Platelet function analyzer (PFA-100): a tool to quantify congenital or acquired platelet dysfunction

Using the Platelet Function Analyzer-100 for monitoring aspirin therapy

INTRODUCTION

The aim of the study was to evaluate the test characteristics of the Platelet Function Analyzer-100 (PFA-100) in patients treated with aspirin.

METHODS AND RESULTS

The study consisted of two sub-studies. In study 1, 10 patients with ischemic heart disease (IHD) and 10 controls had platelet function assessed by optical platelet aggregation and the PFA-100 method in two 5-week periods. Patients with IHD were treated with aspirin 150 mg/day (first 5-week period), and 300 mg/day (second 5-week period), whereas the controls only received aspirin (150 mg/day) during the second 5-week period. From the results of study 1, we found that a cut-off value for the PFA-100 collagen/epinephrine cartridge <165 s identified patients not taking aspirin (sensitivity 0.91, specificity 1.00). A good agreement between the PFA-100 method and optical platelet aggregation was found. Within-subject variation for the PFA-100 collagen/epinephrine cartridge was +/-28%, as compared to +/-17% for the optical platelet aggregation. Study 2 included 298 aspirin treated patients who were admitted with symptoms suggestive of an acute myocardial infarction. Platelet function was assessed in duplicate by the PFA-100 collagen/epinephrine cartridge, and a 95% Limit of Agreement interval at [-65%, 65%] indicated a limited precision.

CONCLUSION

We defined a cut-off value below which patients not taking aspirin can be identified. However, due to imprecision of the PFA-100 method repeated duplicate assessment of the collagen/epinephrine Closure Time is recommended.

A review of inherited platelet disorders with guidelines for their management on behalf of the UKHCDO

The inherited platelet disorders are an uncommon cause of symptomatic bleeding. They may be difficult to diagnose (and are likely to be under-diagnosed) and pose problems in management. This review discusses the inherited platelet disorders summarising the current state of the art with respect to investigation and diagnosis and suggests how to manage bleeding manifestations with particular attention to surgical interventions and the management of pregnancy.

Platelet function in patients with acute coronary syndrome (ACS) predicts recurrent ACS

BACKGROUND

Platelet hyperfunction contributes to acute coronary syndromes (ACS). Thus, we hypothesized that platelet function under high shear stress predicts recurrent ACS during long-term follow-up of ACS patients.

PATIENTS AND METHODS

Consecutive ACS patients (n = 208) were prospectively followed-up for an average of 28 months. Platelet function was measured with the platelet function analyzer (PFA-100; Dade Behring, Marburg, Germany) at baseline for collagen/adenosine diphosphate closure times (CADP-CT) and for collagen/epinephrine closure times (CEPI-CT) after infusion of a uniform dose of 250 mg aspirin.

RESULTS

Of the conventional risk factors, only the prevalence of diabetes was higher in ACS patients with re-events. However, use of clopidogrel and use of beta blockers were also slightly lower in patients with re-events (P < 0.05). The unadjusted risk hazard ratio (HR) for re-events was 3.3 [95% confidence interval (95% CI): 1.4-7.4; P = 0.005] in those patients with the shortest CADP-CT values (lowest quartile). Similarly, the risk was 2.0-fold higher (95% CI: 1.1-3.6; P = 0.02) in ACS patients with CEPI-CT < 300 s as compared with CEPI-CT >or = 300 s. Inclusion of diabetes, clopidogrel and beta blockers in a multivariate Cox regression model enhanced the predictive value of CEPI-CT (HR: 2.7). Inclusion of von Willebrand factor levels did not alter the HR for recurrent ACS (HR: 2.1; 95% CI: 1.1-5.2; P = 0.03) for CEPI-CT < 300 s, but reduced the HR for CADP-CT (HR: 2.8, 95% CI: 0.8-9.8; P = 0.11).

CONCLUSION

Shortened CT values reflect biologically relevant platelet hyperfunction in patients with ACS because they predict recurrent ACS.

POINT OF CARE TESTING OF ASPIRIN RESISTANCE IN PATIENTS WITH VASCULAR DISEASE

INTRODUCTION

The reported range in rates of aspirin resistance (5.5-60%) may reflect difficulties in studying platelet function and the variety of tests used. This study used a platelet function analyzer (PFA-100) to prospectively document aspirin resistance in a cohort of patients with arterial disease.

METHODS

Patients with internal carotid artery (ICA) stenosis or intermittent claudication (IC) were recruited. Exclusion criteria were contraindications to aspirin, prescription of other medication with known antiplatelet effects or known platelet abnormalities. After prescription of 100 mg aspirin/day for 2 weeks an uncuffed venous blood sample was taken and analysed with the PFA-100. Aspirin resistance was defined as closure time (CT) less than the upper limit of normal (158 s collagen/epinephrine agonist; 118 s collagen/adenosine diphosphate (ADP) agonist).

RESULTS

Thirty-three patients with IC and 12 patients with ICA stenosis were recruited (n = 45). Median (range) age was 74 years (49-85) and the male to female ratio was 1.5:1. The median (range) CT was >300 (85 to >300) s with collagen/epinephrine and 100 (52 to >300) s with collagen/ADP agonist. Twelve patients (27%) in the collagen/epinephrine group had normal CT despite treatment with 100 mg aspirin, indicating resistance. Of the 33 patients with collagen/epinephrine CT prolonged by aspirin, 10 patients also had prolonged collagen/ADP CT, suggesting excessive platelet inhibition.

CONCLUSION

A significant proportion of patients taking aspirin do not show laboratory evidence of platelet inhibition and may not be protected from atherothrombotic events. The PFA-100 appears to be a useful tool to screen for both aspirin resistance and excessive aspirin mediated platelet inhibition.

