Summary. Background: In patients undergoing percutaneous coronary intervention (PCI), a link between bleeding and excess mortality has been demonstrated. A potential association of platelet response to clopidogrel and bleeding has not been well established yet. Objectives: The aim of the present study was to assess the impact of clopidogrel responsiveness on the risk of bleeding in clopidogrel-treated patients undergoing PCI. Methods: Patients (n = 2533) undergoing PCI after pretreatment with 600 mg of clopidogrel were enrolled in this study. Blood was obtained directly before PCI. Adenosine-diphosphate (ADP)-induced platelet aggregation was assessed on a Multiplate analyzer. The primary endpoint was the incidence of in-hospital Thrombolysis in Myocardial Infarction (TIMI) major bleeding and the secondary endpoint was in-hospital TIMI minor bleeding. Receiver-operator curve (ROC) analysis was used to derive the optimal platelet aggregation value defining enhanced clopidogrel responders for the association of measurements with major bleeding. Results: Thirty-four (1.3%) major bleeding events and 137 (5.4%) minor bleeding events were observed. The risk of a major bleeding was significantly higher in patients (n = 975) with an enhanced response to clopidogrel as compared with the remaining patients (n = 1558) (2.2 vs. 0.8%, unadjusted odds ratio (OR) 2.6, 95% confidence interval (CI) 1.3–5.2, P = 0.005; adjusted OR 3.5, 95% CI 1.6–7.3, P = 0.001). No significant differences between both groups were observed for the occurrence of minor bleeding events (P = 0.68). Conclusions: Enhanced clopidogrel responsiveness is associated with a higher risk of major bleeding. Whether guidance of antiplatelet treatment based on platelet function testing proves useful for avoiding bleeding events warrants further investigation.
In patients undergoing coronary stent placement, different potent anticoagulants and antiplatelet agents are routinely administered during the procedure and also afterwards to reduce the risk of thrombotic complications [1–4]. Advances in anticoagulant  and antiplatelet treatment regimens  have been associated with a significant risk reduction in the occurrence of thrombotic events. However, this risk reduction is accompanied by an increased risk of bleeding during and after the procedure [6–10]. In the setting of percutaneous coronary interventions (PCI), periprocedural bleeding events are among the most frequent complications [8,11–14]. Evidence exists that the occurrence of bleeding during or after PCI has a similar impact on patients’ mortality risk as compared with the occurrence of a post PCI myocardial infarction (MI) [8,14,15].
For patients undergoing coronary stent placement, a dual antiplatelet treatment regimen consisting of aspirin and clopidogrel is routinely administered to prevent thrombotic events [1,16]. A large inter-individual variability of platelet response to a fixed dose of clopidogrel is a frequently described phenomenon [17–19]. Contrariwise to reducing thrombotic events, clopidogrel treatment significantly contributes to the occurrence of bleeding complications .
Whereas a bulk of evidence is available linking an attenuated platelet response to clopidogrel with the occurrence of thrombotic events including stent thrombosis [17,21–28], a large gap of knowledge exists in the understanding of how bleeding events may be related to an enhanced response to clopidogrel. Recently, we were able to show in a study population consisting of 1608 patients undergoing PCI after pretreatment with clopidogrel, that a low response to clopidogrel is a strong and independent predictor for the occurrence of stent thrombosis . In that study, we assessed clopidogrel responsiveness using multiple electrode aggregometry (MEA) on a Multiplate analyzer. Using MEA, platelet function is assessed in whole blood according to the principles of impedance aggregometry . In the same study population  we also report the incidence of bleeding events. An approximately 2-fold increase in the risk of major bleeding was observed in the group of patients defined as high responders as compared with the remaining patients. The primary aim of that study , however, was to assess the relationship of clopidogrel response and the incidence of stent thrombosis and regarding bleeding the total number of major bleeding events was low.
The significant impact of bleeding events on overall mortality in the setting of PCI  warrants further investigation in to this issue in larger study populations to (i) better estimate the relationship between an enhanced clopidogrel response and bleeding and to (ii) test for an independent association of this relationship. Thus, the aim of the present study was to assess the impact of periprocedural adenosine-diphosphate (ADP)-induced platelet aggregation measurements on the risk of bleeding events in a large cohort of clopidogrel-treated patients undergoing PCI.
Materials and methods
Coronary artery disease (CAD) patients with planned drug-eluting stent placement, who had platelet function measurements available at the time point of the PCI procedure, were consecutively enrolled in this study in the two participating centers between February 2007 and December 2008 (Deutsches Herzzentrum München and I. Medizinische Klinik, Klinikum rechts der Isar, Technische Universität München, Munich, Germany).
