled for surgery with a coronary stent, and receiving 1 or more antiplatelet drugs, has many controversies. The premature discontinuation of antiplatelet drugs substantially increases the risk of stent thrombosis (ST), myocardial infarction, and cardiac death, and surgery under an altered platelet function could also lead to an increased risk of bleeding in the perioperative period. Because of the conflict in the recommendations, this article reviews the current antiplatelet protocols after positioning a coronary stent, the evidence of increased risk of ST associated with the withdrawal of antiplatelet drugs and increased bleeding risk associated with its maintenance, the different perioperative antiplatelet protocols when patients are scheduled for surgery or need an urgent operation, and the therapeutic options if excessive bleeding occurs.

Aspirin resistance: Fact or fiction? A point of view

Aspirin is a wonder drug that has been used for well over 100 years for its analgesic and antipyretic effects. For the past three decades, it has increasingly been used for the prevention of primary and secondary cardiovascular events. Lately, it has been suggested that a significant number of individuals taking aspirin have become resistant to this drug. The phenomenon of "aspirin resistance" is based on the observation of clinical events in some patients taking aspirin, and/or a diminished platelet aggregation inhibitory response to aspirin therapy. Unfortunately, laboratory assays used to monitor the efficacy of aspirin are far from accurate and the results are not reproducible. Furthermore, results of different platelet function tests are often not congruent. In addition, platelet aggregation studies show marked inter-individual and intra-individual variability. Patients with coronary heart disease take many drugs that interfere with the effect of aspirin on platelet aggregation. Besides inhibiting formation of thromboxane A(2) from arachidonic acid, aspirin has a host of platelet-independent effects that complement its platelet inhibitory effects. Laboratory assays designed to measure platelet function do not take into account these pleiotropic effects of aspirin. In our view, use of the term "aspirin resistance" based on inadequate knowledge of imperfect laboratory tests does a disservice to physicians and patients.

Aspirin resistance and compliance with therapy

INTRODUCTION

Aspirin resistance is associated with increased cardiovascular risk in aspirin-treated patients. Poor compliance may explain many cases of "resistance," yet few clinical studies have used objective measurement of therapy compliance. We did so in a case-controlled study.

METHODS

We enrolled patients within 24 h of ischemic stroke and a group of controls taking aspirin who had never suffered a vascular event on therapy. All claimed to be compliant. We assessed platelet function using platelet function analyser (PFA)-100 and rapid platelet function analyser (RPFA) devices, applying standard definitions of resistance. We used high-performance liquid chromatography for levels of aspirin metabolites in the urine to confirm compliance with therapy. We compared rates of resistance in stroke patients and controls, and performed subgroup analysis restricted to patients with objective confirmation of recent aspirin ingestion.

RESULTS

We recruited 90 cases and 90 controls. Complete platelet function tests were available in 177. Resistance rates seen in cases and controls, respectively, were: resistance on one or more test, 30 (34%) versus 21 (25%), P= 0.19; on PFA-100 testing only, 28 (32%) versus 15 (18%), P= 0.031; on RPFA testing only, 16 (18%) versus 12 (14%), P= 0.54; resistance on both tests, 12 (14%) versus 5 (6%), P= 0.037. When only patients with objective evidence of recent aspirin ingestion were considered (n = 71), rates were similar regardless of definition of resistance used.

CONCLUSION

Aspirin resistance is common but poor compliance accounted for nearly half of cases of apparent aspirin "failure." Objective measures to assess compliance are essential in studies of aspirin resistance.

Surgical revascularisation in patients with severe limb ischaemia induces a pro-thrombotic state

Platelet and coagulation activation are implicated in the increased incidence of ischaemic events seen in patients with peripheral arterial disease. This study aimed to assess the effect of surgical revascularisation on platelet aggregation and coagulation in patients with severe limb ischaemia (SLI). Twenty-two patients had blood samples taken: prior to surgery, on reperfusion, 2, 24 and 48 h post-surgery. Platelet aggregation through COX-mediated and thrombin receptor activator peptide (TRAP)-stimulated GPIIb/IIIa pathways was measured by the Ultegra point of care system. Thrombin-antithrombin III Complex (TAT) and D-dimer were measured by ELISA. COX-mediated aggregation increased significantly at reperfusion and remained elevated at 24 h [median increase from baseline of 9% (range -16 to 33%) P = 0.011]. TRAP-stimulated aggregation increased significantly at reperfusion and remained elevated at 2 h post-surgery [median increase 18% (range -71 to 45%); P = 0.007]. TAT levels were significantly elevated from reperfusion and remained so at 48 h (P < 0.003), whereas D-dimer only increased at 24 h (P = 0.014). For the first time, we have demonstrated that in patients with SLI, platelet aggregation is increased following surgery and there is a mismatch in the balance between the coagulation and fibrinolytic pathways despite the use of aspirin and heparin. Thus in the early post-operative these patients exhibit a pro-thrombotic state.

