Aspirin resistance: is it real? Is it clinically significant?

Serum thromboxane B2 but not soluble P-selectin levels identify ischemic stroke patients with persistent platelet reactivity while on aspirin therapy

BACKGROUND

Aspirin non-response due to persistent platelet reactivity has been associated with adverse vascular events. Light transmission aggregometry (LTA), the 'gold standard' for measuring the platelet response to aspirin therapy, is a cumbersome procedure and a simple and reliable alternative is required. Our aim was to explore whether serum thromboxane B2 (sTXB2) and soluble P-selectin can be used to identify patients who are at risk of increased platelet reactivity while on aspirin.

METHODS AND RESULTS

We recruited 293 ischemic stroke patients, taking aspirin for more than seven days, and performed LTA to classify them. Based on therapeutic serum salicylate levels, 63 patients were excluded due to suspected non-compliance, followed by ELISA measurement of TXB2 and P-selectin in serum. Accordingly, patients were classified into 'Responders' (n = 122, 53%), 'Semi-responders' (n = 76, 33%) and 'Non-responders' (n=32, 14%) by LTA. Patients who had platelet aggregation of ≥70% with 10μM ADP and ≥20% with 0.5mM AA were defined as 'Non-responders'. In comparison with 'Responders', 'Non-responders' had 8.63-fold increased risk of secondary vascular events (p = 0.008). ROC curve analysis revealed that sTXB2, at a cut-off level of >4.15 ng/mL, could distinguish the patient group with elevated platelet reactivity with a sensitivity of 84.3% (AUC = 0.84), and was in fair agreement with the LTA-based classification of patients. Soluble P-selectin levels, on the other hand, had no discriminatory ability.

CONCLUSION

We suggest sTXB2 measurement as an alternative to the LTA approach for identifying aspirin-treated ischemic stroke patients who are at risk of enhanced platelet reactivity and subsequent vascular events.

Low Dose Aspirin in high-risk pregnancies: The volatile effect of acetylsalicylic acid on the inhibition of platelets uncovered by G. Born's light transmission aggregometry

Preeclampsia still represents a life-threatening pregnancy complication, associated with severe maternal and neonatal morbidity and mortality. Low-dose Aspirin is advised to avoid preeclampsia in high-risk pregnancies worldwide. As Aspirin does not cover all women at risk, the prescription raises questions concerning optimal target population, dosage, and onset of therapy. The aim of this study was to test platelet responsiveness on Aspirin by optical aggegrometry, to gain robust biochemically assessment data of Aspirin in an obstetric cohort. 248 women at high risk for development of preeclampsia were included in the study. Aspirin-prophylaxis was administered either in 100 mg (n = 229) or 150 mg (n = 90) daily. Dosing of 100 mg Aspirin was maintained if testing revealed a sufficient platelet inhibition. If platelet inhibition was insufficient, dosage was increased to 150 mg Aspirin and re-testing was advised. 91 patients (91/229 = 39.7%) presented a sufficient inhibitory Aspirin effect at a dosage of 100 mg, but in 138 patients LTA showed an inadequate Aspirin response (138/229 = 60.3%). In 19 women 150 mg Aspirin was administered as starting dose due to new recommendations. Of all women at 150 mg Aspirin 64 did not properly respond (35.4%). The overall rate of sufficient responding women regardless the Aspirin dose was 64.6%. This study demonstrates still an insufficient inhibition of platelet aggregation in about 1/3 of women even with a dosage of 150 mg Aspirin daily, who might potentially benefit from further increase. These data show, that there is a need for further research to allow a personalized approach for individualized Aspirin therapy, maximizing the preventive benefit for mother and child.

Low miR-19b-1-5p Expression Is Related to Aspirin Resistance and Major Adverse Cardio- Cerebrovascular Events in Patients With Acute Coronary Syndrome

