Assessment of platelet function in healthy sedated cats using three whole blood platelet function tests

Determination of clopidogrel effect in cats using point-of-care Plateletworks ADP and shipped samples for PFA-200 analysis in a clinical practice setting

OBJECTIVES

Clopidogrel is the recommended first-line antithrombotic in cats for a variety of conditions; however, it is ineffective in 15-20% of cats. The determination of clopidogrel effectiveness with platelet function assays has historically been limited to specialty centers; however, recent work has suggested that in-hospital or shipped analyses of samples may be feasible. The aim of the present study was to investigate the utility of an in-house analysis and shipping of blood samples collected in primary practices for the determination of clopidogrel effectiveness.

METHODS

Citrated blood samples were collected from cats receiving clopidogrel therapy by veterinarians in clinical practices across Canada, a median of 304.4 km from the reference laboratory (range 8-4425). Samples were analyzed in-house using Plateletworks ADP and shipped for remote analysis using PFA-200 P2Y and COL/ADP cartridges.

RESULTS

A total of 30 samples were collected from 25 cats. Of these, the percentage of samples analyzable for the presence or absence of the clopidogrel effect was 86% for Plateletworks ADP, 90% for PFA-200 P2Y and 87% for PFA-200 COL/ADP. There was no significant difference in the number of samples unable to be analyzed by each modality (P = 0.689) due to flow obstruction or other sample characteristics. The prevalence of absence of clopidogrel effectiveness on platelet function assays was 8% with the PFA-200 COL/ADP assay, 25% with the PFA-200 P2Y assay and 30% with the Plateletworks ADP assay.

CONCLUSIONS AND RELEVANCE

The results of this study confirm that samples of feline blood can be collected in clinical practices and shipped to a reference laboratory for PFA-200 analysis with a high rate of success, comparable to point-of-care analysis.

Effect of leukoreduction on the metabolism of equine packed red blood cells during refrigerated storage

BACKGROUND

Understanding of the biochemical and morphological lesions associated with storage of equine blood is limited.

OBJECTIVE

To demonstrate the temporal sequences of lipid and metabolic profiles of equine fresh and stored (up to 42 days) and leukoreduced packed red blood cells (LR-pRBC) and non-leukoreduced packed RBC (nLR-pRBC).

ANIMALS

Packed RBC units were obtained from 6 healthy blood donor horses enrolled in 2 blood banks.

METHODS

Observational study. Whole blood was collected from each donor using transfusion bags with a LR filter. Leukoreduction pRBC and nLR-pRBC units were obtained and stored at 4°C for up 42 days. Sterile weekly sampling was performed from each unit for analyses.

RESULTS

Red blood cells and supernatants progressively accumulated lactate products while high-energy phosphate compounds (adenosine triphosphate and 2,3-Diphosphoglycerate) declined. Hypoxanthine, xanthine, and free fatty acids accumulated in stored RBC and supernatants. These lesions were exacerbated in non-LR-pRBC.

CONCLUSION AND CLINICAL IMPORTANCE

Leukoreduction has a beneficial effect on RBC energy and redox metabolism of equine pRBC and the onset and severity of the metabolic storage lesions RBC.

Platelet function analyzer-200 closure curve analysis and assessment of flow-obstructed samples

BACKGROUND

The Platelet function analyzer-200 can determine the effect of clopidogrel in cats. Flow obstruction is an error that causes uninterpretable results. Closure curves and parameters initial flow rate (IF) and total volume (TV) are displayed by the PFA-200 and may allow interpretation of results in cases of flow obstruction. The primary hemostasis components (PHC) are calculated values that normalize these parameters.

OBJECTIVES

To determine if closure curves and research parameters allow detecting the effect of clopidogrel in cases of flow obstruction.

METHODS

A review of closure curves identified those with flow obstruction and paired analysis that did not. Non-flow-obstructed curves were used to categorize curves with respect to clopidogrel effects. IF, TV, PHC(1), and PHC(2) were evaluated to determine if these could be used to categorize if a sample exhibited the effects of clopidogrel. Curves were visually analyzed, and characteristics identified that were more common with or without the effect of clopidogrel. Visual analysis of curves was performed by blinded observers to determine if a visual analysis was able to predict the effect of clopidogrel.

