Assessment of platelet response to clopidogrel with multiple electrode aggregometry, the VerifyNow P2Y12 analyzer and platelet Vasodilator-Stimulated Phosphoprotein flow cytometry

High plateletcrit is associated with early loss of patency after open and endovascular interventions for chronic limb ischemia

OBJECTIVE

Plateletcrit (PCT) reflects the total platelet mass in blood and can be calculated from a complete blood count. We examined the effect of PCT on outcomes of endovascular and open interventions for chronic limb ischemia.

METHODS

Patients who underwent revascularization for chronic limb ischemia (Rutherford categories 3-6) between June 2001 and December 2014 were retrospectively identified. PCT on admission was recorded. Patients and limbs were divided into tertiles of low (0.046-0.211), medium (0.212-0.271), and high (0.272-0.842) PCT. Patency, limb salvage, major adverse limb events, major adverse cardiac events, and survival rates were calculated using Kaplan-Meier analysis and compared with log-rank test. Cox regression analysis was used for multivariate analysis.

RESULTS

A total of 1431 limbs (1210 patients) were identified and divided into low PCT (477 limbs in 407 patients), medium PCT (477 limbs in 407 patients), and high PCT (477 limbs in 396 patients) groups. The patients in the high tertile were 2 years older that the patients in the other two tertiles (P = .009). Five-year primary patency was 65% ± 3% in the low-PCT group compared with 55% ± 3% and 51% ± 3% in the medium and high PCT groups, respectively (P = .004). Five-year secondary patency was 81% ± 2% in the low PCT group compared with 82% ± 2% and 72% ± 3% in the medium and high PCT groups, respectively (P = .02). Five-year limb salvage rate was 86% ± 2% in the low PCT group compared with 79% ± 3% and 74% ± 3% in the medium PCT and high PCT groups, respectively (P = .004). Multivariate regression analysis showed that low PCT was independently associated with primary patency after endovascular interventions (hazard ratio, 0.67 [0.47-0.95]; P = .02) but not after open interventions (hazard ratio, 0.72 [0.43-1.21]; P = .21).

CONCLUSIONS

High PCT is associated with poor patency and limb salvage rates after interventions for lower extremity chronic limb ischemia. Multivariate regression analysis confirmed association of low PCT with improved primary patency after endovascular interventions but not after open interventions. High PCT may be a marker of increased platelet reactivity and could be used to identify patients at high risk for early thrombosis and failure after interventions.

High on-treatment platelet reactivity – definition and measurement

In the last decade, several studies revealed inter-patient response variability to antiplatelet agents: patients who display negligible or no responses to these drugs are considered poor responders, or “resistant” to treatment. In order to identify poor responders to an antiplatelet drug, laboratory tests of platelet function that specifically explore the platelet activation pathway that is targeted by the drug should be utilised. In addition, they should be performed both at baseline and during treatment: however, most studies explored platelet function during antiplatelet treatment, in order to identify those patients with “high on-treatment platelet reactivity” (HPR), which exposes them to increased risk of major adverse cardiovascular events (MACE). Many tests of platelet function have been used, most of which are able to identify patients at risk of MACE. Unfortunately, universal cut-off values for HPR have not been clearly established yet. In addition, the concordance among different tests in the identification of patients at risk is very poor and the most effective and safe treatment for patients at risk is still unknown.

Malignant stroke in a ticagrelor non-responder as a complication following aneurysm treatment with the Pipeline Embolization Device™

The Pipeline Embolization Device™ (PED; Covidien Neurovascular Inc, Irvine, CA, USA) is a flow-diverting stent often used for the endovascular treatment of large or giant, wide-necked intracranial aneurysms of the internal carotid artery. Because of the inherent thrombogenicity of intracranial stents, dual-antiplatelet therapy is initiated after placement, which has been shown to decrease morbidity and mortality related to perioperative ischemic events in neurointerventional procedures. However, in some series, as much as 50% of patients demonstrate clopidogrel non-responsiveness. In these non-responders, alternate agents such as ticagrelor can be used to achieve adequate anticoagulation. Compared with clopidogrel, a prodrug requiring Cytochrome P450 enzymolysis for activation, ticagrelor directly and reversibly inhibits the P2Y12 ADP receptor. The absorption of the prodrug and the formation of its active metabolite is comparatively quicker ( t_max 1.3-2 hours; 1.5-3 hours, respectively). To date, there have been no documented cases of ticagrelor non-responsiveness involving patients undergoing placement of flow-diverting stents or other endovascular neuro-interventional procedures.