Perioperative management of antiplatelet agents in noncardiac surgery

It is common that patients who are scheduled for surgery are treated with antiplatelet agents (APAs) due to their wide indications. The management of these APAs in the perioperative period (acetylsalicylic acid alone, a thienopyridine alone or, in most cases, a combination of them) has a dual perspective: the risk of bleeding when the patient is operated under the effect of the APA against the risk of thrombosis if it has been withdrawn. The main challenges for the anaesthesiologist and the surgeon include patients with a coronary stent (mainly, new drug-eluting coronary stents), those undergoing urgent surgery and those undergoing high bleeding risk surgery. We review current protocols and discuss the most recent proposals for the management of APAs in patients undergoing noncardiac surgery. Current recommendations include the maintenance of aspirin if possible throughout the perioperative period, in order to limit the risks of cardiological, vascular or neurological postoperative events, although this makes it necessary to assume a small risk for haemorrhagic complications in some patients. Nevertheless, there are many circumstances that are not clear yet and, in this situation, it is crucial that patients are treated with a multidisciplinary approach (anaesthesiologists, surgeons, cardiologists and haematologists).

[Incidence of aspirin resistance in the patient group of a university hospital in Korea]

BACKGROUND

Aspirin is the most common drug used for the prevention of arterial thrombosis. However, platelet responsiveness to aspirin is variable among individuals and it is important to detect aspirin resistance to improve clinical outcome. We analyzed the changes of platelet reactivity before and after aspirin treatment. We also investigated the incidence and influencing factors of aspirin resistance in Korean.

METHODS

We tested platelet function in 198 patients who had been treated with aspirin in a Korean university hospital, and 59 of these patients were tested for platelet function before and after aspirin treatment. We also analyzed platelet reactivity in 136 patients who had not been treated with aspirin. Platelet function was tested using the VerifyNow Aspirin Assay (Accumetrics, USA). Platelet reactivity was expressed as aspirin reaction unit (ARU) and > or =550 ARU was defined as aspirin resistance.

RESULTS

Platelet reactivity of 136 patients who had not been treated with aspirin was 632.2plusmn;46.3 ARU (meanplusmn;SD) (range, 462-675). Platelet reactivity of 198 patients who had been treated with aspirin was 472.5plusmn;60.0 (338-666) ARU, and 10.1% of patients were aspirin-resistant. The difference of platelet reactivity before and after aspirin treatment was 128.3plusmn;68.7 (-40-248) ARU. Hb level was lower and platelet count was higher in aspirin-resistant group than in aspirin-sensitive group (P<0.05).

CONCLUSIONS

We demonstrated the distribution of platelet reactivity before and after aspirin treatment using the VerifyNow Aspirin Assay. The incidence of aspirin resistance was 10.1%, and low Hb level and high platelet count were related with aspirin resistance.

Dual antiplatelet therapy monitoring for neurointerventional procedures using a point-of-care platelet function test: a single-center experience

BACKGROUND AND PURPOSE

Growing evidence of the relationship between poor antiplatelet response and occurrence of clinical events elicited the need of monitoring the response which has not been part of our daily practice. We present our initial experience with a new point-of-care antiplatelet-function test (VerifyNow assay) in neurointerventional procedures.

MATERIALS AND METHODS

Among the 106 consecutive patients from July 2006 to April 2007, ninety-eight met the inclusion criteria. Our preferred antiplatelet regimen was aspirin (325 mg daily) and clopidogrel (300 mg of loading dose followed by 75 mg daily) starting 5-10 days before the procedure. The test results were reported as aspirin-reaction unit (ARU) for aspirin and P2Y(12) reaction units (PRU), baseline (BASE), and percentage inhibition for the P2Y(12) assay and were summarized as mean +/- SD of the values. We analyzed the effects of several factors of poor clopidogrel response (<40% inhibition). The occurrence of thrombotic events was recorded.