All patients were uniformly pretreated with a loading dose of 600 mg of clopidogrel, which was recommended to be given at least 2 h before the procedure. Coronary interventions were performed according to current standard guidelines . Intravenous anticoagulant treatment with unfractionated heparin (UFH) was given to the large majority of patients and only a few patients received bivalirudin. A small subset of the patients (< 5%) received intravenous (i.v.) antiplatelet therapy with the glycoprotein (GP) IIb/IIIa inhibitor abciximab (0.25 mg kg−1 of body weight bolus, followed by a 0.125 μg kg−1 min−1 infusion for 12 h) in addition to a reduced dose of UFH. After the procedure, all patients received in-hospital treatment with aspirin (200 mg day−1) and clopidogrel (75 mg twice a day until discharge but no longer than 3 days, followed by a daily dose of 75 mg). Other cardiac medications including coumarine derivatives were prescribed at the physician’s discretion. As patients were consecutively enrolled in this trial, patients were considered eligible irrespective of the clinical presentation. Patients with stable or unstable angina were enrolled as well as patients with a myocardial infarction at the time point of the procedure. Patients presenting with an acute ST-segment elevation MI did not receive thrombolytic therapy prior to the procedure. Exclusion criteria were contraindications to dual antiplatelet treatment with aspirin or clopidogrel and prior administration of GP IIb/IIIa inhibitors during the 10 days before the procedure. The present study complies with the Declaration of Helsinki and was approved by the institutional ethics committee. All patients gave written informed consent for the intervention and for platelet function testing prior to study inclusion.
Platelet function testing
For platelet function testing with MEA, whole blood was obtained from the arterial sheath of all patients after diagnostic angiography and directly before PCI. In all cases, blood was taken before administration of intravenous (i.v.) PCI-related anticoagulative treatment. Blood was placed in 4.5-mL plastic tubes containing the anticoagulant lepirudin (25 μg mL−1, Refludan, Hirudin blood collection tubes; Dynabyte, Munich, Germany). ADP (6.4 μm)-induced platelet aggregation in whole blood was assessed with MEA using a Multiplate analyzer (Dynabyte). Details of this method have been reported previously [21,29]. Aggregation measured with MEA was quantified by the area under the curve (AUC) of aggregation units (AU × min). All material used for platelet function testing was obtained from the manufacturer (Dynabyte).
Study end points and definitions
For the analysis of the impact of periprocedural ADP-induced platelet aggregation values on the risk for bleeding, patients were stratified according to their platelet aggregation value using a cut-off point derived with receiver-operator characteristic (ROC) curve analysis. Patients with an ADP-induced platelet aggregation value below this cut-off point were defined as enhanced clopidogrel responders. The primary endpoint of the study was the occurrence of an in-hospital major bleed defined according to the Thrombolysis in Myocardial Infarction (TIMI) criteria (TIMI major bleeding). The secondary endpoint was the occurrence of an in-hospital TIMI minor bleeding. In addition to the assessment of bleeding events, we also report on the incidence of early (≤ 30 days) definite stent thrombosis (ST), which was defined according to the Academic Research Consortium (ARC) criteria  as the occurrence of an acute coronary syndrome with either angiographic or pathological confirmation of thrombosis. Events were adjudicated by an event adjudication committee blinded to the platelet function measurements of the patients.
Variables are presented as mean ± standard deviation (SD), counts (percentages) or median with interquartile range [IQR]. Categorical variables were compared using the χ2-test. The Kolmogorov–Smirnov test was used to check for normal distribution of continuous data. Normally distributed continuous data were compared between groups using the t-test. Non-normally distributed continuous data were compared between groups using Wilcoxon’s test. Platelet function data obtained with MEA were not normally distributed and are presented as median [IQR].
A ROC curve analysis was calculated to determine the ability of MEA measurements to distinguish between patients with and without an in-hospital major bleeding event. The optimal cut-off value was calculated by determining the value for ADP-induced platelet aggregation (in AU × min) that provided the greatest sum of sensitivity and specificity. ROC curve analysis was also calculated to determine the ability of MEA to distinguish between patients with and without definite ST in the first 30 days after the procedure.