Monitoring ASA and P2Y12-specific platelet inhibition--comparison of conventional (single) and multiple electrode aggregometry

OBJECTIVE

Several platelet function test systems exist for the evaluation of the platelet inhibitory effect in patients on P2Y12 inhibitors and/or acetylsalicylic acid (ASA, aspirin) therapy. Studies comparing different available assays found only a poor correlation. The objective of the present study was to evaluate the correlation and agreement between single electrode (SEA) and multiple electrode (MEA) aggregometry.

METHODS AND RESULTS

In whole blood arachidonic acid (AA) and adenosine diphosphate (ADP)-induced platelet aggregation was measured simultaneously using SEA (Chrono-Log) and MEA (Multiplate). We analyzed a total of 226 measurements taken from 58 patients on single ASA therapy or dual antiplatelet therapy with ASA and a thienopyridine. A cut-off value for clopidogrel/prasugrel high on-treatment platelet reactivity (HPR) of > 47 units (U) was chosen for MEA testing using hirudin and > 5 Ohm for SEA with citrate anticoagulated blood samples. The respective cut-off values for ASA HPR were > 30 U for the MEA assay and > 1 Ohm for SEA testing. There was a good correlation of the prevalence of thienopyridine-HPR in both whole blood assays (Spearman rank correlation coefficient r = 0.698) and a good inter-rate accordance (Cohen's Kappa statistic κ = 0.648). For AA-induced aggregation, the correlation of the results obtained was significant (r = 0.536; p < 0.001) and detecting ASA-HPR revealed a moderate (κ = 0.482) correlation between both impedance aggregometry assays.

CONCLUSION

Platelet function testing using SEA and MEA provided both good accordance and correlation and therefore study results obtained by these two assays similarly enabled the detection of HPR of thienopyridine (and ASA) therapy.

Influence of Different β-Blockers on Platelet Aggregation in Patients With Coronary Artery Disease on Dual Antiplatelet Therapy

INTRODUCTION

The use of β-blockers in the treatment of patients with coronary heart disease is associated with a decrease in the frequency of angina pectoris and mortality of patients. Due to the severity of the disease and previous cardiovascular interventions, many patients with coronary artery disease (CAD) use dual antiplatelet therapy to achieve greater inhibition of platelet aggregation. The influence of β-blockers on platelet aggregation in patients using antiplatelet therapy is not well understood.

OBJECTIVE

To examine the effect of different β-blockers on platelet aggregation in patients on dual antiplatelet therapy.

METHODOLOGY

The study included 331 patients who were treated at the Department of Cardiology, Clinical Center Kragujevac during 2011. Patients were divided into 4 groups depending on the type of β-blockers that were used (bisoprolol, nebivolol, metoprolol, and carvedilol). Platelet aggregation was measured using the multiplate analyzer and expressed through the value of adenosine diphosphate (ADP) test (to assess the effect of clopidogrel), ASPI test (to assess the effect of acetyl salicylic acid), TRAP test (to assess baseline platelet aggregation), and the ratio of ADP/TRAP and ASPI/TRAP ASPI/TRAP (ASPI - aranchidonic acid induced aggregation, TRAP - thrombin receptor activating peptide) representing the degree of inhibition of platelet aggregation compared to the basal value. In consideration were taken the representation of demographic, clinical characteristics, laboratory parameters, and cardiovascular medications between the groups.