Background Because of a nonresponse to aspirin (aspirin resistance), patients with acute coronary syndrome (ACS) are at increased risk of developing recurrent event. The in vitro platelet function tests have potential limitations, making them unsuitable for the detection of aspirin resistance. We investigated whether miR-19b-1-5p could be utilized as a biomarker for aspirin resistance and future major adverse cardio-cerebrovascular (MACCE) events in patients with ACS. Methods and Results In this cohort study, patients with ACS were enrolled from multiple tertiary hospitals in Christchurch, Hong Kong, Sarawak, and Singapore between 2011 and 2015. MiR-19b-1-5p expression was measured from buffy coat of patients with ACS (n=945) by reverse transcription quantitative polymerase chain reaction. Platelet function was determined by Multiplate aggregometry testing. MACCE was collected over a mean follow-up time of 1.01±0.43 years. Low miR-19b-1-5p expression was found to be related to aspirin resistance as could be observed from sustained platelet aggregation in the presence of aspirin (-Log-miR-19b-1-5p, [unstandardized beta, 44.50; 95% CI, 2.20-86.80; P<0.05]), even after adjusting for age, sex, ethnicity, and prior history of stroke. Lower miR-19b-1-5p expression was independently associated with a higher risk of MACCE (-Log-miR-19b-1-5p, [hazard ratio, 1.85; 95% CI, 1.23-2.80; P<0.05]). Furthermore, a significant interaction was noted between the inverse miR-19b-1-5p expression and family history of premature coronary artery disease (P=0.01) on the risk of MACCE. Conclusions Lower miR-19b-1-5p expression was found to be associated with sustained platelet aggregation on aspirin, and a higher risk of MACCE in patients with ACS. Therefore, miR-19b-1-5p could be a suitable marker for aspirin resistance and might predict recurrence of MACCE in patients with ACS.

Comparison of acetylsalicylic acid and clopidogrel non-responsiveness assessed by light transmittance aggregometry and PFA-100® in patients undergoing neuroendovascular procedures

Objectives

Dual platelet inhibition is commonly used for prevention of cardiovascular events in patients undergoing neuroendovascular procedures. Non-responsiveness to platelet inhibitors may be associated with adverse outcomes. The aim of this study was to evaluate the reliability of the platelet function analyzer PFA-100® in comparison to light transmittance aggregometry (LTA) for monitoring clopidogrel and acetylsalicylic acid (ASA) non-responsiveness in a cohort of patients treated for intracranial aneurysm or cranial artery stenosis.

Methods

Non-responsiveness to clopidogrel and ASA was assessed by LTA using adenosine diphosphate (ADP) and arachidonic acid and by PFA-100® with the ADP/prostaglandin E1 (PGE1) and collagen/epinephrine cartridges, respectively.

Results

A total of 203 patients (145 females; median age, 57 years) were analyzed. Agreement between the two tests was poor for clopidogrel non-responsiveness (ƙ=0.19) and not better than chance for ASA non-responsiveness (ƙ=0.01). Clopidogrel non-responsiveness by LTA and PFA-100® was associated with higher von Willebrand factor antigen and activity levels. ADP-induced platelet disaggregation was lower in patients with clopidogrel non-responsiveness as assessed by PFA-100®. Clopidogrel non-responsiveness by LTA was associated with a higher prevalence of diabetes and a higher body mass index (BMI). Adverse outcomes (death, thromboembolism, or in-stent thrombosis) occurred in 13% (n=26) of all patients independently of ASA and clopidogrel non-responsiveness as assessed by both devices.

Conclusions

Our results show that LTA and PFA-100® are not interchangeable in the assessment of ASA and clopidogrel non-responsiveness in patients undergoing neuroendovascular interventions.

Platelet function in stroke/transient ischemic attack patients treated with tocotrienol

The purpose of this study was to characterize the effects of tocotrienol form of vitamin E (TCT) on platelet function in patients with stroke or transient ischemic attack (TIA). A double blind, randomized, single center phase II clinical trial was conducted comparing placebo (PBO) and 400 and 800 mg TCT daily for a year in 150 patients with a sentinel ischemic stroke or TIA event in the prior 6 months. Platelet function was measured at baseline and then, at 3 month intervals for a year, using light transmission aggregometry. The incidence of aspirin resistance in aspirin-treated patients or platelet inhibition in patients on clopidogrel alone was compared between the three treatment groups. Results showed that in patients taking aspirin and clopidogrel, the incidence of aspirin resistance was significantly decreased from 40% in PBO-treated patients to 9% in the 400 mg TCT group and 25% in the TCT 800 mg group (P = .03). In conclusion, patients on aspirin and clopidogrel had a higher incidence of aspirin resistance than all patients treated with aspirin alone and TCT decreased the frequency of aspirin resistance in this group.

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.

Platelet function testing: state of the art

Platelet function testing has evolved from crude tests, such as the bleeding time, to tests that permit a relatively sophisticated evaluation of platelet activity. Nonetheless, these tests are hampered by lack of specificity and sensitivity, and poor standardization of methods and techniques. The bleeding time, which has long been a staple of hemostasis testing, has been dropped from the test menu at many laboratories. In its place, tests such as the Platelet Function Analyzer-100 are increasingly used to screen patients with possible bleeding disorders. Older tests, such as platelet aggregometry and lumiaggregometry, are still used frequently because they provide insight into receptor, signaling pathway and granule release mechanisms. Flow cytometry is available in some specialized laboratories and allows for quantitative and qualitative assessment of some platelet functions, although the expense of testing is often prohibitive. Finally, the wider availability of platelet function testing has stimulated interest and demand for monitoring the effect of platelet inhibitory drugs, such as aspirin and clopidogrel. As platelet function pathways become better understood, the demand for these type of monitoring tests is likely to increase.