RESULTS

The mean ARU of aspirin assays was 438.3 +/- 47.9 (range, 350-632), and the response was poor in 2 patients (2.1%). For clopidogrel, the mean of the BASE, PRU, and percentage inhibition was 356.8 +/- 56.3 (range, 234-495), 198.9 +/- 104.4 (range, 8-401), and 45.2 +/- 27.1% (range, 0-98), respectively. Forty-two patients (42.9%) showed poor response. Multivariate analysis showed greater body weight (81.9 Kg +/- 19.1 kg versus 69.9 +/- 15 kg) in the poor-response group. All 3 cases of intraprocedural thrombosis (3.1%) were observed only in the poor-response group.

CONCLUSION

We observed a high frequency of poor clopidogrel responses in the neurointerventional setting. Routine monitoring of the drug response would be helpful for the early identification of poor antiplatelet responders so that we may modify the regimen and/or treatment plan.

The role of aspirin resistance in the treatment of acute coronary syndromes

The TIMI Risk Score recognizes prior aspirin use as an independent risk factor for adverse outcomes in subjects presenting with an acute coronary syndrome. The etiology of this increased risk awaits clarification, but prior aspirin use may be associated with altered thrombus composition which is more resistant to current treatment modalities as compared to thrombus formation in subjects without prior aspirin use. Post hoc analysis of acute coronary syndrome trials has shown that prior aspirin users treated with unfractionated heparin are at particularly high risk. The addition of glycoprotein IIb/IIIa receptor inhibitor to unfractionated heparin or substitution of low-molecular-weight heparin significantly improves outcomes in prior aspirin users. The prognostic significance of prior aspirin use in acute coronary syndromes has important implications not only in clinical practice, but also in the design and interpretation of clinical trials.

Monitoring antiplatelet therapy during peripheral vascular and coronary interventions

Platelets play a central role in the initiation and propagation of thrombus formation. The use of antiplatelet and antithrombotic medications during peripheral vascular and coronary interventions helps reduce the likelihood of intravascular thrombus formation and adverse ischemic events. As formation of intravascular thrombus and subacute stent thrombosis are thrombin- and platelet-mediated phenomenon, achieving optimal activated clotting time and platelet inhibition (PI) during the interventional procedure is critical. However, as a quick and easy measure of platelet function has previously not been available in the interventional laboratory, cardiovascular interventions are routinely performed after administration of oral or intravenous antiplatelet agents without evaluating platelet function. Recently, point-of-care rapid platelet function assays have become available that allow quick and reproducible measure of platelet function in the interventional laboratory after administration of aspirin, thienopyridines, and glycoprotein IIb/IIIa inhibitors. Though PI can now be routinely measured during vascular interventions, considerable inconsistencies exist in the management of patients based on these results. We present an algorithm for the management of antiplatelet therapy during cardiovascular interventions based on rapid evaluation of platelet function in the interventional laboratory.

Prevalence of persistent platelet reactivity despite use of aspirin: a systematic review

BACKGROUND

The absolute risk of recurrences among patients using aspirin for prevention of cardiovascular events remains high. Persistent platelet reactivity despite aspirin therapy might explain this in part. Reported prevalences of this so-called aspirin resistance vary widely, between 0% and 57%.

OBJECTIVES

The aim of the study was to systematically review all available evidence on prevalence of aspirin resistance and to study determinants of reported prevalence.

METHODS

Using a predefined search strategy, we searched electronic databases MEDLINE, EMBASE, CENTRAL, and Web of Science. To be included in our analysis, articles had to contain a laboratory definition of aspirin resistance, use aspirin as secondary prevention, and report associated prevalence.

RESULTS

We included 34 full-text articles and 8 meeting abstracts. The mean prevalence of aspirin resistance was 24% (95% CI 20%-28%). After adjustment for differences in definition, used dosage, and population, a statistically significant higher prevalence was found in studies with aspirin dosage < or =100 mg compared with > or =300 mg (36% [95% CI 28%-43%] vs 19% [95% CI 11%-26%], P < .0001). Studies measuring platelet aggregation using light aggregometry with arachidonic acid as an agonist had a pooled unadjusted prevalence of 6% (95% CI 0%-12%). In studies using point-of-care platelet function-analyzing devices, the unadjusted prevalence was significantly higher, at 26% (95% CI 21%-31%).

CONCLUSIONS

Prevalences widely differ between studies reporting on aspirin resistance. Both aspirin dosage and the method of defining aspirin resistance strongly influence estimated prevalence, which explains found heterogeneity among studies. On average, it appears that about 1 in 4 individuals may express biochemically defined aspirin resistance.