RESULTS

Patients who used nebivolol had a significantly lower value of the ratio of ADP/TRAP (0.39 ± 0.30) compared to patients who used bisoprolol (0.48 ± 0.26; P = .038), and trend toward the lower values of ADP test (328.0 ± 197.3 vs 403.7 ± 213.2; P = .059), while there was no statistically significant difference in values of other laboratory parameters of platelet function between other groups.

CONCLUSION

Patients with CAD on dual antiplatelet therapy who used nebivolol had significantly lower levels of residual ADP-induced platelet aggregation compared to baseline than patients who used bisoprolol.

The clinical utility of Multiplate analyser measurement in platelet function testing following stroke and transient ischaemic attack

BACKGROUND

Platelet responsiveness to aspirin in people with cerebrovascular disease is poorly understood.

OBJECTIVES

To determine: (i) normal reference range, imprecision and reproducibility of the Multiplate instrument in healthy volunteers naive to aspirin; (ii) imprecision and reproducibility of the Multiplate instrument in acute stroke and transient ischaemic attack (TIA); (iii) the relationship between aspirin responsiveness and clinical outcome.

MATERIALS AND METHODS

We evaluated platelet function response to three agonists [Adenosine Diphosphate (ADP), Arachidonic Acid (AA), Collagen (Col)] using the Mulitplate platelet function analyser in a two-phase pilot study. In phase 1, we recruited healthy volunteers to determine the normal reference range and imprecision of the Multiplate instrument. In phase 2, we assessed platelet function in acute stroke or TIA patients presenting to hospital. These patients were bled within 24 h of presentation and between 12 and 24 h after ≥75 mg dose of Aspirin. Patients were followed up to 1 yr to assess mortality and recurrent cardiovascular event.

RESULTS

Overall, 29 healthy volunteers and 81 stroke/TIA patients were recruited. On assessing components of variance, Multiplate testing is reproducible and precise in volunteers and stroke/TIA patients. In stroke patients receiving aspirin, Bland-Altman plots show initial day 1 measurement provided a reliable measure of continuing response to aspirin at day 3. We defined one-third of patients as aspirin resistant [31.8% (95% CI: 22.1%-42.8%)] using cut off mean aggregation of ≥23.08% for AA and mean aggregation of ≥80.76% for ADP.

CONCLUSION

The Multiplate device gives reproducible, precise results in volunteers and stroke/TIA patients.

Poor agreement between light transmission aggregometry, Verify Now P2Y₁₂ and vasodilatator-stimulated phosphoprotein for clopidogrel low-response assessment: a potential explanation of negative results of recent randomized trials

Clopidogrel low response as assessed by several different biological tests correlates with poor prognosis after percutaneous coronary intervention (PCI). However, recent randomized clinical trials (RCT) testing the strategy of individual antiplatelet therapy tailoring based on one sole test have all shown negative results. Poor correlation between the different tests may explain the difficulties of patient selection and identification of "true poor responders" to clopidogrel. In this prospective study, clopidogrel response was assessed in 100 consecutive patients between 18 and 24 hours after a 600 mg clopidogrel loading dose using three different tests: light transmission aggregometry with 10 μmol ADP (LTA, results expressed as platelet aggregation percentage: PAP), Verify Now P2Y12 (VN, results expressed as P2Y12 reaction unit: PRU) and vasodilatator-stimulated phosphoprotein (VASP, results expressed as platelet reactivity index: PRI). Patients under chronic clopidogrel therapy were excluded. The mean PAP, PRU and PRI values were 38.6%, 176.1 PRU and 52.4%, respectively. When clopidogrel response was analyzed as continuous variable, there was a good correlation between the different tests: LTA/VN (R(2 )= 0.642, p < 0.001), LTA/VASP (R(2 )= 0.409, p < 0.001) and VN/VASP (R(2 )= 0.616, p < 0.001). However, when clopidogrel response was analyzed as pre-specified cut-off points to define patients as "poor or good responders" (according to the literature: 50% PAP for LTA, 235 PRU for VN and 50% PRI for VASP), only 47% of the patients were defined as "good" or "poor responders" by the three tests. Altogether, 33% of the patients were defined as "poor responders" by only one test, 20% by two tests and only 16% by the three tests. The correlation between the different tests is good when clopidogrel response is analyzed as continuous variable. Each individual is however rarely (less than 50%) defined as "poor or good responder" by all the three tests when pre-specified cut-off values are used. A sole test might not be sufficient to manage antiplatelet therapy in an individual patient and these results may explain the results of recent RCT showing the lack of benefit of systematic antiplatelet therapy monitoring strategy.