A multiple logistic regression model was used to test for an independent association of periprocedural ADP-induced platelet aggregation measurements with the primary study endpoint (in-hospital major bleeding). The occurrence of a major bleeding was defined as the dependent variable. Multivariate analysis was performed (i) by including platelet aggregation measurements as a continuous independent variable (per 100 decrease in AU × min) and also (ii) in a second model as a dichotomous independent variable (presence of enhanced clopidogrel responsiveness). To control for possible confounding variables, the models included demographic and clinical variables with a difference between enhanced responders and the remaining patients at a P-value < 0.20 (see Table 1). In addition, we also adjusted for procedural characteristics of patients that may have a relevant impact on the risk for bleeding [use of abciximab, use of intra-aortic balloon pumping (IABP), number of lesions treated, lesion complexity defined according to the ACC/AHA lesion class]. The odds ratio (OR) and the corresponding 95% confidence interval (CI) was calculated for each variable included in the multivariate model. All analyses were performed using the S-PLUS software package (Insightful Corp, Seattle, WA, USA). A P-value < 0.05 was considered statistically significant.
Table 1. Baseline characteristics of the study population
Enhanced responders (n = 975)
Remaining patients (n = 1558)
Data presented are means ± SDs or numbers of patients (percentages). CAD, coronary artery disease; IABP, intra-aortic balloon pumping; STEMI, ST-elevation myocardial infarction.
67.2 ± 10.6
68.2 ± 10.3
Woman, n (%)
Body mass index (kg m−2)
27.1 ± 4.1
27.8 ± 4.4
Renal failure, n (%)
Diabetes mellitus, n (%)
Active smokers, n (%)
Arterial hypertension, n (%)
Hypercholesterolemia, n (%)
Family history of CAD, n (%)
Previous MI, n (%)
Previous bypass surgery, n (%)
Multivessel disease, n (%)
Non-STEMI/STEMI, n (%)
Use of abciximab, n (%)
Use of IABP, n (%)
Number of lesions per patient
1.9 ± 0.9
1.9 ± 1.0
Complex (type B2/C) lesions, n (%)
Number of stents per lesion
1.2 ± 0.6
1.3 ± 0.6
Medication ad admission, n (%)
A total of 2533 patients were enrolled in this trial. The median [IQR] value of periprocedural ADP-induced platelet aggregation after administration of 600 mg clopidogrel was 225 [142–374] AU × min. Figure 1 shows the distribution of measurements in the entire study population. Baseline characteristics of the study population are shown in Table 1. Patients with an enhanced response to clopidogrel were younger, had a lower body mass index, were less frequently diabetic patients, presented less frequently with MI and differed in medication at admission and the use of IABP as compared with the remaining patients.
Platelet aggregation and bleeding events
In-hospital major bleeding, the primary endpoint of the study, was observed in a total of 34 (1.3%) patients. Figure 1 shows the distribution of major bleeding events across the entire spectrum of ADP-induced platelet aggregation values. The optimal cut-off value according to ROC analysis to predict the occurrence of an in-hospital major bleeding was 188 AU × min. With this cut-off value, MEA had 62% sensitivity, 62% specificity, an area under the ROC curve of 0.61 (95% CI: 0.51–0.70; P = 0.017), a positive predictive value of 2.2% and a negative predictive value of 99.2%. Applying this cut-off value to the study population, 975 patients (38.0%) had an MEA value < 188 AU × min and were defined as enhanced responders.
The incidence of major bleeding was significantly higher in enhanced clopidogrel responders (n = 975) as compared with the remaining patients (n = 1558) [21 (2.2%) vs. 13 (0.8%); OR 2.6, 95% CI 1.3–5.2; P = 0.005] (Fig. 2). In-hospital minor bleeding, the secondary endpoint of the study, was observed in a total of 137 (5.4%) patients. No significant differences were observed for the incidence of minor bleedings in patients (n = 975) with an enhanced response to clopidogrel treatment as compared with the remaining patients (n = 1558) [55 (5.6%) vs. 82 (5.3%); OR 1.1, 95% CI 0.8–1.5; P = 0.68]. For the combined incidence of minor or major bleedings, no significant differences were observed for the incidence of bleeding in patients with an enhanced response to clopidogrel treatment as compared with the remaining patients [76 (7.8%) vs. 95 (6.1%); OR 1.3, 95% CI 0.95–1.8; P = 0.10].
Multivariate analysis of bleeding events
For a multivariate analysis with major bleeding as the dependent variable, platelet aggregation measurements were included: (i) as a continuous independent variable and (ii) as a dichotomous independent variable (presence of enhanced clopidogrel responsiveness). Other variables included into the model were age, body mass index, diabetes, renal failure, presence of an MI at admission, treatment at admission with aspirin, a thienopyridine, a statin or a coumarin derivate, use of abciximab, use of intra-aortic balloon pumping, number of lesions treated and complex lesions (defined as type B2/C lesions according to AHA/ACC lesions morphology).