Antiplatelet activity during coadministration of the selective serotonin reuptake inhibitor paroxetine and aspirin in male smokers: a randomized, placebo-controlled, double-blind trial

Depression is associated with an increased incidence of vascular events and develops after stroke and myocardial infarction. Beside potential clinical outcome benefits of selective serotonin reuptake inhibitors for vascular diseases, bleeding events were reported. We investigated whether paroxetine and aspirin synergistically inhibit platelet function. Paroxetine (20 mg/d) was administered over 18 days to 20 men in a randomized, placebo-controlled, crossover design. Aspirin (100 mg/d) was coadministered within the last 4 study days. Platelet function was assessed by the platelet function analyzer and by flow cytometry. Paroxetine prolonged epinephrine-dependent predictive index within 14 days (P<.02). Aspirin enhanced the predictive index (P<.004 vs baseline and P>.05 between periods). A trend toward decreased thrombin receptor-activating peptide-induced CD62P expression after paroxetine was further enhanced by aspirin treatment (P>.05 between periods). The combination of paroxetine and aspirin did not further inhibit platelet plug formation under high shear stress in male smokers.

Coagulation Defects

The present understanding of the coagulation process emphasizes the final common pathway and the proteolytic systems that result in the degradation of formed clots and the prevention of unwanted clot formations, as well as a variety of defense systems that include tissue repair, autoimmune processes, arteriosclerosis, tumor growth, the spread of metastases, and defense systems against micro-organisms. This article discusses diagnosis and management of some of the most common bleeding disorders. The goals are to provide a simple guide on how best to manage patients afflicted with congenital or acquired clotting abnormalities during the perioperative period, present a brief overview of the methods of testing and monitoring the coagulation defects, and discuss the appropriate pharmacologic or blood component therapies for each disease.

Prevalence of aspirin resistance in patients with an evolving acute myocardial infarction

OBJECTIVE

To study the prevalence and importance of aspirin resistance in patients with an evolving acute myocardial infarction (AMI) by use of the Platelet Function Analyzer-100.

INTRODUCTION

Previous studies have demonstrated the existence of aspirin resistance, but the clinical relevance of the phenomenon remains to be clarified. If aspirin resistant patients comprise a high-risk subgroup, it might be expected that the prevalence of aspirin resistance in patients with AMI would be higher than in patients without AMI. We hypothesized that the prevalence of aspirin resistance in patients with AMI was twice the prevalence in patients without AMI.

METHODS

We included 298 consecutive patients with known cardiovascular disease who were admitted to hospital with symptoms suggestive of an AMI. All had been taking aspirin 150 mg/day for at least 7 days prior to hospital admission. Platelet function was measured immediately at admission, and aspirin resistance was defined as a collagen/epinephrine Closure Time (CT(CEPI))<165 s.

RESULTS

We found that 70 (23.5%) patients were aspirin resistant, and 70 (23.5%) patients ended up with the diagnosis of an AMI. The prevalence of aspirin resistance was significantly higher in patients with AMI as compared to patients without (36% versus 20%, OR 2.26, CI 95% 1.19-4.22, p=0.0058). The CT(CEPI) measured at admission was an independent factor associated with an AMI.

CONCLUSIONS

Aspirin resistance is present in almost one fourth of patients admitted to hospital with symptoms suggestive of an AMI, and aspirin resistance is significantly associated with the diagnosis of a definite AMI.

Central laboratory and point of care assessment of perioperative hemostasis

PURPOSE

To review laboratory investigations required for the diagnosis of a constitutional or acquired defect of hemostasis before surgery, or during the perioperative period in context of rapid evolution and possible therapeutic adjustment.

METHODS

A review of the literature.

PRINCIPAL FINDINGS

Systematic preoperative screening is poorly efficient. It should be restricted to patients selected on clinical history and physical examination. Intra- and postoperative investigation is oriented by the clinical circumstances. In most instances, laboratory investigation is organized as hierarchical steps, the first one including a platelet count, a prothrombin time and activated partial thromboplastin time. According to the results of these assays and to the clinical circumstances, further tests specific of primary hemostasis, coagulation or fibrinolysis are performed. Point of care (POC) monitoring has been developed more recently for the investigation of a perioperative bleeding. Several tests examine platelet functions, others measure whole blood activated partial thromboplastin time or prothrombin time and some explore global hemostasis, allowing the detection of excessive fibrinolysis. Point of care testing provides a rapid and valuable answer but, if one accepts the monitoring of unfractionated heparin during extracorporeal circulation, most assays have not been properly validated.

CONCLUSION

The investigation of hemostasis at the central laboratory and POC testing have distinct objectives. The utility of the former for the diagnosis and the adjustment of therapeutics have been well demonstrated. In contrast, the experience with POC testing is relatively recent, and its utility for patient management remains to be demonstrated in proper clinical trials.

Validation of rotation thrombelastography in a model of systemic activation of fibrinolysis and coagulation in humans

BACKGROUND

Thrombelastography (TEG) is a whole blood assay to evaluate the viscoelastic properties during blood clot formation and clot lysis. Rotation thrombelastography (e.g. ROTEM) has overcome some of the limitations of classical TEG and is used as a point-of-care device in several clinical settings of coagulation disorders. Endotoxemia leads to systemic activation of the coagulation system and fibrinolysis in humans.

OBJECTIVES

We validated whether ROTEM is sensitive to endotoxin induced, tissue factor-triggered coagulation and fibrinolysis and if its measures correlate with biohumoral markers of coagulation and fibrinolysis.

PATIENTS AND METHODS

Twenty healthy male volunteers participated in this randomized placebo-controlled trial. Volunteers received either 2 ng kg(-1) National Reference Endotoxin or saline.

RESULTS

Endotoxemia significantly shortened ROTEM clotting time (CT) by 36% (CI 0.26-0.46; P < 0.05) with a strong inverse correlation with the peak plasma levels of prothrombin fragments (F(1 + 2)) (r = -0.83, P < 0.05). Additionally, endotoxin infusion enhanced maximal lysis (ML) 3.9-fold (CI: 2.5-5.2) compared with placebo or baseline after 2 h (P < 0.05). Peak ML and peak tissue plasminogen activator (t-PA) values correlated excellently (r = 0.82, P < 0.05). ROTEM parameters clot formation time and maximal clot firmness were not affected by LPS infusion, whereas platelet function analyzer (PFA-100) closure times decreased.