[Assessment of platelet function in man]

Assessment of platelet function was primarily designed to explore patients with hemostatic disorders, but is becoming important for the monitoring of anti platelet agents, mostly aspirin and clopidogrel. Beside platelet counting, morphological analysis and bleeding time, a number of dedicated platelet function instruments are now available, generally allowing a rapid evaluation of platelet function in whole blood. The other tests including aggregometry and ELISA measurement of activation markers are generally restricted to specialized laboratories. Although aggregometry is still considered as the "gold standard", the recently developed flow cytometric-based platelet function analysis provides a wide choice of tests that assess the number of surface receptors, the measure of secretion and aggregation, the quantification of microparticules and leukocyte-platelet aggregates. It also allows the measure of the function of the ADP receptor P2Y12 by the phosphorylation level of the VASP protein, method currently under evaluation to monitor the platelet response to clopidogrel treatment.

Assessment of biochemical aspirin resistance at rest and immediately after exercise testing

Some aspirin-treated patients experience thromboembolic events, a phenomenon termed 'aspirin resistance', which may be clinical or biochemical by definition. Physical exercise is known to enhance platelet secretion and aggregability. To evaluate the presence of biochemical aspirin resistance at rest and immediately after exercise in individuals with stable coronary artery disease or coronary artery disease risk factors. We prospectively enrolled 101 patients who had received 100 or 300 mg/day enteric-coated aspirin for at least 7 days. Biochemical aspirin resistance (defined as normal collagen-epinephrine closure time < 165 s) was studied using the standardized platelet function analyzer. Of the 101 patients, 63 were aspirin sensitive both at rest and immediately after exercise, 18 exhibited biochemical aspirin resistance both at rest and after exercise, and 20 were aspirin sensitive at rest but exhibited biochemical aspirin resistance immediately after exercise. The results of exercise testing were similar in all three groups (each P > 0.05). Our results indicate that in almost 20% of the patients, aspirin did not seem to protect against exercise-induced platelet activation, despite the presence of aspirin sensitivity at rest. We did not, however, determine the extent to which the biochemical aspirin resistance noted in our study applied to clinical events.

Measuring antiplatelet drug effects in the laboratory

This review discusses the advantages and disadvantages of currently available tests for the monitoring of antiplatelet therapy (especially aspirin and clopidogrel). Many tests of platelet function are now available for clinical use, and some of these tests have been shown to predict clinical outcomes after antiplatelet therapy. However, in most of these studies, the number of major adverse clinical events was low. No published studies address the clinical effectiveness of altering therapy based on the results of monitoring antiplatelet therapy. Therefore, the correct treatment, if any, of "resistance" to antiplatelet therapy is unknown and, other than in research trials, monitoring of antiplatelet therapy in patients is not generally recommended. A clinically meaningful definition of "resistance" to antiplatelet drugs needs to be developed, based on data linking drug-dependent laboratory tests to clinical outcomes in patients.

Biological effects of aspirin and clopidogrel in a randomized cross-over study in 96 healthy volunteers

BACKGROUND

Some data suggest that biological 'resistance' to aspirin or clopidogrel may influence clinical outcome.

OBJECTIVE

The aim of this study was to evaluate the relationship between aspirin and clopidogrel responsiveness in healthy subjects.

METHODS

Ninety-six healthy subjects were randomly assigned to receive a 1-week course of aspirin 100 mg day(-1) followed by a 1-week course of clopidogrel (300 mg on day 1, then 75 mg day(-1)), or the reverse sequence, separated by a 2-week wash-out period. The drug effects were assessed by means of serum TxB2 assay, platelet aggregation tests, and the PFA -100 and Ultegra RPFA -Verify Now methods.

RESULTS

Only one subject had true aspirin resistance, defined as a serum TxB2 level > 80 pg microL(-1) at the end of aspirin administration and confirmed by platelet incubation with aspirin. PFA-100 values were normal in 29% of the subjects after aspirin intake, despite a drastic reduction in TxB2 production; these subjects were considered to have aspirin pseudo-resistance. Clopidogrel responsiveness was not related to aspirin pseudo-resistance. Selected polymorphisms of platelet receptor genes were not associated with either aspirin or clopidogrel responsiveness.

CONCLUSIONS

In healthy subjects, true aspirin resistance is rare and aspirin pseudo-resistance is not related to clopidogrel responsiveness.