When ADP-induced platelet aggregation (in AU × min) was included as a continuous variable, calculations were done for an absolute decrease of 100 AU × min. Results of this analysis demonstrated that the ADP-induced platelet aggregation value (per decrease of 100 AU × min) was an independent predictor of a major bleeding event (OR 1.3, 95% CI 1.1–1.7; P = 0.017). For dichotomous analysis, an enhanced response to clopidogrel (ADP-induced platelet aggregation value < 188 AU × min) was found to be an independent predictor of an in-hospital major bleeding event (OR 3.5, 95% CI 1.6–7.3; P = 0.001). Other independent predictors observed were age (OR 1.9, 95% CI 1.3–2.8, calculated per 10 year increase in age; P = 0.002), use of intra-aortic balloon pumping (OR 9.6, 95% CI 1.5–60.7; P = 0.02) and the number of lesions treated (OR 1.9, 95% CI 1.4–2.5, calculated per one additional treated lesion; P < 0.001). The remaining variables were not independently associated with the occurrence of a major bleeding event (P ≥ 0.18).
Platelet aggregation and stent thrombosis
Sixteen cases (0.6%) of definite stent thrombosis were observed in this cohort of patients in the first 30 days after the procedure. Based on the cut-off value of 468 AU × min determined by ROC analysis, 428 (17%) patients showed an ADP-induced platelet aggregation value ≥ 468 AU × min. Nine (2.1%) of these 428 patients had a definite ST as compared with seven cases (0.3%) of definite ST that were recorded among the remaining patients (n = 2105) with an ADP-induced platelet aggregation value < 468 AU × min (P < 0.001).
Figure 3 illustrates the incidence of adverse events (combined for both major bleeding and definite stent thrombosis) and its relation to ADP-induced platelet aggregation values.
To the best of our knowledge this is the first study reporting on a significant impact of an enhanced response to clopidogrel treatment on the occurrence of bleeding events in a large population of consecutively recruited and prospectively followed patients undergoing PCI after pretreatment with clopidogrel. The major finding of our study is that an enhanced response to clopidogrel is associated with an increased risk of bleeding; mostly in its major form.
By stratifying the patients into two groups – enhanced responders vs. remaining patients – according to periprocedural ADP-induced platelet aggregation measurements, we were able to demonstrate that the risk of a major bleeding was about 3-fold higher in patients with an enhanced response to clopidogrel. The association of ADP-induced platelet aggregation values in general and major bleeding was strengthened by a multiple logistic regression model, which also included known predictors of bleeding such as age, body mass index and renal failure [7,12,15], were ADP-induced platelet aggregation was included as a continuous variable and was found to be an independent predictor of a major bleeding event.
Stratification of patients according to ADP-induced platelet aggregation values has been the primary approach in the majority of previous trials in the setting of clopidogrel response and clinical outcome [21,24,31,32]. Such stratifications can be practical and useful for the clinician to define a population being at risk for a certain adverse event. In fact, the cut-off value according to ROC analysis (188 AU × min) below which the risk for bleeding substantially increases, was found to be very close to the cut-off value defining the lowest tertile (33%) of patients (172 AU × min), which supports defining approximately one-third of patients as patients at risk of bleeding after clopidogrel treatment and undergoing PCI.
Concerning the occurrence of stent thrombosis, we were able to confirm the findings of our previous trial that included 1608 patients undergoing PCI in the present larger study population of 2533 patients. A highly significant association of clopidogrel low-responsiveness and the occurrence of ST was found and a cut-off value of 468 AU × min derived by ROC analysis was observed, above which the risk for stent thrombosis substantially increases. From the entire study population, 17% of patients were found to be above this cut-off value and therefore at risk of stent thrombosis. Taking both cut-off values derived by ROC analysis into account (188 AU × min for bleeding and 468 AU × min for stent thrombosis), a cohort of patients can be defined with an ADP-induced platelet aggregation value in between this range, where the risk of both bleeding and stent thrombosis is relatively low.
The TIMI definition of bleeding has been used in a number of large-scale clinical trials [8,33] and shows a strong relationship with 1-year mortality . The observed differences in bleeding risk between patients with and without an enhanced response to clopidogrel treatment were more pronounced for TIMI major bleeding. Minor bleeding remained less influenced by platelet response to clopidogrel and it is unclear why. It has to be acknowledged, however, that results reported here are observational, and the amount to which clopidogrel responsiveness impacts risk of major and minor bleeding warrants further investigation.