CONCLUSIONS

Rotation thrombelastography (ROTEM) detects systemic changes of in vivo coagulation activation, and importantly it is a point of care device, which is sensitive to changes in fibrinolysis in humans. The ex vivo measures CT and ML correlate very well with established in vivo markers of coagulation activation (F(1 + 2)) and fibrinolysis (t-PA), respectively.

Platelet function analyzer (PFA)-100 closure time in the evaluation of platelet disorders and platelet function

BACKGROUND

Closure time (CT), measured by platelet function analyzer (PFA-100) device, is now available to the clinical laboratory as a possible alternative or supplement to the bleeding time test.

AIM

On behalf of the Platelet Physiology Subcommittee of the Scientific and Standardization Committee of the International Society on Thrombosis and Haemostasis (ISTH-SSC), a working Group was formed to review and make recommendations on the use of the PFA-100 CT in the evaluation of platelet function within the clinical laboratory.

METHODS

The Medline database was searched to review the published information on the PFA-100 CT in the evaluation of platelet disorders and platelet function. This information, and expert opinion, was used to prepare a report and generate consensus recommendations.

RESULTS

Although the PFA-100 CT is abnormal in some forms of platelet disorders, the test does not have sufficient sensitivity or specificity to be used as a screening tool for platelet disorders. A role of the PFA-100 CT in therapeutic monitoring of platelet function remains to be established.

CONCLUSIONS

The PFA-100 closure time should be considered optional in the evaluation of platelet disorders and function, and its use in therapeutic monitoring of platelet function is currently best restricted to research studies and prospective clinical trials.

Duration of effects of aspirin on platelet function in healthy volunteers: an analysis using the PFA-100

STUDY OBJECTIVE

The aim of the study was to determine the duration and effects of aspirin on platelet function.

STUDY DESIGN

Prospective investigation.

SETTING

Blood samples from volunteers. PATIENTS AND MONITORING: Ten healthy male volunteers took part in this investigation. After having measured baseline hemostatic parameters, 2000-mg aspirin was taken orally. Subsequently, the hemostatic profile had been compiled daily for a duration of 4 days after ingestion.

MEASUREMENTS AND MAIN RESULTS

Platelet function was analyzed (1) after 3 hours and (2) daily for 4 days by the platelet function analyzer (PFA-100, Dade Co, Miami, Fla), which represents a sensitive investigation method for measuring platelet function and dysfunction. Routine hemostatic parameters were investigated. Three hours after ingestion of aspirin, abnormal PFA-100 values could be detected. Further detectable elevated values were found during the following 3 days.

CONCLUSION

Effects of single-dose aspirin 2000 mg on platelet function were detected after 3 hours and had been lasting for the following 3 days. This result shows that PFA-100 may help by evaluating hemostasis during the preoperative period.

Platelet activation predicts recurrent ischemic events after percutaneous coronary angioplasty: a 6 months prospective study

INTRODUCTION

An increasing amount of evidence indicates that platelet reactivity, despite a standard anti-thrombotic therapy, is a potential risk factor for recurrent myocardial ischemia in patients with coronary artery disease. We now hypothesize that this condition, measured by collagen-epinephrine (CEPI) or collagen-ADP (CADP) closure times (CT) by Platelet Function Analyzer (PFA-100), may predict the recurrence of coronary events after percutaneous coronary intervention (PCI).

MATERIALS AND METHODS

CEPI and CADP-CT were measured 30+/-8 h after PCI in 175 consecutive patients admitted with a diagnosis of stable angina (n=94) or acute coronary syndromes (n=81) and prospectively followed up for a mean period of 6 months. We stratified the patients in accordance to both the CEPI-CT (<or> 190 s), reflecting the intensity of cycloxygenase inhibition by aspirin and the distribution into quartiles for CADP-CT.

RESULTS

CEPI-CT<190 s as well as CADP-CT<82 s were associated with a higher rate of clinical recurrence (hazard ratio 8.5, p<0.001 and 22.9, p<0.001, respectively). Multivariate analysis after adjustment for other risk factors confirmed that the lowest CADP-CT quartile significantly correlates with the risk of recurrent coronary events (hazard ratio 36.5, p<0.01), as well as CEPI-CT<190 s (hazard ratio 6.7, p=0.01).

CONCLUSIONS

An enhanced platelet function after PCI when measured under high shear rates by PFA-100 is an independent predictor of a worst clinical outcome, even during a short term follow-up and may help in patients risk stratification.

The role of exercise on platelet aggregation in patients with stable coronary artery disease: exercise induces aspirin resistant platelet activation

OBJECTIVES

The aim of our study was to determine the relation between exercise stress test and aspirin resistance in patients with stable coronary artery disease.

BACKGROUND

Clinically aspirin resistance is defined as having thrombotic and embolic cardiovascular events despite regular aspirin therapy.

METHODS

We studied platelet functions of 62 patients with stable coronary artery disease and 20 subjects with normal coronary arteries by Platelet Function Analyzer (PFA-100, Dade Behring, Germany) at rest and after exertion with collagen and/or epinephrine (Col/Epi) and collagen and/or ADP cartridges. Closure time (CT)<186 seconds was defined as aspirin resistance with Col/Epi cartridges of PFA-100. Symptom limited treadmill stress test (protocol of Bruce) was performed with Oxford Streslink TD-1 system.

RESULTS

8 (12.9%) patients were aspirin resistant by PFA-100 (CT<186s despite regular aspirin therapy) at rest. At the first minute of the recovery period of exercise stress test 14 (22.5%) patients were aspirin resistant by PFA-100. CTs with Col/ADP were respectively 89+/-6 s (83--100s) and 89+/-5 s (82--104s) at rest and after exercise (p=0.107). 20.3% (11/54) of patients known as in vitro aspirin sensitives at rest had shorter CTs and 11.1% (6/54) had aspirin resistance after exercise (p=0.004). There was no statistically significiant difference in platelet functions in the control group after exertion.

CONCLUSION

We conclude that 11.1% of in vitro aspirin sensitive subjects at rest had aspirin resistance after exercise by PFA-100. In some individuals, exercise induced platelet activation is aspirin insensitive at usual antiplatelet doses. We need further clinical trials to optimize antiplatelet therapy in patients with coronary artery disease.