A Preliminary Study on the Effects of Exercising to Maximum Walking Distance on Platelet and Endothelial Function in Patients with Intermittent Claudication

BACKGROUND

Platelet and endothelial activation has been shown to be increased in patients with intermittent claudication (IC). Recent studies have suggested that exercise may induce further platelet activation. The aims of this study were to investigate the effect of exercising to maximum walking distance on platelet and endothelial function in patients with intermittent claudication who were receiving statin and aspirin therapy compared with age matched healthy controls.

METHODS

Platelet aggregation through COX-mediated and thrombin receptor activator peptide (TRAP)-stimulated GPIIb/IIIa pathways was measured by the Ultegra point of care system in 20 patients with IC on aspirin and 20 healthy volunteers before, immediately and 1h after exercising to treadmill maximal walking distance (MWD). Soluble P-selectin, vWF and sICAM were measured using an enzyme linked immuno-sorbent assay technique.

RESULTS

Baseline platelet aggregation was significantly reduced in patients with IC compared to volunteers (p<0.05). In patients, exercising to MWD significantly reduced platelet aggregation (COX, median -5% [range -24 to 13%]; p = 0.02; GPIIIa/IIb, median -13% [range -72 to 33%]; p = 0.02) immediately post-exercise which returned to baseline values at 1 h. There was no change in the healthy volunteers following the same median duration of exercise. Baseline sP-selectin levels were higher in the patients with IC compared to the healthy volunteers [Median values (interquartile range), 42.72 (33.28-54.24) versus 29.16 (24.40-34.10), p = 0.0003] but there were no differences in vWF levels. Both sP-selectin and vWF levels increased significantly in the control and patient group following exercise (p<0.005). sICAM were higher at baseline in the patients with IC but were unchanged following exercise [Median values (interquartile range),560.9 (405.5-739.4) versus 467.0 (325.7-643.4), p<0.05].

CONCLUSION

This study is the first to show that platelet aggregation is reduced immediately following treadmill exercise to maximum walking distance in patients with IC despite a rise in sP-selectin and vWF, suggesting endothelial activation. The inhibition of platelet aggregation after exercise in subjects on antiplatelet and statin therapy suggests that exercise is unlikely to exacerbate platelet thrombus formation in patients with IC.

Aspirin resistance

Aspirin resistance is the inability of aspirin to reduce platelet production of thromboxane A2 and thereby platelet activation and aggregation. Increasing degrees of aspirin resistance may correlate independently with increasing risk of cardiovascular events. Aspirin resistance can be detected by laboratory tests of platelet thromboxane A2 production or platelet function that depend on platelet thromboxane production. Potential causes of aspirin resistance include inadequate dose, drug interactions, genetic polymorphisms of COX-1 and other genes involved in thromboxane biosynthesis, upregulation of non-platelet sources of thromboxane biosynthesis, and increased platelet turnover. Aspirin resistance can be overcome by treating the cause or causes, and reduced by minimising thromboxane production and activity, and blocking other pathways of platelet activation. Future research is aimed at defining aspirin resistance, developing reliable tests for it, and establishing the risk of associated cardiovascular events. Potential mechanisms of aspirin resistance can then be explored and treatments assessed.

Aspirin Resistance: A Review Of Diagnostic Methodology, Mechanisms, and Clinical Utility

Ingestion of a daily aspirin in patients with coronary artery disease decreases the rate of occlusive atherosclerotic events by about 25 percent. Some patients whose platelets are minimally inhibited by aspirin are categorized as aspirin resistant. Three reports document an increased risk for future vascular events in aspirin resistant patients. Aspirin's platelet inhibitory effect is measured using a variety of techniques. The demarcation between minimal and expected aspirin inhibition of platelets is arbitrarily determined by each investigator which leads to confusion in translating these reports to patient care. The focus of this report is the relative merits of the different techniques and their utility for defining patients with minimal aspirin induced platelet inhibition. The clinically useful mechanisms underlying decreased aspirin induced platelet inhibition include failure of a patient to take their daily aspirin, poor compliance, and nonsteroidal anti-inflammatory drugs (NSAIDs) interference with aspirin's ability to get to its binding site on the cyclooxygenase enzyme-1 (COX-1)]. Compliance is best assessed by comparing the results obtained with arachidonic acid (AA) stimulated light aggregation at two time points. The first time point is while the patient is supposedly taking their usual daily aspirin and the second time point is 2 hours after the patient is observed to ingest 325 mg of aspirin. After observed ingestion of aspirin, those patients with minimal aspirin inhibition of platelets are best detected using light aggregation stimulated by a new platelet agonist platelet prostaglandin agonist (PPA). In order for the results of a particular technique to be clinically meaningful it must be shown that those patients with minimal aspirin inhibition of platelets have an increased risk for a future vascular event that is independent from known major cardiovascular risk factors.