RESULTS

Analysis of parameters was able to predict closure or non-closure in flow-obstructed curves. TV, PHC(1), and PHC(2) had area under the curve of the receiver operating characteristics of 0.79, 0.79, and 0.87. Visual curve analysis was unable to predict closure, with an average accuracy of only 55%, among three reviewers. Agreement between reviewers was poor (Fleiss' Kappa 0.06).

CONCLUSIONS

Visual curve analysis was unable to determine the effect of clopidogrel in flow-obstructed samples. Numerical parameters were able to detect the effect of clopidogrel with a high degree of accuracy in flow-obstructed samples.

Validation of storage and shipping of feline blood samples for analysis on the Platelet Function Analyzer-200 for determining the effect of clopidogrel

BACKGROUND

The Platelet function analyzer-200 (PFA-200) can determine the effect of clopidogrel in cats, but analysis traditionally must be performed at point-of-care (POC). The ability to ship samples of blood to a laboratory would allow widespread access.

OBJECTIVES

We aimed to validate the shipping of blood samples for PFA-200 analysis in cats to determine the effect of clopidogrel.

METHODS

Twenty healthy cats and 10 cats receiving clopidogrel were recruited. Blood was collected from cats and aliquoted into two samples, one was analyzed at POC within 2 hours using the PFA-200, and the other was packaged and transported to a location 4 km away, stored, and transported back to the lab for analysis the following day.

RESULTS

Median closure times (CTs) with the collagen/adenosine diphosphate (COL/ADP) cartridge in healthy cats were 51.5 seconds (POC) and 78.8 seconds (shipped), which were significantly different (P < 0.001), and for cats on clopidogrel, median CTs were 147.5 seconds (POC) and 190 seconds (shipped), which were not significantly different (P = 0.131). Median CTs with the P2Y cartridge in healthy cats were 50.5 seconds (POC) and 64.9 seconds (shipped), which were significantly different (P = 0.03), and in cats receiving clopidogrel, median CTs were 300 seconds (POC) and 300 seconds (shipped) which were not significantly different (P = 1.000). Reference intervals for CTs differed for COL/ADP at POC (19.8-89.7 seconds) and shipped (50.9-161.6 seconds) and for P2Y at POC (35.5-118.8 seconds) and shipped (35.1-108.9 seconds). Receiver operating characteristics showed similar areas under the curve (AUCROCs) regarding the effect of clopidogrel for COL/ADP at POC (0.994 seconds) and shipped (0.932) and for P2Y at POC (0.904 seconds) and shipped (0.975 seconds). When classifying for the presence of clopidogrel effects, Cohen's Kappa was 0.62 for COL/ADP and 1.00 for P2Y.

CONCLUSIONS

Shipping blood samples for PFA analysis are feasible with similar performance to POC analyses for determining the effect of clopidogrel in cats.

Investigation of Platelet Function Analyzer 200 platelet function measurements in healthy cats and cats receiving clopidogrel

The Platelet Function Analyzer 200 (PFA-200; Siemens) is an in vitro substitute for in vivo bleeding time that is designed to investigate platelet function in a more physiologic manner than traditional aggregometry. The analyzer reports a closure time (CT) as a marker of platelet function, and may also report the calculated platelet function measurement primary hemostasis components, PHC1 and PHC2. These incorporate the measured total volume (TV) of blood aspirated and the initial flow rate (IF). We determined, for the COL/ADP and P2Y cartridges, the median total volume (TVmedian), and RIs for CT, IF, TV, PHC1, and PHC2, and investigated the sensitivity and specificity of those parameters at the determined interpretation thresholds in determination of the clopidogrel effect. Healthy client-owned cats were recruited prospectively to determine RIs for CT, IF, TV, PHC1, and PHC2. Healthy blood-donor cats and cats on clopidogrel therapy were included retrospectively to determine test performance. In 20 healthy cats, RIs for COL/ADP were CT (19.5-87.2 s), IF (199-278 µL/min), TV (199-332 µL), PHC1 (94-106%), and PHC2 (52-148%); and for P2Y, CT (4.2-94.3 s), IF (112-208 µL/min), TV (151-294 µL), PHC1 (35-178%), and PHC2 (90-109%). CVs were calculated for all of these values. Specificity for detection of the clopidogrel effect was calculated from a group of healthy blood donors, and sensitivity for detection of the clopidogrel effect from a group of cats with known clopidogrel effect. Sensitivity and specificity were, for COL/ADP: CT (83.3%, 66.6%), IF (41.4%, 83.3%), TV (83.3%, 100%), PHC1 (100%, 100%) and PHC2 (100%, 83.3%); and for P2Y: CT (100%, 94.4%), IF (30%, 44.4%), TV (100%, 94.4%), PHC1 (100%, 100%), and PHC2 (100%, 97.7%). These PFA-200 values may be beneficial in the determination of platelet function in cats.