Platelet hyper-function in acute coronary syndromes

Previous studies have demonstrated shortened bleeding times in patients with acute coronary syndromes, especially in myocardial infarction (MI). In this study we have investigated platelet hyper-function using the PFA-100 with collagen/adenosine diphosphate and collagen/epinephrine cartridges in 78 patients presenting with acute chest pain. Patients were classified into MI, unstable angina (UA) and non-specific chest pain. All patients received 300 mg aspirin (ASA) more than 2 h before blood samples were collected. Twenty healthy normal subjects were also tested before and 2 h after 300 mg ASA (n = 10). The collagen/adenosine diphosphate closure time was significantly shorter in MI patients (median, 71 s; P = 0.0237) but not in UA patients (median, 81 s; P > 0.05) compared with normal subjects (median, 92.5 s). The collagen/epinephrine closure times were significantly longer in UA patients (median, 233 s) than in untreated controls (median, 125 s; P < 0.0001), as expected, but there was no difference in MI patients (median, 149.24 s; P > 0.05), suggesting that the MI patients were not all responding to ASA. Analysis of a subset of the apparent ASA non-responders (n = 5) by platelet aggregation demonstrated that this was not related to failure of ASA to block cyclo-oxygenase activity. Von Willebrand factor levels were significantly elevated in both UA and MI patients compared with normal subjects (mean, 175.5 and 248.9 versus 89.1 s; P < 0.0001 and P < 0.0001, respectively) and were also significantly higher in the MI group compared with the UA group (P < 0.05). There is evidence for platelet hyper-function and elevated von Willebrand factor levels in the MI group that could explain their decreased responsiveness to ASA on the collagen/epinephrine cartridge.

The PFA-100

The PFA-100 is a laboratory test designed to measure platelet function. Adequate platelet function depends upon the platelet's ability to adhere to the site of endothelial injury, activate surface receptors to attract other platelets and aggregate or clump together to form a platelet plug. This process is necessary to stop bleeding but may be harmful if it causes occlusion of a vessel. Platelet altering medication therapy is widely used in the prevention and treatment of coronary artery disease, peripheral arterial occlusive disease, and cerebrovascular ischemia. The PFA-100 provides the clinician with valuable information about platelet function. This information is helpful in determining the therapeutic effectiveness of antiplatelet medications, assisting in evaluating risk of bleeding and identifying primary platelet dysfunction.

Variables influencing Platelet Function Analyzer-100 closure times in healthy individuals

We investigated the relationship between platelet function analyzer (PFA-100) closure times (CT) and bleeding time (BT), platelet aggregation (PA) induced by ADP, arachidonic acid, and collagen, blood cell counts, and von Willebrand factor (VWF) in 120 well-characterised healthy individuals. Pre-analytical and analytical conditions were standardised comprehensively. In a substantial number of cases the differences between duplicate measurements exceeded 15%. The reference range (5th and 95th percentiles) for CT with the collagen/epinephrine (CEPI) and the collagen/ADP (CADP) cartridge was 93-223 s and 64-117 s respectively. Re-examination of 11 individuals with CEPI-CT above the 95th percentile revealed considerable batch-to-batch variation of CEPI-CT. Males had significantly longer CADP than females (P = 0.002). CEPI and CADP-CT measured pm were significantly longer than corresponding values determined am (P = 0.003 and P < 0.0001 respectively). Blood group O was associated with greater CEPI and CADP-CT and lower VWF levels compared with non-O blood groups (P = 0.008, P = 0.0003 and P < 0.0001 respectively). Linear regression analysis revealed association between CEPI-CT, CADP-CT and VWF (P < 0.0001), but no relationship was found between CT and BT or between CT and PA. We conclude that VWF plasma levels modulate PFA-100 CT to a greater extent than platelet function. Establishment of reliable reference ranges and careful standardisation of pre-analytical and analytical conditions is a prerequisite for obtaining reliable PFA-100 results. Duplicate measurements are necessary.

Evaluation of platelet function under high shear condition in the small-sized collagen bead column

We previously reported that platelet retention rates as measured with collagen-coated bead columns (the conventional column) reflect the processes of platelet adhesion and aggregation under low shear stress, and that this system could serve as an easy-to-use platelet aggregometry. With this column, platelet glycoprotein (GP) VI and GPIIb/IIIa, but not the GPIb-von Willebrand factor (VWF) interaction, play major roles in platelet activation. To develop a system that can better reflect the GPIb-VWF interaction under high shear stress, we designed a column containing small-sized beads (125-212 microm) coated with porcine collagen type I. As expected, the GPIb-VWF interaction played a crucial role in platelet retention rates at higher flow rates. Adenosine 5'-diphosphate, but not thromboxane A2, appears to support platelet activation in this system. The platelet retention rates among healthy individuals with the new columns are in the range wider than the conventional columns, and this diversity could be attributed to the broad range of the VWF antigen and/or its activity. It is suggested that this new column can serve as an easy-to-use method for evaluating the VWF antigen levels and its activity and for monitoring patients with thrombotic or bleeding disorders related to the VWF-GPIb interaction.

No alteration in the PFA-100 in vitro bleeding time induced by the Ginkgo biloba special extract, EGb 761, in elderly patients with mild cognitive impairment

EGb 761 is widely used in the management of mild cognitive impairment in the elderly population. Elucidation of the effects of EGb 761 on primary haemostasis via PFA-100 could represent an important step for better understanding of the haemostatic safety of EGb 761. The purpose of this prospective study is to assess the effects of Ginkgo biloba special extract, EGb 761, on PFA-100 in vitro bleeding time in elderly patients with mild cognitive impairment. A total of 40 elderly patients aged 65-79 years who were referred for geriatric assessment and who were diagnosed as having mild cognitive impairment were included. Patients were started on 80 mg EGb-761 three times daily. The complete set of PFA-100 in vitro bleeding time and coagulation parameters including prothrombin time, activated partial thromboplastin time and International Normalized Ratio were assessed before and on the seventh day of treatment with EGb 761. There was no statistically significant prolongation in PFA-100 in vitro bleeding time or coagulation parameters in patients receiving EGb 761 after 7 days. The data about the safety of EGb 761 from the point of primary haemostasis in our elderly patient population with mild cognitive impairment casts hope for the future management of this 'difficult-to-treat' population with the promising Ginkgo extracts.