Primary and secondary prevention of cardiovascular disease in diabetes with aspirin

Diabetes is associated with an increased cardiovascular risk. The role for aspirin in diabetes is of high clinical interest. Guidelines recommend that primary prevention of cardiovascular disease (CVD) in diabetes with aspirin should be based on the individual risk for CVD. New mechanistic studies suggest that enhanced platelet turnover may partly contribute to the fact the primary prevention studies found unequivocal results in diabetes. There is initial evidence that a potential future modification of dosages in diabetes may counteract the enhancement in platelet turnover in diabetes. The use of aspirin in diabetic patients for secondary prevention of CVD is supported by key evidence. The aim of the review is to present recent studies on aspirin for prevention of CVD in diabetes and to highlight its role also in view of new mechanistic and clinical studies with aspirin. Novel aspects of aspirin, e.g. its potential role for the prevention of cancer, are also presented.

Point of controversy: perioperative care of patients undergoing pheochromocytoma removal-time for a reappraisal?

Adrenalectomy for pheochromocytoma is reported with a mortality close to zero in recent studies. The dogma of preoperative fluid and hypotensive drug administrations is widely applied in patients scheduled for pheochromocytoma removal and is assumed to have a beneficial effect on operative outcomes. This paradigm is only based on historical studies of non-standardized practices and criteria for efficacy, with no control group. Pre- and intraoperative hypovolemia have never been demonstrated in patients scheduled for pheochromocytoma removal. Recent improvements in outcome of patients undergoing adrenalectomy for pheochromocytoma could also be the result of improvement in surgical techniques and refinement in anesthetic practices. Whether better knowledge of the disease, efficiency of available intravenous short-acting vasoactive drugs, and careful intraoperative handling of the tumor make it possible to omit preoperative preparation in most patients scheduled for pheochromocytoma removal is presently questionable. We reviewed available literature in this respect.

Anti-platelet drug resistance in the prediction of thromboembolic complications after neurointervention

OBJECTIVE

The aim of this study was to analyze the correlation between thromboembolic complications and antiplatelet drugs before and after neurointervention.

METHODS

Blood samples and radiographic data of patients who received a neurointervention (coil embolization, stent placement or both) were collected prospectively. Rapid platelet function assay-aspirin (RPFA-ASA) was used to calculate aspirin resistance in aspirin reaction units (ARU). For clopidogrel resistance, a P2Y12 assay was used to analyze the percentage of platelet inhibition. ARU > 550 and platelet inhibition < 40% were defined as aspirin and clopidogrel resistance, respectively.

RESULTS

Both aspirin and clopidogrel oral pills were administered in fifty-three patients before and after neurointerventional procedures. The mean resistance values of all patients were 484 ARU and < 39%. Ten (17.0%) of 53 patients showed resistance to aspirin with an average of 597 ARU, and 33 (62.3%) of 53 patients showed resistance to clopidogrel with an average of < 26%. Ten patients demonstrated resistance to both drugs, 5 of which suffered a thromboembolic complication after neurointervention (mean values : 640 ARU and platelet inhibition < 23%). Diabetic patients and patients with hypercholesterolemia displayed mean aspirin resistances of 513.7 and 501.8 ARU, and mean clopidogrel resistances of < 33.8% and < 40.7%, respectively.

CONCLUSION

Identifying individuals with poor platelet inhibition using standard regimens is of great clinical importance and may help prevent cerebral ischemic events in the future. Neurointerventional research should focus on ideal doses, timing, choices, safety, and reliable measurements of antiplatelet drug therapy, as well as confirming the clinical relevance of aggregometry in cerebrovascular 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.

Platelet aspirin resistance in ED patients with suspected acute coronary syndrome

BACKGROUND

Platelet aspirin resistance is reported to be as high as 45%. The prevalence of emergency department (ED) platelet aspirin resistance in suspected acute coronary syndrome (ACS) is not described. Our purpose was to determine the prevalence of platelet aspirin resistance.