At present, a large gap of knowledge exists in the understanding of how bleeding events may be related to platelet response to clopidogrel treatment. An early identification of patients being at a high risk of developing a bleeding event by platelet function testing may offer the opportunity to adjust antithrombotic treatment regimens accordingly. The emerging relevance of bleeding for overall outcome success in PCI-treated patients gave rise to the inclusion of bleeding as the component of the primary endpoint in different large-scale clinical trials [4,34,35], which has been termed as net adverse clinical events (NACE). Moreover, the advent of new and more potent antiplatelet drugs targeting the P2Y12 ADP receptor such as prasugrel  and ticagrelor  sets the need to study the relationship of antiplatelet treatment and risk of bleeding thoroughly. Further on, a number of studies have demonstrated that mortality may result from both ischemic events such as MI but also from hemorrhage [8,14,15,33]. In the present study, we were able to identify an enhanced response to clopidogrel as an independent predictor of postprocedural bleeding in addition to other known and established factors that affect bleeding risk such as age, gender or body mass index.
The MEA technique applied for platelet function testing in this study is moderately complex and can be applied in routine laboratory or near-patient settings. Platelet function measurements obtained immediately before the procedure may help tailoring of antiplatelet treatment to ensure optimal balance between ischemic and bleeding complications in patients undergoing coronary stent placement. Results reported here and the results of other studies establish the need to further define a therapeutic window for oral antiplatelet treatment with drugs that target the P2Y12 ADP-receptor of platelets. More data from outcome driven and specifically designed randomized studies, however, are needed to answer the question whether an individual and tailored antiplatelet treatment regimen based on platelet function testing is able to improve the overall outcome of patients. Not only should these studies focus on reducing thrombotic events but also on the risk for bleeding that comes along with intensified treatment regimens. In this context, the focus of previous studies was mostly based on the rationale that a certain antiplatelet treatment would constitute of a dose being too low for certain patients. It should, however, be considered as well, that a standard dosing regimen, could be a dosing regimen being too high for a certain subgroup of patients.
The present study has limitations that merit mention. Here, we only assessed the periprocedural platelet response to clopidogrel treatment and the response to oral antiplatelet treatment with aspirin was not assessed. We are therefore unable to estimate in how far the response to aspirin treatment contributes to the occurrence of bleeding events. In addition, we only assessed the incidence of in-hospital bleeding complications in relation to clopidogrel response status. Whether the platelet response to clopidogrel maintenance treatment is linked to higher post-discharge bleeding events warrants investigation in separate studies. The overall frequency of in-hospital major bleedings was rare in the present study population, and a comprehensive analysis on all its predictors requires thousands of patients. This was beyond the scope of our study, which was primarily designed to assess the relationship of platelet function measurements and major bleeding. Finally, the present study is observational and, therefore, it is subject to limitations inherent to all such analysis.
Enhanced clopidogrel responsiveness is associated with a higher risk of major bleeding. Whether guidance of antiplatelet treatment based on platelet function testing proves useful for avoiding bleeding events has to be tested in specifically designed trials.
D. Sibbing: conception and study design, analysis and interpretation of data, manuscript drafting, revising of the manuscript and final approval .S. Schulz: conception and study design, collection of data, analysis and interpretation of data, manuscript drafting, revising of the manuscript and final approval. S. Braun: conception and study design, analysis and interpretation of data, collection of data, manuscript drafting, revising of the manuscript and final approval. T. Morath: collection of data, analysis and interpretation of data, revising of the manuscript and final approval. J. Stegherr: collection of data, analysis and interpretation of data, revising of the manuscript and final approval. J. Mehilli: conception and study design, collection of data, revising of the manuscript and final approval. A. Schömig: conception and study design, analysis and interpretation of data, revising of the manuscript and final approval. N. von Beckerath: conception and study design, analysis and interpretation of data, revising of the manuscript and final approval. A. Kastrati: conception and study design, analysis and interpretation of data, manuscript drafting, revising of the manuscript and final approval.
Source of funding
Material for platelet function analysis on the Multiplate device were provided free of charge from Dynabyte (Munich, Germany). The sponsors had no role in the design and conduct of the study; collection, management, analysis and interpretation of the data; or preparation, review or approval of the manuscript.
Disclosure of Conflict of Interests
D. Sibbing reported receiving speaker fees from Dynabyte and fees for advisory board activities from Eli Lilly.