Aspirin Resistance: An Evaluation of Current Evidence and Measurement Methods

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

The role of PFA-100R testing in the investigation and management of haemostatic defects in children and adults

The PFA-100 provides a simple global measure of high shear-dependent platelet function, and as such is not diagnostic or specific to any disorder. Prolonged closure times must be interpreted in conjunction with a full blood count, von Willebrand factor (VWF) screen and other platelet tests. The PFA-100 may also give false negative results with relatively common platelet defects. If clinical suspicion is high, further detailed platelet function testing and VWF screening are required to exclude abnormal platelet function, even if the PFA-100 is normal. In more recent studies the PFA-100 has been used for preoperative identification and management of surgical patients with haemostatic defects and for assessing the clinical effectiveness of platelet transfusion therapy. This review highlights the up to date, evidence-based, advantages and disadvantages of the PFA-100 test in the investigation and management of haemostatic disorders in both children and adults.

Screening for aspirin responsiveness after transient ischemic attack and stroke: comparison of 2 point-of-care platelet function tests with optical aggregometry

BACKGROUND AND PURPOSE

Recent studies suggest that patients who do not respond to aspirin (ASA) therapy may be at increased risk of ischemic vascular events. The availability of simple to use point-of-care (POC) platelet function tests now potentially allows aspirin nonresponsiveness to be identified in routine clinical practice. However, there are very few data on whether the different tests produce consistent results. We therefore compared 2 POC tests (PFA-100 device and the Ultegra-RPFA [RPFA]) with conventional light transmission aggregometry (LTA).

METHODS

Platelet function was assessed by all 3 tests in 100 patients receiving low-dose ASA therapy after transient ischemic attack (TIA) or ischemic stroke.

RESULTS

The incidence of ASA nonresponsiveness was 17% by the RPFA and 22% by the PFA-100, compared with only 5% by LTA (ie, as defined with both arachidonic acid and ADP). Agreement between the RPFA and the PFA-100 and arachidonic acid induced LTA was poor (kappa=0.16, 95% CI, -0.08 to 0.39, P=0.11; and kappa=0.09 -0.12 to 0.30, P=0.32, respectively). Agreement between the 2 POC tests was also poor (kappa=0.14, -0.08 to 0.36, P=0.15). Only 2% of patients were aspirin nonresponders by all 3 tests.

CONCLUSIONS

The prevalence of apparent ASA nonresponsiveness was higher with both the POC tests than with LTA. However, agreement between the tests was poor and very few patients were ASA nonresponsive by all 3 tests. Aspirin nonresponsiveness is therefore highly test-specific and large prospective studies will be required to determine the prognostic value of each of the separate tests.

The role of platelet glycoprotein IIIa polymorphism in the high prevalence of in vitro aspirin resistance in patients with intracoronary stent restenosis

OBJECTIVES

The aim of our study was to determine the relation between platelet glycoprotein IIIa (Pl A ) polymorphism and aspirin resistance in patients with intracoronary stent restenosis.

BACKGROUND

Clinically, aspirin resistance is defined as having thrombotic and embolic cardiovascular events despite regular aspirin therapy. Platelet glycoprotein IIIa polymorphism is said to be a possible mechanism of aspirin resistance.

METHODS

We studied the prevalence of aspirin resistance in 204 previously intracoronary stent-implanted patients with stable coronary artery disease. In 102 of these patients, intracoronary stent restenosis was present. Platelet functions were analyzed in a platelet function analyzer (PFA-100, Dade Behring, Germany) with collagen and/or epinephrine (Col/Epi) and collagen and/or adenosine diphosphate cartridges. Closure time <186 seconds was defined as aspirin resistance with Col/Epi cartridges of PFA-100. The Pl A polymorphisms of 43 aspirin-resistant and 51 aspirin-sensitive subjects were determined with polymerase chain reaction and restriction fragments length polymorphism.

RESULTS

A total of 31.3% (n = 32) of patients with intracoronary stent restenosis and 10.7% (n = 11) of patients with patent intracoronary stents were resistant to aspirin by PFA-100. The Pl A1,A1 allele of glycoprotein IIIa was present in 36 subjects (83.7.%) and the Pl A1,A2 allele was present in 7 subjects (16.2.%) in the aspirin-resistant patients group. The Pl A1,A1 allele of glycoprotein IIIa was present in 37 subjects (72.5%) and the Pl A1,A2 allele was present in 14 subjects (27.5%) in the aspirin-sensitive patients group ( P = .195).

CONCLUSION

Our results suggest that platelets of patients with intracoronary stent restenosis with or without Pl A2 heterozygosity of glycoprotein IIIa are more likely to be resistant to low-dose aspirin therapy.

Additive effects between platelet concentrates and desmopressin in antagonizing the platelet glycoprotein IIb/IIIa inhibitor eptifibatide

BACKGROUND

Platelet (PLT) glycoprotein (GP) IIb/IIIa receptor antagonists have demonstrated efficacy in decreasing ischemic complications of percutaneous coronary intervention and/or unstable angina. In case of bleeding, the drug can be stopped and PLT transfusions can be given.

STUDY DESIGN AND METHODS

This crossover study tested the additive effects of PLT concentrates (PCs) after desmopressin (DDAVP) infusion in antagonizing the anti-PLT effects of GPIIb/IIIa inhibitors and aspirin. After eptifibatide and aspirin infusion (at standard dosages), 10 healthy volunteers received DDAVP or placebo. Thereafter, increasing amounts of PLTs from fresh single-donor apheresis concentrates were added in vitro to blood samples of all volunteers to increase PLT counts by 30 x 10(9), 60 x 10(9), or 120 x 10(9) per L.