Platelet reactivity tests for assessing antiplatelet drug response: what the clinician needs to know

Antiplatelet therapy is a cornerstone in the treatment of cardiovascular disease to prevent ischemic events. Various tests have become clinically available to measure platelet function after antiplatelet treatment. A wide interpatient variability in the magnitude of platelet inhibition has been demonstrated in numerous studies, especially in response to clopidogrel. Several reasons including clinical, pharmacological and genetic factors have been identified. High on-clopidogrel platelet reactivity has been linked to adverse clinical outcome, in particular to stent thrombosis after percutaneous coronary interventions. New antiplatelet drugs including prasugrel and ticagrelor have been advocated to overcome the limitations of clopidogrel. Several studies addressed the concept of tailored antiplatelet treatment according to the results of platelet function testing. Within this review, we summarize the current status of personalized antiplatelet therapy for cardiovascular disease.

Predictive values of post-clopidogrel platelet reactivity assessed by different platelet function tests on ischemic events in East Asian patients treated with PCI

Abstract An accumulating number of studies are revealing that platelet reactivity above specific cut-off scores leads to exponentially increased rates of post-percutaneous coronary intervention (PCI) ischemic events. To evaluate the optimal predictive values for three different platelet function measurement assays of platelet reactivity on early clinical outcomes in Korean patients undergoing PCI, we enrolled 228 patients receiving clopidogrel prior to PCI. Platelet reactivity was measured by light transmittance aggregometry (LTA), VerifyNow P2Y12 assay, and multiple electrode platelet aggregometry (MEA). The primary endpoint was the 30-day occurrence of ischemic events after PCI. MACE occurred in 36 patients (15.8%), including 35 patients (15.4%) with periprocedural MI and the death of one patient (0.4%). ADP-induced LTA and VerifyNow values (pre- and post-PCI) were significantly higher in patients with the subsequent occurrence of periprocedural MI, but the MEA assay data (PCI and post-PCI) displayed no significant differences (pre-PCI p=0.25 and post-PCI p=0.33). ROC curve analysis demonstrated HPR values for LTA (pre-PCI, >66% and post-PCI, >53 %, all p<0.001), VerifyNow (pre-PCI, >347 PRU and post-PCI >272 PRU, all p<0.001) and MEA (pre-PCI, >50 U and post-PCI >39 U, all p>0.05). The platelet reactivity measurements by LTA and the VerifyNow assay can discriminate the risk of 30-day ischemic events after PCI. The predictive cut-off values for adverse events are dependent on sampling time.