Aspirin resistance

Treatment failures occur with any drug and aspirin is no exception. Evidence is growing to indicate that there are subpopulations that do not respond to antithrombotic action of aspirin. The term 'aspirin resistance' has been used to describe a number of different phenomena, including inability of aspirin to: (i) protect against cardiovascular events despite its regular intake; (ii) to affect various laboratory tests, reflecting platelet activity. Research on aspirin resistance yielded interesting results in clinical pharmacology and pharmacogenetics. Future studies will show whether genotyping for polymorphisms might be of value in everyday clinical use of aspirin. Present data indicate that in survivors of recent myocardial infarction or unstable angina, patients receiving coronary artery bypass grafts, as well as in subjects with hypercholesterolemia, aspirin resistance has to be considered when implementing antithrombotic therapy. However, in individual patients the available laboratory tests are of no particular use to predict reliably the clinical outcome or to guide in making therapeutic decision. Prospective clinical trials seem necessary to reach such conclusions.

Aspirin Resistance: An Evaluation of Current Evidence and Measurement Methods

Aspirin resistance is a poorly characterized phenomenon, whereby certain patients do not benefit from the antithrombotic effect of aspirin. The frequency of aspirin resistance is unknown, but estimates range from 5-60%. The mechanism of aspirin resistance also is unknown; proposed mechanisms are poor patient compliance, poor aspirin absorption, increased isoprostane activity, platelet hypersensitivity to agonists, increased cyclooxygenase-2 activity, a cyclooxygenase-1 polymorphism, and the platelet alloantigen 2 polymorphism of platelet glycoprotein IIIa. Aspirin resistance appears to be dose related in some patients and therefore may be overcome with high doses. Evidence indicates that aspirin resistance is a dynamic state, with significant intrapatient variability in aspirin sensitivity with time. To date, a sensitive and specific assay of aspirin effect that reliably predicts treatment failure has not emerged. However, several commercially available products are being marketed for this purpose without convincing clinical data. Despite a wealth of literature on the topic, aspirin resistance remains an enigma. Further investigation is needed regarding strategies to identify and treat patients resistant to aspirin.

Low-dose aspirin increases aspirin resistance in patients with coronary artery disease

PURPOSE

We sought to investigate the association of aspirin dose and aspirin resistance in stable coronary artery disease patients measured by a point-of-care assay.

METHODS

We studied 468 consecutive stable coronary artery disease patients in a referral cardiac center who were taking aspirin 80 to 325 mg daily for > or =4 weeks. The VerifyNow Aspirin (Ultegra RPFA-ASA, Accumetrics Inc, San Diego, Calif) was used to determine aspirin responsiveness. An aspirin reaction unit (ARU) > or =550 indicates the absence of aspirin-induced platelet dysfunction, based on correlation with epinephrine-induced light transmission aggregometry. Demographic and clinical data were collected to analyze the predictors of aspirin resistance.

RESULTS

Aspirin resistance was noted in 128 (27.4%) patients. Univariate predictors of aspirin resistance include elderly (P = 0.002), women (P <0.001), anemia (P <0.001), renal insufficiency (P = 0.009) and aspirin dose < or =100 mg (P = 0.004). Multivariate analysis revealed hemoglobin (odds ratio [OR] 0.6; 95% confidence interval [CI] 0.51 to 0.69; P <0.001) and aspirin dose < or =100 mg (OR 2.23; 95% CI 1.12 to 4.44; P = 0.022) to be independent predictors of aspirin resistance. Daily aspirin dose < or = 100 mg was associated with increased prevalence of aspirin resistance compared with 150 mg and 300 mg daily (30.2% vs 16.7% vs 0%, P = 0.0062).

CONCLUSION

A 100 mg or less daily dose of aspirin, which may have lower side effects, is associated with a higher incidence of aspirin resistance in patients with coronary artery disease. Prospective randomized studies are warranted to elucidate the optimal aspirin dosage for preventing ischemic complications of atherothrombotic disease.