RESULTS

Adding platelets in vitro further improved PLT function after DDAVP: it shortened collagen-adenosine diphosphate closure times (p < 0.01), to normal ranges as measured by the PLT function analyzer (PFA-100). In contrast, normal PLT function could not be restored even when PLT counts were increased by 50 percent (120 x 10(9)/L) in the placebo group.

CONCLUSION

Combined use of PLTs from fresh apheresis PC and DDAVP additively enhances recovery of normal PLT function after eptifibatide infusion. Such a strategy may help to avoid excessive transfusion of PC.

Platelet function analysis

Since the last guidelines for BCSH platelet function testing were written in the late 1980s, many new tests have become available. Previously most platelet function tests were traditionally utilized to aid in the diagnosis and management of patients with platelet and haemostatic disorders. Most traditional tests were also largely restricted to the specialized laboratory or centre. However, nowadays there is also much renewed interest in monitoring the efficacy of anti-platelet therapy and measuring platelet hyper-function. A number of dedicated platelet function instruments have now become available that are much simpler to use and are beginning to be utilized as point of care instruments. These can now provide measurement of platelet function within whole blood without the requirement of sample processing. Some are also beginning to be incorporated into routine clinical use and can be utilized as not only as general screening tests of platelet function but to monitor anti-platelet therapy and to potentially assess both risk of bleeding and/or thrombosis. Modern flow cytometric-based platelet function analysis now also provides a wide variety of specific tests that can assess different aspects of platelet biology that are useful for diagnostic purposes. This review will highlight some of these of new tests/instruments and discuss their potential utility both within the haemostasis laboratory but also as potential point of care instruments.

Short- and long-term effects of therapy with interferon-alpha and pegylated interferon-alpha/ribavirin on platelet plug formation and von Willebrand factor release in patients with chronic hepatitis C

BACKGROUND

A pegylated interferon-alpha-induced decrease in platelet counts may become a limiting factor for continuation of therapy.

AIM

To evaluate the effect of pegylated interferon-alpha administration on platelet plug formation and von Willebrand factor antigen release in patients with chronic hepatitis C.

METHODS

Thirty patients with chronic hepatitis C (genotype 1; fibrosis 1-3: n = 16, cirrhosis: n = 14) received a single dose of 9 MU interferon-alpha2a, followed by weekly administration of 180 mug of pegylated interferon-alpha2a/ribavirin for 48 weeks. Platelet counts, platelet function (collagen-epinephrine-induced closure time) and von Willebrand factor antigen were measured.

RESULTS

Platelet counts and collagen-epinephrine-induced closure time decreased by 13% and 16%, respectively, 24 h after the first dose of interferon-alpha2a, and von Willebrand factor antigen levels increased by 31% (P < 0.01) compared with baseline. During a 48-week observation period, platelet counts decreased by a maximum of 33% (P < 0.001), von Willebrand factor antigen levels increased by 69% (P < 0.001) whereas collagen-epinephrine-induced closure time did not change. In noncirrhotic patients, the increase of von Willebrand factor antigen levels was maintained throughout therapy without a change in collagen-epinephrine-induced closure time. In contrast, in cirrhotics, von Willebrand factor antigen levels did not increase, while collagen-epinephrine-induced closure time was prolonged.

CONCLUSION

Single-dose interferon-alpha decreases platelet counts but improves platelet function, possibly by the release of von Willebrand factor antigen. Accordingly, long-term antiviral treatment had no effect on collagen-epinephrine-induced closure time, despite the decrease in platelet count in noncirrhotic patients. Such a compensation of decreased platelet counts by increased von Willebrand factor antigen level did not occur in cirrhotics.

Platelets and anti-platelet therapy

Platelets play a central role in the hemostatic process and consequently are similarly involved in the pathological counterpart, thrombosis. They adhere to various subendothelial proteins, exposed either by injury or disease, and subsequently become activated by the thrombogenic surface or locally produced agonists. These activated platelets aggregate to form a platelet plug, release agonists which recruit more platelets to the growing thrombus, and provide a catalytic surface for thrombin generation and fibrin formation. These platelet-rich thrombi are responsible for the acute occlusion of stenotic vessels and ischemic injury to heart and brain. A range of anti-platelet drugs are currently used, both prophylactically and therapeutically, in regimens to manage thrombo-embolic disorders. These include inhibitors of the generation, or effects, of locally produced agonists; several large clinical trials have supported roles for cyclooxygenase inhibitors, which prevent thromboxane generation, and thienopyridine derivatives, which antagonize ADP receptors. Similarly intravenous alpha IIb beta 3 antagonists have been shown to be effective anti-thrombotics, albeit in highly selective situations; in contrast, to date studies with their oral counterparts have been disappointing. Recent advances in understanding of platelet physiology have suggested several novel, if yet untested, targets for anti-platelet therapy. These include the thrombin receptor, the serotonin handling system, and the leptin receptor.

Platelet function predicts myocardial damage in patients with acute myocardial infarction

BACKGROUND

Platelet activation is a hallmark of acute coronary syndromes. Numerous lines of evidence suggest a mechanistic link between von Willebrand factor or platelet hyperfunction and myocardial damage in patients with acute coronary syndromes. Thus, we assessed whether platelet function under high shear rates (collagen adenosine diphosphate closure times [CADP-CTs]) measured with the platelet function analyzer (PFA-100) may be enhanced in patients with myocardial infarction (MI) and whether it may predict the extent of myocardial damage as measured by creatine kinase (CK-MB) or troponin T (TnT) levels.

METHODS AND RESULTS

Patients with acute chest pain or symptoms suggestive of acute coronary syndromes (n=216) were prospectively examined at an emergency department. CADP-CT was significantly shorter in patients with MI, particularly in those with an ST-segment-elevation MI (STEMI) compared with the other patient groups (unstable angina, stable coronary artery disease, or controls). Furthermore, CADP-CT and collagen epinephrine-CT at presentation were independent predictors of myocardial damage as measured by CK-MB or TnT. Patients with MI whose CADP-CT values fell in the first quartile had 3-fold higher CK-MB and TnT levels than those in the fourth quartile.