Coagulation management in patients undergoing mechanical circulatory support

The incidence of bleeding and thrombo-embolic complications in patients undergoing mechanical circulatory support therapy remains high and is associated with bad outcomes and increased costs. The need for anticoagulation and anti-platelet therapy varies widely between different pulsatile and non-pulsatile ventricular-assist devices (VADs) and extracorporeal membrane oxygenation (ECMO) systems. Therefore, a unique anticoagulation protocol cannot be recommended. Notably, most thrombo-embolic complications occur despite values of conventional coagulation tests being within the targeted range. This is due to the fact that conventional coagulation tests such as international normalised ratio (INR), activated partial thromboplastin time (aPTT) and platelet count cannot detect hyper- or hypofibrinolysis, hypercoagulability due to tissue factor expression on circulating cells or increased clot firmness, and platelet aggregation as well as response to anti-platelet drugs. By contrast, point-of-care (POC) whole blood viscoelastic tests (thromboelastometry/-graphy) and platelet function tests (impedance or turbidimetric aggregometry) reflect in detail the haemostatic status of patients undergoing mechanical circulatory support therapy and the efficacy of their anticoagulation and antiaggregation therapy. Therefore, monitoring of haemostasis using POC thromboelastometry/-graphy and platelet function analysis is recommended during mechanical circulatory support therapy to reduce the risk of bleeding and thrombo-embolic complications. Notably, these haemostatic tests should be performed repeatedly during mechanical circulatory support therapy since thrombin generation, clot firmness and platelet response may change significantly over time with a high inter- and intra-individual variability. Furthermore, coagulation management can be hampered in non-pulsatile VADs by acquired von Willebrand syndrome, and in general by acquired factor XIII deficiency as well as by heparin-induced thrombocytopenia. In addition, POC testing can be used in bleeding patients to guide calculated goal-directed therapy with allogeneic blood products, haemostatic drugs and coagulation factor concentrates to optimise the haemostasis and to minimise transfusion requirements, transfusion-associated adverse events and to avoid thrombo-embolic complications, as well. However, coagulation management in patients undergoing mechanical circulatory support therapy is somehow like navigating between Scylla and Charybdis, and development of protocols based on POC testing seems to be beneficial.

Adjunctive cilostazol versus high maintenance dose of clopidogrel in patients with hyporesponsiveness to chronic clopidogrel therapy

PURPOSE

Whether addition of cilostazol is superior to increasing dose of clopidogrel in patients with hyporesponsiveness to chronic clopidogrel therapy is unknown.

MATERIALS AND METHODS

We studied 73 patients with hyporesponsiveness to clopidogrel on standard dual antiplatelet therapy for more than 2 weeks. Clopidogrel hyporesponsiveness was defined as percent inhibition of P2Y12 reaction units (PRU) <30% on VerifyNow P2Y12 assay. Patients were randomly assigned to increased dose of clopidogrel (aspirin 100 mg+clopidogrel 150 mg daily: group A, n=38) or to receiving additional cilostazol (aspirin 100 mg+clopidogrel 75 mg+cilostazol 100 mg bid daily: group B, n=35).

RESULTS

Baseline percent inhibition of PRU and PRU was similar between 2 groups (13.0 ± 10.2% versus 11.8 ± 9.7%, p = 0.61, and 286.3 ± 54.7 versus 295.7 ± 53.7, p=0.44, respectively). At follow-up, percent inhibition of PRU was higher and PRU was lower significantly in group B than in group A (38.5 ± 17.9% versus 28.3 ± 16.6%, p=0.02, and 207.3 ± 68.2 versus 241.3 ± 76.7, p=0.050, respectively). Among those still showing hyporesponsiveness to clopidogrel at follow-up (21 patients in group A, 10 patients in group B), 12 patients completed further crossover study. Compared to the baseline, magnitude of change in percent inhibition of PRU and PRU showed an improved tendency after the crossover (from 2.7 ± 8.7% to 15.8 ± 18.4%, p=0.08, and from -18.6 ± 58.0 to -61.9 ± 84.3, p=0.08).

CONCLUSION

Adjunctive cilostazol improved clopidogrel responsiveness better than the higher maintenance dose of clopidogrel in hyporesponsive patients with chronic clopidogrel therapy.

Doubling the clopidogrel dose in patients with reduced responsiveness to the standard dose is associated with a limited effectiveness as evaluated by impedance aggregometry

BACKGROUND

Different methods are available for quantifying platelet function inhibition. Measuring vasodilator-stimulated phosphoprotein (VASP) phosphorylation is currently the most specific method for assessing the clopidogrel effect. The aim of our study was to compare different tests in view of a clinically applicable bedside test. Further, we examined whether doubling the clopidogrel dose to 150mg/d in clopidogrel low-responder would lead to a reduction in platelet reactivity.