Validation of Plateletworks ADP for the ProCyte Dx analyzer

BACKGROUND

Platelet function testing in cats allows determination of clopidogrel effect. Plateletworks assesses aggregation based on decreasing platelet counts on hematology analyzers in response to agonists. It has not been validated for the IDEXX ProCyte Dx analyzer. Ideal time to perform analysis and the utility of other platelet parameters have not been fully assessed.

OBJECTIVES

To validate Plateletworks ADP on the ProCyte Dx, to investigate the utility of various platelet parameters using Plateletworks ADP, and determine the ideal time to perform analysis.

ANIMALS

Twenty healthy cats recruited from the general population used for transference of reference intervals to a new analyzer, and 10 cats receiving clopidogrel to determine clopidogrel effect.

METHODS

Plateletworks ADP using the ProCyte Dx and ADVIA 2120i analyzer was run simultaneously in both healthy cats and cats receiving clopidogrel, and CBC results at different timepoints were compared between analyzers.

RESULTS

Aggregation was significantly different (P < .001) between analyzers. Cohen's kappa showed almost perfect agreement for determination of clopidogrel effect, and the area under the curve of the receiver operating characteristic was 1.0. Lower limits of the aggregation reference interval in healthy cats were 28.8% on the ProCyte Dx and 12.5% on the ADVIA 2120i. Coefficients of variation for platelet parameters were not different between analyzers. No significant changes in mean platelet volume, plateletcrit, large platelets, and mean platelet component were identified. No significant change in aggregation was observed within the first hour after phlebotomy.

CONCLUSIONS AND CLINICAL IMPORTANCE

Our study validated the Plateletworks ADP system on the ProCyte Dx analyzer. Samples may be analyzed up to 1 h after collection.

Platelet Priming and Activation in Naturally Occurring Thermal Burn Injuries and Wildfire Smoke Exposure Is Associated With Intracardiac Thrombosis and Spontaneous Echocardiographic Contrast in Feline Survivors

Wildfires pose a major health risk for humans, wildlife, and domestic animals. We previously discovered pathophysiologic parallels between domestic cats with naturally occurring smoke inhalation and thermal burn injuries and human beings with similar injuries; these were characterized by transient myocardial thickening, cardiac troponin I elevation and formation of intracardiac thrombosis. While the underlying mechanisms remain unclear, results from murine models suggest that platelet priming and activation may contribute to a global hypercoagulable state and thrombosis. Herein, we evaluated and compared the degree of platelet activation, platelet response to physiologic agonists and levels of platelet-derived microvesicles (PDMV) in 29 cats with naturally occurring wildfire thermal injuries (WF), 21 clinically healthy cats with subclinical hypertrophic cardiomyopathy (HCM) and 11 healthy cats without HCM (CC). We also quantified and compared circulating PDMVs in WF cats to CC cats. In addition, we examined the association between thrombotic events, severity of burn injuries, myocardial changes, and the degree of platelet activation in cats exposed to wildfires. Flow cytometric detection of platelet surface P-selectin expression showed that WF cats had increased platelet response to adenosine diphosphate (ADP) and thrombin compared to the two control groups indicating the presence of primed platelets in circulation. In addition, cats in the WF group had increased circulating levels of PDMV, characterized by increased phosphatidylserine on the external leaflet. Cats in the WF group with documented intracardiac thrombosis had elevated platelet activation and platelet priming in the presence of ADP. While high dose arachidonic acid (AA) mostly resulted in platelet inhibition, persistent response to AA was noted among cats in the WF group with intracardiac thrombosis. Univariate and multiple logistic regression analyses demonstrated that increased platelet response to AA was independently associated with thrombotic events. This is the first study reporting the significant association between platelet priming and intracardiac thrombosis in domestic cats with naturally occurring wildfire-related injuries and smoke inhalation. Further studies are required to delineate additional mechanisms between inflammation and thrombosis, especially regarding platelet primers and the cyclooxygenase pathway.