CONCLUSIONS

Patients with STEMI have significantly enhanced platelet function when measured under high shear rates. CADP-CT is an independent predictor of the severity of MI, as measured by markers of cardiac necrosis. Measurement of platelet function with the PFA-100 may help in the risk stratification of patients presenting with MI.

High incidence of defective high-shear platelet function among platelet donors

BACKGROUND

Because single-donor plateletpheresis concentrates now account for a large percentage of PLT concentrates, the PLT quality of individual donations is increasingly important. There has been little previous work on PLT function in blood donors.

STUDY DESIGN AND METHODS

The aim of this study was to investigate the prevalence of defective PLT function among 100 healthy UK PLT donors on 3 consecutive donation days. Citrated blood samples were taken before plateletpheresis. High-shear PLT function testing was performed by a PLT function analyzer (PFA-100, Dade Behring), within both collagen-ADP (CADP) and collagen-epinephrine (CEPI) cartridges.

RESULTS

Sixteen percent of PLT donors (mainly over the 3 donating days) had prolonged CEPI closure times (CTs) only, indicative of an aspirin-like pattern; 9 percent had a severe defect with prolonged CEPI CTs between 200 and 300 seconds, and 4 percent had maximal CEPI CTs of greater than 300 seconds. The majority of prolonged CEPI CTs appeared transient in nature.

CONCLUSION

PLT dysfunction as detected by the PFA-100 is surprisingly common within a voluntary PLT donor population. The majority of CEPI defects appeared transient in nature and are indicative of surreptitious intake of cyclooxygenase inhibitors (e.g., aspirin). Identification of defective PLT function within donors would provide a simple means to eradicate defective PLTs from being clinically utilized.

A multicenter clinical evaluation of the Clot Signature Analyzer

BACKGROUND

The Clot Signature Analyzer (CSA) was designed to assess global hemostasis as a screening assay using non-anticoagulated whole blood. Three different measurements are produced by the instrument: platelet hemostasis time (PHT), clot time (CT), and collagen-induced thrombus formation (CITF).

OBJECTIVES

The purpose of the present study was to determine normal ranges for these measurements and assess the performance of the CSA in patients with well-characterized hemostatic disorders and in normal subjects.

PATIENTS AND METHODS

Four institutions participated in the study. Each established their own normal reference ranges. Patients with well-characterized hemostatic disorders and concurrent normal controls were subsequently examined.

RESULTS

Normal ranges between institutions were similar although statistically different. One hundred and eight patients were examined: 46 individuals with von Willebrand disease (VWD) (type 1, 26; type 2A, 11; type 2B, six; type 3, three); 38 patients with a coagulation factor deficiency; 13 individuals with platelet function defects; 10 patients taking warfarin; and one individual on low-molecular-weight heparin. Of these patients, 89% had at least one abnormality by CSA: 42/46 VWD patients, 35/38 coagulation protein defect patients, 9/13 patients with platelet function defects, 9/10 patients on warfarin and 1/1 patient on low-molecular-weight heparin. Of 116 normal subjects, 103 (89%) fell within the centers' normal range. These data suggest that the CSA has a good sensitivity for bleeding disorders.

Variable inhibition of high-shear-induced platelet plug formation by eptifibatide and tirofiban under conditions of platelet activation and high von Willebrand release: a randomized, placebo-controlled, clinical trial

BACKGROUND

Glycoprotein (GP) IIb/IIIa antagonists have become a mainstay for the treatment of acute coronary syndromes. Yet, they have rarely been evaluated under relevant pathophysiologic conditions, for example, high shear rates in the presence of physiologic calcium concentrations. We compared the efficacy of eptifibatide and tirofiban versus placebo on high shear-induced platelet plug formation in a model in which healthy subjects exhibit von Willebrand factor concentrations and platelet activation comparable to patients with acute coronary syndromes.

METHODS

Thirty male volunteers received 2 ng/kg endotoxin and standard doses of eptifibatide, tirofiban, or placebo over a period of 5 hours in a randomized, double-blinded, placebo-controlled, double-dummy parallel-group trial. Platelet inhibition was measured with the Platelet Function Analyzer-100 (PFA-100) and the Ultegra method.

RESULTS

Although bolus infusion of both GPIIb/IIIa antagonists inhibited high shear-induced platelet plug formation, continuous infusion of eptifibatide prolonged closure times more effectively than did tirofiban (P <.008). Interestingly, tirofiban had only placebo-like effects on platelet plug formation after 2 hours. However, when additional drug was exogenously added, closure time values were maximally prolonged in all cases.

CONCLUSIONS

Standard doses, particularly of tirofiban, have limited impact on high shear-induced platelet plug formation at physiologic Ca(2+) concentrations.

Desmopressin antagonizes the in vitro platelet dysfunction induced by GPIIb/IIIa inhibitors and aspirin

Whereas bleeding is the most frequent adverse event encountered in patients receiving glycoprotein (GP) IIb/IIIa inhibitors, there are currently no recommendations for how to treat such patients. The present study tested the hypothesis that infusion of desmopressin (DDAVP) reverses the in vitro platelet dysfunction induced by GPIIb/IIIa inhibitors (+l-aspirin). Study group 1 (10 healthy volunteers) received a DDAVP infusion to establish dose-response curves for the in vitro inhibition of platelet function by eptifibatide, abciximab, and tirofiban together with l-aspirin before and after DDAVP. In a randomized, double-blind, placebo-controlled, crossover study (group 2) volunteers received l-aspirin and a standard eptifibatide infusion. Thereafter, DDAVP or a physiologic saline infusion was given over 30 minutes. In group 1, all GPIIb/IIIa inhibitors prolonged collagen-epinephrine (CEPI) and collagen-adenosine diphosphate (CADP) closure times (CTs), measured with the platelet function analyzer 100 (PFA-100). DDAVP caused a shift in the concentration response curves to the right of all 3 GPIIb/IIIa inhibitors. In group 2, DDAVP accelerated the normalization of CADP-CT and CEPI-CT after the stop of eptifibatide infusion with a maximum effect at 1.5 hours to 2 hours. In contrast, CEPI-CT remained above normal in the placebo group for more than 4 hours. In conclusion, DDAVP accelerates normalization of the in vitro platelet dysfunction induced by GPIIb/IIIa inhibitors (+l-aspirin).