METHODS AND RESULTS

ADP-, ADP Hs-, and TRAP-induced platelet aggregation were measured by impedance aggregometry in 100 patients with CAD and 18 healthy controls. Moreover, platelet aggregation was assessed by flow cytometrical detection of VASP-phosphorylation and surface P-selectin in a subgroup of 34 patients and in healthy controls. Another 10 patients with CAD, identified as low-responder, were treated with a clopidogrel dose of 150mg/d. Thereafter, ADP-induced platelet aggregation was assessed by impedance aggregometry. Significant correlations were observed between ADP-induced platelet aggregation assessed by VASP-phosphorylation and by impedance aggregometry. Doubling the dose of clopidogrel to 150mg/d was associated with a reduction of ADP-induced platelet aggregation in only 60% of the patients.

CONCLUSIONS

Impedance aggregometry is a valuable bedside test to assess platelet function inhibition. Doubling the clopidogrel dose is not effective to reduce high on-treatment platelet reactivity in almost half of these patients, pointing to the need of a more powerful platelet inhibitor.

Response variability to clopidogrel: is tailored treatment, based on laboratory testing, the right solution?

Clopidogrel is an antithrombotic prodrug, whose active metabolite inhibits platelet function by irreversibly binding to the platelet receptor for adenosine diphosphate, P2Y(12). Wide inter-individual variability of response to clopidogrel has been reported in several studies: a significant proportion of treated patients (about one-third) exhibit a suboptimal inhibition of platelet function. Genetic and environmental factors that influence the absorption and/or the extent of metabolism of clopidogrel to its active metabolite account for the observed variability of response. Tailored treatment based on the results of laboratory tests of platelet function has been proposed as a solution to this problem, which has important clinical implications. Although it is often considered a desirable evolution of modern medicine, tailored treatment based on laboratory tests is actually an old remedy (of yet unproven efficacy, in the case of antiplatelet therapy) for the problem of response variability to antithrombotic drugs with unpredictable bioavailability. When possible, the use of alternative drugs with more uniform and predictable bioavailability, and with favourable profiles in terms of risk/benefit and cost-benefit ratios should be preferred. Moreover, tailored treatment with laboratory tests must be validated in randomized clinical trials before its implementation can be recommended. We still need to identify and standardize the laboratory test for this purpose, as well as answer basic questions on its clinical utility and cost-effectiveness, before tailoring clopidogrel therapy based on laboratory tests can be recommended in clinical practise.

Evaluation of multiple electrode aggregometry in whole blood using Multiplate Mini Test cells

Multiple electrode aggregometry (MEA) enables rapid platelet function testing in whole blood using 600 μL disposable standard test cells (STC). However, newly available 350 μL mini test cells (MTC) could potentially be advantageous in some clinical settings where sample volume is limiting. In order to evaluate the diagnostic performance of MTC, we have estimated assay imprecision, correlation and agreement between area under curve (AUC) determined using MTC and STC in whole blood from healthy donors and from 119 cardiac surgery patients. Imprecision was similar with ADP, AA and TRAP test reagents using STC and MTC, but was markedly higher with the unvalidated ADR reagent. AUC determined using MTC and STC and the ADP, AA and TRAP reagents correlated strongly although MTC yield consistently lower AUC values reflecting fewer platelets in the smaller test cell. Agreement between AUC from STC and MTC was less strong, probably reflecting a composite effect of imprecision from both assay formats. MTC and STC are equally valid for MEA but AUC values obtained using one test format cannot be directly transformed to the other. Therefore, STC and MTC cannot be used interchangeably and AUC results must be compared to separately determined reference intervals.

The thrombin inhibitors hirudin and Refludan(®) activate the soluble guanylyl cyclase and the cGMP pathway in washed human platelets