A genetic polymorphism in P2RY1 impacts response to clopidogrel in cats with hypertrophic cardiomyopathy

Clopidogrel is converted to its active metabolite by cytochrome P450 isoenzymes and irreversibly inhibits platelet activation by antagonizing the adenosine-diphosphate (ADP) receptor. It is frequently used in cats with hypertrophic cardiomyopathy (HCM) to prevent thromboembolic complications. However, significant interpatient variability of the response to clopidogrel therapy has been suspected. In this study, we assessed the impact of single nucleotide polymorphisms (SNPs) within ADP receptor (P2RY1, P2RY12) and cytochrome P450 isoenzyme (CYP2C41) genes on platelet inhibition by clopidogrel administration in cats with HCM. Forty-nine cats completed the study, and blood samples were obtained before and after clopidogrel therapy to assess the degree of platelet inhibition based on flow cytometry and whole blood platelet aggregometry. Plasma concentrations of clopidogrel metabolites were measured after the last dose of clopidogrel. Whole blood platelet aggregometry revealed a significant reduction of platelet inhibition by clopidogrel in cats with the P2RY1:A236G and the P2RY12:V34I variants. The association with the P2RY1:A236G variant and clopidogrel resistance remained significant after adjustment for multiple comparisons. This study demonstrated that a genetic polymorphism in the P2RY1 gene altered response to clopidogrel therapy and suggests that clinicians may consider alternative or additional thromboprophylactic therapy in cats with the P2RY1:A236G variant.

Platelet function in cats with hyperthyroidism

OBJECTIVES

Cats with hyperthyroidism have been reported to develop thromboembolism, with and without echocardiographic abnormalities consistent with hyperthyroidism. The objective of this study was to compare platelet function in cats with hyperthyroidism with euthyroid age-matched cats. We hypothesized that cats with hyperthyroidism have shortened collagen and adenosine diphosphate (C-ADP) closure times as measured with the platelet function analyzer (PFA-100) in comparison with healthy, age-matched controls.

METHODS

Sixteen hyperthyroid and nine euthyroid healthy cats >7 years of age were recruited from the hospital population. Platelet function, measured using the C-ADP closure times by the PFA-100, and platelet count were measured in healthy euthyroid cats and cats with hyperthyroidism.

RESULTS

Mean ± SD closure times were not significantly different between control (66.3 ± 9.6 s) and hyperthyroid cats (65.9 ± 11.5 s; P = 0.75). The mean ± SD closure times of hyperthyroid cats that either were untreated or received methimazole for ⩽3 weeks (n = 6; mean 68.5 ± 15.4 s) was not different than that of cats treated for >3 weeks (n = 10; mean 64.3 ± 8.9 s; P = 0.57). The mean automated platelet count was higher in the hyperthyroid group than in the control group (P = 0.023).

CONCLUSIONS AND RELEVANCE

Platelet function, as measured by closure time under high shear conditions using C-ADP as an agonist, was not affected by hyperthyroidism in this group of cats. Further research is needed to determine if a hypercoagulable state exists in hyperthyroid cats and the potential roles platelets and von Willebrand factor may have.

Abnormal platelet activity in dogs and cats - impact and measurement

Abnormal platelet activity can either lead to bleeding tendencies or inappropriate thrombus formation and can occur secondarily to a wide variety of disease processes, with a range of clinical consequences and severity. This article will discuss the pathophysiology of platelet function abnormalities and consider a logical diagnostic approach applicable to veterinary practice. Recent advances in platelet function testing will then be discussed, with regards to detection of platelet dysfunction and tailoring of pharmacological manipulation. Although many of these tests are still confined to research or academic institutions, techniques for indirectly assessing platelet function are starting to become more widely available. Although we still require further research to develop guidelines for the use of these tests in clinical decision-making, the recent advances in this field are an exciting step forward in being able to detect and manage platelet dysfunction in both primary care and referral practice.

Influence of different supplementation on platelet aggregation in patients with rheumatoid arthritis

INTRODUCTION

Long-chain n-3 polyunsaturated fatty acids (n-3 PUFAs; eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA)) have been reported to reduce platelet aggregation. Our aim was to prospectively assess the potential influence of different supplementation omega-3 PUFA on the antiplatelet effects in rheumatoid arthritis (RA) patients.