METHODS

We determined platelet aspirin resistance in a convenience sample of ED suspected ACS patients. Eligible patients had longer than 10 minutes of chest pain or an ischemic equivalent. Two hours after receiving 325 mg of aspirin, blood was assessed for platelet function (Accumetrics, San Diego, CA). Definitions are as follows: aspirin resistance, at least 550 aspirin reaction units; positive troponin T, greater than 0.1 ng/mL; significant coronary lesion, at least 70% stenosis. The composite end point was prospectively defined as a 30-day revisit, positive cardiac catheterization, or hospital length of stay (LOS) longer than 3 days.

RESULTS

Of 200 patients, 50.5% were male, 50.0% were black, troponin T was positive in 7.5%, cardiac catheterization was done in 10.5%, and 33.3% had a significant stenosis. Final diagnoses were noncardiac in 83.4%, stable angina in 8.0%, and unstable angina in 8.5%. Overall, 6.5% were resistant to aspirin; and high-risk patients trended to more aspirin resistance than non-high-risk patients (23.1% [3] vs 9.1% [17]; P value 95% confidence interval [CI], -0.0929 to 0.373). One-month follow-up found ED revisits in 12.5% of aspirin-resistant vs 4.9% of non-aspirin-resistant patients (95% CI, -0.114 to 0.182) and rehospitalization in 12.5% of resistant patients vs 4.3% of nonresistant patients (P value 95% CI, -0.108 to 0.187). Although LOS was similar at index admission, if rehospitalized, LOS was 6.5 for aspirin-resistant patients vs 3.2 days in nonresistant patients (P < .0001).

CONCLUSION

This first report of platelet aspirin resistance in patients presenting to the ED with suggested ACS finds that it is present in 6.5% of patients.

Resistance to low-dose aspirin therapy among patients with acute coronary syndrome in relation to associated risk factors

BACKGROUND

A substantial proportion of patients have recurrence of vascular events despite daily intake of low-dose aspirin therapy. Therefore, different patients may require different aspirin dosages to achieve complete inhibition of platelet function.

OBJECTIVE

The aim of this work was to measure the response to low-dose aspirin therapy (150 mg/day) among patients with unstable angina or non-ST-segment elevation myocardial infarction and to find out whether titrating aspirin dosage to 300 mg/day, would provide a better therapeutic response in the resistant cases. Moreover, we also aimed to study any association between aspirin non-responsiveness and atherothrombotic risk factors.

METHODS

The antiplatelet effect of 150 mg/day aspirin was studied prospectively in 50 consecutive patients with unstable angina or non-ST-segment elevation myocardial infarction. Platelet aggregation was measured using optical platelet aggregometry and serum thromboxane B(2) level. Aspirin resistance was defined as collagen (1 μg/mL) and adenosine diphosphate (ADP) (5 μmol/L)-induced platelet aggregation of ≥ 40% when compared with control values. Twenty healthy age- and sex-matched individuals were taken as a control group. All patients were subjected to complete medical history (risk factors, medications), thorough clinical examination, ECG, coronary angiography and laboratory investigations including: complete haemogram, coagulation, kidney, liver and lipid profiles, fasting blood glucose and glycated haemoglobin (HbA(1C) ).

RESULTS

Eleven of 50 patients (22%) were found to be aspirin resistant. A highly significant difference was found between the mean values of ADP, collagen-induced platelet aggregation percentage and thromboxane B(2) level after aspirin 150 mg/day when compared with the corresponding mean values after aspirin 300 mg/day among the resistant patients (66 ± 7.01%, 62 ± 4.34% and 620 ± 64.58 pg/mL, respectively, vs. 26.87 ± 2.85%, 16.5 ± 3.8% and 77 ± 11.3 pg/mL) indicating enhanced response to aspirin after escalating the dose. The presence of atherothrombotic risk factors (hypertension, smoking, family history of ischaemic heart disease and previous MI) were not statistically different between aspirin-resistant and aspirin-sensitive patients. However, there was a highly significant difference between the aspirin sensitive and the resistant patients regarding the other risk factors (diabetes mellitus and dyslipidaemia) (P < 0.01).

CONCLUSION

There is inter-individual variability in response to the antiplatelet effect of standard doses of aspirin (150, 300 mg/day). The response to aspirin 300 mg/day is enhanced in resistant patients when compared to 150 mg/day. There was a significant association between aspirin resistance and atherothrombotic risk factors (diabetes, hyperlipidaemia and obesity).

Aspirin resistance: an update

Aspirin resistance (AR) still lacks a universally accepted definition, but it may be discussed as either a laboratory phenomenon or a clinical presentation. Laboratory resistance is mainly defined as abnormal platelet response to aspirin, whereas the clinical manifestation is the failure of aspirin to prevent cardiovascular events. Although there is evidence of an association, it appears that a laboratory abnormality in platelet function is not the only risk factor for the clinical manifestation of AR. Therapies for primary and secondary prevention of AR still need to be elucidated, but there are some data to suggest that in an acute episode of aspirin failure because of AR, different therapeutic interventions need to be considered.