A number of direct thrombin inhibitors are successfully used clinically and experimentally as novel antithrombotics and specific anticoagulants. They are also used as anticoagulants in certain blood collection tubes for the analysis of platelet function. A series of platelet function tests have emerged to measure adequate responses to antiplatelet therapy. For comparative and practical reasons, it would be of advantage to use the same anticoagulant in blood collection tubes for different methods, e.g. thrombin inhibitors. However, there are little data on the effects of thrombin inhibitors on platelet signalling pathways that could influence results. We examined the applicability of thrombin inhibitor containing blood for platelet reactivity index (PRI) measurements of the VASP assay and investigated the effects of two thrombin inhibitors (hirudin and lepirudin) on cAMP- and cGMP-mediated signalling pathways in washed human platelets. We show that induction of VASP phosphorylation by PGE1 is markedly reduced in lepirudin containing blood samples. In consequence, PRI levels were highly variable compared to routinely used citrated blood. Surprisingly, in vitro incubation of platelets with thrombin inhibitors increases platelet cGMP levels and induces NOS independent sGC/PKG-mediated VASP phosphorylation. We conclude that thrombin inhibitors activate sGC/PKG-dependent pathways resulting in an increase of VASP phosphorylation which contributes to deviations in PRI measurements. These effects of thrombin inhibitors on sGC- and cGMP-mediated pathways including increased VASP phosphorylation may indicate the presence of an important additional platelet-based mechanism for the reduction of thrombus formation and thromboembolism by thrombin inhibitors.

The role of laboratory monitoring in antiplatelet therapy

In the last decade, several studies reported a high inter-individual variability in the pharmacological response to antiplatelet drugs. Suboptimal response to aspirin, as determined by specific tests (serum thromboxane B(2)), is rare and, when present, it appears to be caused by poor compliance in most instances. In contrast, studies that used specific tests to measure the pharmacological effect of clopidogrel showed a wide variability of response, with about 1/3 of treated subjects who are very poor responders. Inter-individual difference in the extent of metabolism of clopidogrel to its active metabolite by cytochrome P450 isoforms is the most relevant cause of the observed inter-individual variability in platelet inhibition. Tailored treatment based on laboratory monitoring of platelet function has been proposed as a solution to poor responsiveness to clopidogrel. However, we still need to identify the ideal laboratory test and to answer basic questions on its clinical utility and cost-effectiveness, before monitoring clopidogrel therapy can be recommended in clinical practice.

Measurement of Shear-Activated Platelet Aggregate Formation in Non-Anticoagulated Blood: Utility in Detection of Clopidogrel–Aspirin-Induced Platelet Dysfunction

We studied the ability of a new instrument, the PlaCor PRT that measures shear-induced platelet aggregation in fingerstick, non-anticoagulated blood without added agonists, to detect platelet dysfunction ex vivo. Platelet reactivity time (PRT) and whole blood aggregation (WBA) were measured in 160 healthy volunteers, before and after aspirin and in 170 participants with established vascular disease or risk factors thereof treated with aspirin ± clopidogrel. Pretreatment PRT and WBA were significantly correlated (collagen r = −.63; arachidonate r = −.65; P < .0001). Following aspirin, the mean PRT increased from 82 to 142 seconds ( P < .0001), and in participants treated with clopidogrel–aspirin, the mean PRT (286 seconds, n = 65) was significantly longer than with aspirin alone (166 seconds, n = 105; P < .001). Only 13% of PRTs of participants treated with clopidogrel and aspirin were within the normal range. We conclude that the PlaCor PRT is a simple, rapid, point-of-care instrument that compares favorably with published descriptions of other platelet function instruments.

Clinical, genetic and confounding factors determine the dynamics of the in vitro response/non response to clopidogrel

Platelet inhibition by clopidogrel varies from one individual to the next. Further, in vitro high on-treatment residual adenosine-diphosphate inducible platelet reactivity (HRPR) is associated with an increased risk for major adverse cardiovascular events after percutaneous coronary intervention (PCI) with stent implantation. Recent studies identified numerous influencing factors for the antiplatelet effect of clopidogrel. Besides genetic predispositions, diverse clinical conditions as well as pharmacological interactions were shown to significantly impair clopidogrel-mediated platelet inhibition. Consequently, these influencing factors may affect clinical outcome after PCI and it is therefore desirable to identify cofounders of HRPR by platelet reactivity testing. It is apparent, that not all assays are sensitive to the same variables, and only cofounders of HRPR that are repeatedly identified by more than one test system may be clinically meaningful. However, treatment adjustment based on platelet function testing has not been associated with improved patients' outcome. This summary shall provide an overview over current knowledge on influencing factors for clopidogrel-mediated platelet inhibition and aid guidance for critical interpretation of in vitro obtained data on HRPR.