METHODS

The study included 60 patients with RA at the Department of Rheumatology, Clinical Center Kragujevac. Patients were divided into three groups depending on who used concentrated fish oil only or concentrated fish oil in combination with evening primrose oil or control group without supplementation in a period of 3 months. Platelet aggregation was measured using the multiplate analyzer and expressed through the value of adenosine diphosphate (ADP) test, aranchidonic acid-induced aggregation (ASPI) test, thrombin receptor-activating peptide (TRAP) test (to assess baseline platelet aggregation), and the ratio of ADP/TRAP and ASPI/TRAP representing the degree of inhibition of platelet aggregation compared to the basal value. The platelet function analysis in whole blood was performed 18-24 h before starting supplementation and after 90 days. Considerations were taken in the representation of demographic, clinical characteristics, and laboratory parameters between the groups.

RESULTS

Patients who used concentrated fish oil only had a significantly lower value of the ratio of ADP/TRAP (0.68 ± 0.20) compared to patients without supplementation (0.83 ± 0.12; p = 0.008), while there was no statistically significant difference in values of other laboratory parameters of platelet function between other groups.

CONCLUSIONS

Co-administration of supplementation-concentrated fish oil may reduce platelet aggregation in adults with RA.

KEY POINTS

• Omega-3 PUFAs are essential for health and are known to possess anti-inflammatory properties, improving cardiovascular health as well as benefiting inflammatory diseases.. • In this paper, we report on anti-aggregation effects n-3 PUFAs and ɤ-linolenic acid in RA. • The risk of cardiovascular morbidity and mortality is increased in RA, and dietary supplementation of n-3 PUFA may have preventive potential for the cardiovascular management in rheumatoid arthritis.

Comparative evaluation of Plateletworks, Multiplate analyzer and Platelet function analyzer-200 in cardiology patients

The objective of this study was to comparatively evaluate three commercial whole-blood platelet function analyzer systems: Platelet Function Analyzer-200 (PFA; Siemens Canada, Mississauga, Ontario, Canada), Multiplate analyzer (MP; Roche Diagnostics International Ltd., Rotkreuz, Switzerland), and Plateletworks Combo-25 kit (PLW; Helena Laboratories, Beaumont, TX, USA). Venipuncture was performed on 160 patients who visited a department of cardiology. Pairwise agreement among the three platelet function assays was assessed using Cohen's kappa coefficient and percent agreement within the reference limit. Kappa values with the same agonists were poor between PFA-collagen (COL; agonist)/adenosine diphosphate (ADP) and MP-ADP (-0.147), PFA-COL/ADP and PLW-ADP (0.089), MP-ADP and PLW-ADP (0.039), PFA-COL/ADP and MP-COL (-0.039), and between PFA-COL/ADP and PLW-COL (-0.067). Nonetheless, kappa values for the same assay principle with a different agonist were slightly higher between PFA-COL/ADP and PFA-COL/EPI (0.352), MP-ADP and MP-COL (0.235), and between PLW-ADP and PLW-COL (0.247). The range of percent agreement values was 38.7% to 73.8%. Therefore, various measurements of platelet function by more than one method were needed to obtain a reliable interpretation of platelet function considering low kappa coefficient and modest percent agreement rates among 3 different platelet function tests.

Comparison of Multiplate, Platelet Function Analyzer-200, and Plateletworks in Healthy Dogs Treated with Aspirin and Clopidogrel

BACKGROUND

Platelet function testing may be warranted to assess response to aspirin and clopidogrel.

HYPOTHESIS/OBJECTIVES

To evaluate the effects of aspirin, clopidogrel, or combination therapy using 3 platelet function tests: Multiplate Analyzer (MP), Platelet Function Analyzer-200 (PFA), and Plateletworks (PW).

ANIMALS

Six healthy laboratory Beagles.

METHODS

Randomized double-blind placebo-controlled study (crossover design). Dogs were given aspirin 1 mg/kg, clopidogrel 2 mg/kg, or combination therapy for 1 week each, with a washout period of 2 weeks. Platelet function was assessed on days 0 and 7 of each phase using MP (adenosine diphosphate [ADP], arachidonic acid [AA], collagen [COL] agonists), PFA (P2Y, COL-ADP [CADP], COL-Epinephrine [CEPI] cartridges), and PW (ADP, AA, COL agonists). Platelet counts were obtained with impedance and optical counters.