Monitoring oral antiplatelet therapy: is it justified?

Platelets play a key role in the initial formation and progression of intravascular thrombus. During coronary and peripheral vascular interventions, antiplatelet therapy is used to impair platelet reactivity in order to minimize adverse ischemic events. Chronic antiplatelet therapy is also administered to decrease the long term risk of major adverse cardiovascular events in patients with high atherothrombotic burden. However, due to a heterogeneous response to antiplatelet agents, a substantial number of patients with cardiovascular disease remain at risk despite adherence to therapy. Since the availability of point-of-care rapid platelet function assays, quick and reproducible evaluation of platelet function after the administration of aspirin, thienopyridines, and glycoprotein IIb/IIIa inhibitors is possible. Various definitions of suboptimal platelet inhibition with oral antiplatelet therapy and associated risk factors are presented here. An algorithm to guide optimal antiplatelet therapy based on rapid platelet function testing during cardiovascular interventions is also presented.

Non-compliance is the predominant cause of aspirin resistance in chronic coronary arterial disease patients

Background

Our previous publication showed that 9% of patients with a history of myocardial infarction MI. could be labeled as aspirin resistant; all of these patients were aspirin resistant because of non-compliance. This report compares the relative frequency of aspirin resistance between known compliant and non-compliance subjects to demonstrate that non-compliance is the predominant cause of aspirin resistance.

Methods

The difference in the slopes of the platelet prostaglandin agonist (PPA) light aggregation curves off aspirin and 2 hours after observed aspirin ingestion was defined as net aspirin inhibition.

Results

After supposedly refraining from aspirin for 7 days, 46 subjects were judged non-compliant with the protocol. Of the remaining 184 compliant subjects 39 were normals and 145 had a past history of MI. In known compliant subjects there was no difference in net aspirin inhibition between normal and MI subjects. Net aspirin inhibition in known compliant patients was statistically normally distributed. Only 3% of compliant subjects (2 normals and 5 MI) had a net aspirin inhibitory response of less than one standard deviation which could qualify as a conservative designation of aspirin resistance. A maximum of 35% of the 191 post MI subjects could be classified as aspirin resistant and/or non-compliant: 9% aspirin resistant because of non-compliance, 23% non-compliant with the protocol and possibly 3% because of a decreased net aspirin inhibitory response in known compliant patients.

Conclusion

Our data supports the thesis that the predominant cause of aspirin resistance is noncompliance.

Biomarkers and Bioassays for Cardiovascular Diseases: Present and Future

Stratification of cardiac patients arriving at the emergency department is now being made according to the levels of acute cardiac biomarkers (i.e. cardiac troponin (cTn) or creatine kinase myocardial band (CK-MB)). Ongoing efforts are undertaken in an attempt to identify and validate additional cardiac biomarkers, for example, interleukin-6, soluble CD40L, and C-reactive protein, in order to further risk stratify patients with acute coronary syndrome. Several studies have also now shown an association of platelet transcriptome and genomic single nucleotide polymorphisms with myocardial infarction by using advanced genomic tools. A number of markers, such as myeloid-related protein 14 (MRP-14), cyclooxygenase-1 (COX-1), 5-lipoxygenase activating protein (FLAP), leukotriene A4 hydrolase (LTA4H) and myocyte enhancing factor 2A (MEF2A), have been linked to acute coronary syndromes, including myocardial infarction. In the future, these novel markers may pave the way toward personalized disease-prevention programs based on a person's genomic, thrombotic and cardiovascular profiles. Current and future biomarkers and bioassays for identifying at-risk patients will be discussed in this review.

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.

Acquired platelet dysfunction

Acquired platelet dysfunction is encountered frequently in clinical practice. The usual clinical presentation is that of mucosal bleeding, epistaxis, or superficial epidermal bleeds. Often, the dysfunctional platelets are related to a medication or a systemic disorder. Normally, when platelets are exposed to damaged endothelium, they adhere to the exposed basement membrane collagen and change their shape from smooth disks to spheres with pseudopodia. Then, they secrete the contents of their granules, a process referred to as the release reaction. Additional platelets form aggregates on those platelets that have adhered to the vessel wall. As a result, the primary hemostatic plug is formed, and bleeding is arrested. This article reviews the various forms of acquired platelet dysfunction that result in decreased platelet aggregation, adhesion, or secretion.