The influence of bisoprolol dose on ADP-induced platelet aggregability in patients on dual antiplatelet therapy

PURPOSE

Dual antiplatelet therapy is recommended after acute coronary syndrome or after percutaneous coronary intervention with coronary stent implantation. Many of the patients on dual antiplatelet therapy receive β-blockers; some of them could have antiaggregatory effect. Bisoprolol is a highly selective adrenoceptor-blocker, which is often used in the settings of percutaneous coronary intervention or acute coronary syndrome in patients on dual antiplatelet therapy. Its antiaggregative effect has not been extensively studied. Therefore, the aim of this study is to investigate the effect of bisoprolol on ADP-induced platelet aggregation in patients on dual antiplatelet therapy.

METHODS

Platelet aggregability has been measured in 100 patients on dual antiplatelet therapy with multiplate analyzer using ADP test in blood samples anticoagulated with heparin. ADP test values have been expressed by arbitrary units/minute. In univariate and multivariate regression analyses, we have investigated the influence of bisoprolol and its dose and also different factors, such as risk factors, concomitant drugs and their dosage, laboratory findings, on ADP test values.

RESULTS

Univariate regression analysis showed significant correlation between the bisoprolol dose and the ADP test value (P=0.046, B=52.55, 95% confidence interval 0.87-104.23), which was also shown in the multivariate regression analysis (P=0.018; B=57.011; 95% confidence interval 10.455-103.567).

CONCLUSION

We have identified a positive correlation between bisoprolol dose and ADP-induced platelet aggregability in patients on dual antiplatelet therapy.

Perioperative assessment of platelet function in patients under antiplatelet therapy

Platelets play a central role in primary hemostasis. Analysis of platelet function is therefore a cornerstone in the global assessment of the coagulation status in the perioperative setting, primarily in patients receiving antiplatelet medication, such as cyclooxygenase-1 inhibitors, adenosine diphosphate antagonists and glycoprotein IIb/IIIa inhibitors. In these patients, knowledge of residual platelet function is highly warranted in order to maintain an optimal and individual balance perioperatively between platelet function and inhibition - that is, bleeding and thrombosis. Traditional laboratory-based assays, such as light-transmission aggregometry and flow cytometry, are the clinical standards of platelet function testing today. Light-transmission aggregometry is one of the most widely used tests to identify and diagnose defects in platelet function. The majority of the conventional laboratory-based techniques are labor intensive, costly and time consuming, and require a high degree of experience and expertise to perform and interpret. Therefore, new automated technologies have been developed to measure platelet function more rapidly and easily, and several techniques can be used at the bedside, including whole blood aggregometry, high shear-induced platelet function assessment or viscoelastic measurement techniques. All methods assessing platelet function are summarized and their limitations are discussed in this article, emphasizing their perioperative use.

Platelet function testing in clinical diagnostics

Although the utility of platelet function testing is still under debate, the necessity to inhibit platelets in patients suffering from cardiovascular and cerebrovascular disease is undoubted and well proven. The wide variety of available platelet function tests often using different methodologies, the apparent lack of standardization, and finally the emerging evidence on the clinical value of platelet function testing are resulting in a considerable uncertainty in the clinical practice, how to deal with the issue of platelet function testing. Platelet function testing might not only yield clinical benefits for the patients but also economical advantages by identifying the right drug at the right dose for the right patient. This article intends to provide an overview of the current platelet function tests such as light transmittance aggregometry, whole blood impedance aggregometry, the PFA-1001 system, the VerifyNow2 system, flow cytometry, as well as other promising technologies like Plateletworks3, IMPACT-R4, PADA, thromboelastography, and the mean platelet component (MPC), briefly addressing strengths, weaknesses and clinical utility of these tests.