RESULTS

For MP, mean aggregation was decreased for COL and AA with combination therapy and for ADP with all treatments. For PFA, mean CT was increased for the CEPI cartridge with aspirin; and for the P2Y and CADP cartridges with clopidogrel or combination therapy. More dogs receiving clopidogrel showed an increase in PFA CT using the P2Y than the CADP cartridge. For PW, mean aggregation was decreased for AA with all treatments; for ADP with clopidogrel or combination therapy; and for COL with clopidogrel. The PW results with the 2 hematology counters showed almost perfect agreement.

CONCLUSION AND CLINICAL IMPORTANCE

All platelet function tests detected treatment effects in some dogs and may have utility for monitoring therapy.

Assessment of platelet function in healthy cats in response to commonly prescribed antiplatelet drugs using three point-of-care platelet function tests

Objectives The objective was to determine if decreased platelet function could be detected after treatment with aspirin and/or clopidogrel in healthy cats using three point-of-care platelet function tests that evaluate platelet function by different methods: Multiplate (by impedance), Platelet Function Analyzer 100 (by mechanical aperture closure) and Plateletworks (by platelet counting). Methods Thirty-six healthy cats were randomly assigned to receive one of three oral treatments over an 8 day period: (1) aspirin 5 mg q72h; (2) aspirin 20.25 mg q72h; or (3) clopidogrel 18.75 mg q24h. Cats treated with 5 and 20.25 mg aspirin also received clopidogrel on days 4-8. Platelet aggregation in response to adenosine diphosphate and collagen ± arachidonic acid was assessed on days 1 (baseline), 4 and 8. Aspirin and clopidogrel metabolites were measured by high-performance liquid chromatography. Platelet function in response to treatment was analyzed by ANCOVA, linear regression and Spearman correlation. Results The only solitary aspirin effect was detected using Plateletworks with collagen in cats treated with 20.25 mg. The only effect detected by Multiplate was using arachidonic acid in cats treated with both aspirin 20.25 mg and clopidogrel. All clopidogrel treatment effects were detected by Platelet Function Analyzer 100, Plateletworks (adenosine diphosphate) and Plateletworks (collagen). Drug metabolites were present in all cats, but concentrations were minimally correlated to platelet function test results. Conclusions and relevance Platelet Function Analyzer 100 and Plateletworks using adenosine diphosphate ± collagen agonists may be used to detect decreased platelet function in response to clopidogrel treatment. Either aspirin is not as effective an antiplatelet drug as clopidogrel, or the tests used were not optimal to measure aspirin effect. Cats with heart disease are commonly prescribed antiplatelet drugs to decrease the risk of aortic thromboembolism. Platelet Function Analyzer 100 and Plateletworks may be useful for confirming clopidogrel treatment in these cats.

Influence of a low dosage of clopidogrel on platelet function in cats as measured by the platelet function analyser PFA-100 and the multiplate analyser

The antiplatelet drug clopidogrel is widely used for prophylaxis of arterial thromboses in cats in a standard dosage of 18.75mg per cat once daily. The aim of the study was to verify if a reduced daily dose of 10mg clopidogrel per cat has a similar antiplatelet effect as the standard dosage. Platelet function was measured with the platelet function analyser PFA-100^® and a novel impedance aggregometer. Suitability of the platelet function analyser was tested in citrated blood samples of 59 healthy cats and reference ranges were established. In addition, agonist concentrations for impedance aggregometry were optimised. In the main experiment two groups of 6 healthy cats received clopidogrel either in a dosage of 10 or 18.75mg per cat over a period of seven days. Analyses were performed on days 1, 2, 3, 5, and 7. In comparison to baseline both clopidogrel dosages showed an inhibitory effect on results of the platelet function analyser and velocity of ADP-induced platelet aggregation. Values at all times were different from baseline, with the exception of day 1 in cats receiving 10mg clopidogrel where the closure time of the platelet function analysis and part of ADP-induced aggregation did not show a significant difference. Significant differences were not found between the two doses. In conclusion, our study indicates that 10mg clopidogrel per day may be as effective as 18.75mg although the latter may be advantageous as an initial loading dosage to achieve effective levels more rapidly.