Mechanisms of platelet retention in the collagen-coated-bead column

Although the glass-bead column has been used to measure platelet adhesion, whether platelet interaction with glass beads represents physiologic processes remains unsettled. In an attempt to obtain more physiologic platelet responses, plastic beads coated with type I collagen have been recently developed to replace glass beads. In this study, we analyzed the factors responsible for platelet retention in the collagen-coated-bead column and investigated its possible clinical applications. We pumped citrated whole-blood samples into columns at a fixed speed with an injection pump and calculated platelet-retention rates by measuring platelet counts before and after passage through the columns. The platelet-retention rates, which were highly reproducible with samples from healthy donors, were reduced in a patient with glycoprotein (GP) VI deficiency but not in patients with type III von Willebrand disease. Anti-GPIIb/IIIa antibody and GRGDS peptide markedly inhibited platelet retention, whereas inhibition of the GPIb-von Willebrand factor or GPIa/IIa-collagen interaction had no effect. Data on the effects of various antiplatelet agents (including the antithrombin agent argatroban, prostacyclin, acetylsalicylic acid, and the ADP scavenger creatine phosphate/creatine phosphokinase) support the usefulness of this assay method in clinical application. Our findings suggest that GPVI and GPIIb/IIIa but not the GPIb-von Willebrand factor interaction are mainly involved in platelet retention in this column.

Decreased Platelet Function in Cavalier King Charles Spaniels with Mitral Valve Regurgitation

With aggregometry, increased platelet activity has been reported in Cavalier King Charles Spaniels (CKCS) without mitral regurgitation (MR). In contrast, dogs with MR have been found to have decreased platelet activity. The purpose of this study was to test an easy bedside test of platelet function (the Platelet Function Analyzer [PFA-100]) to see if it could detect an increase in platelet activity in CKCS without MR and a decrease in platelet activity in CKCS with MR. This study included 101 clinically healthy dogs > 1 year of age: 15 control dogs of different breeds and 86 CKCS. None of the dogs received medication or had a history of bleeding. The PFA-100 evaluates platelet function in anticoagulated whole blood under high shear stress. Results are given as closure times (CT): the time it takes before a platelet plug occludes a hole in a membrane coated by agonists. The CT with collagen and adenosine-diphosphate as agonists was similar in control dogs (median 62 seconds; interquartile interval 55-66 seconds) and CKCS with no or minimal MR (55; 52-64 seconds). The CT was higher in CKCS with mild MR (regurgitant jet occupying 15-50% of the left atrial area) (75; 60-84 seconds; P = .0007) and in CKCS with moderate to severe MR (jet > 50%) (87: 66-102 seconds; P < .0001). CKCS with mild, moderate, and severe, clinically inapparent MR have decreased platelet function. The previous finding of increased platelet reactivity in nonthrombocytopenic CKCS without MR could not be reproduced with the PFA-100 device.

Platelet function assays

The roles of platelets in hemostasis and arterial thrombosis involve their adherence to sites of vessel injury or ruptured atherosclerotic plaques, aggregation to form hemostatic plugs or thrombi, and acceleration of the coagulation cascade leading to the formation of thrombin. These roles of platelets are described in this review, hereditary platelet defects and other abnormalities associated with bleeding disorders are listed, and the various aggregating agents are discussed. A number of tests of platelet function are reviewed, including a description of their advantages and disadvantages: bleeding time determination; measurement of platelet aggregation in citrated platelet-rich plasma by recording changes in light transmission; measurement of platelet aggregation in citrated whole blood by impedance aggregometry; measurement of platelet-related hemostasis with the high shear Platelet Function Analyzer (PFA-100) system and the Ultegra Rapid Platelet Function Assay; use of the Cone and Plate(let) Analyzer to measure platelet adherence and aggregation under conditions of high shear; measurement of secretion of granule contents (ATP, 14C-serotonin, platelet factor 4, beta-thromboglobulin) and the formation of thromboxane B(2); and use of flow cytometry to assess the state of platelet activation (including conformational changes in membrane glycoproteins and surface expression of P-selectin and phosphatidylserine) ex vivo and in vitro following addition of agonists, and to measure levels of platelet membrane glycoproteins in the detection of inherited deficiencies.

Platelet function under aspirin, clopidogrel, and both after ischemic stroke: a case-crossover study

BACKGROUND AND PURPOSE

Combined antiplatelet agents may offer additive protection over single drugs after stroke. We investigated whether platelet activation is reduced under combined aspirin and clopidogrel compared with each drug alone.

METHODS

In a case-crossover study, 31 patients with previous atherothrombotic or lacunar stroke who were treated with aspirin (100 to 300 mg/d) received clopidogrel (75 mg/d) and both aspirin and clopidogrel for 4 weeks. Platelet function in whole blood was studied after each treatment period and in healthy control subjects to assess activation-dependent antigens CD62p and CD63 by flow cytometry and collagen/epinephrine (CEPI-CT) and collagen/ADP (CADP-CT) closure times with the platelet function analyzer PFA-100, which investigates platelet-related function under shear stress.

RESULTS

CD62p expression and CD63 expression were not different under the 3 treatment regimens. CD63 but not CD62p expression was lower in control subjects than in stroke patients regardless of the antiplatelet treatment (P<0.05). CEPI-CT was prolonged under aspirin and aspirin plus clopidogrel compared with clopidogrel monotherapy (P<0.0001). CADP-CT was longer under combination therapy than under aspirin (P=0.0009) or clopidogrel (P=0.0074) or in control subjects (P=0.0010), mainly because of strong prolongation in a patient subgroup (28%).

CONCLUSIONS

CD63 expression reflecting the release of platelet lysosomes is consistently increased after stroke and incompletely suppressed by treatment with aspirin, clopidogrel, or both. The strong prolongation of CADP-CT under combined aspirin and clopidogrel in a patient subgroup may indicate a lower risk of thrombosis but also a higher risk of hemorrhage. The predictive value of platelet activation parameters requires investigation in prospective studies.

Monitoring aspirin 100 mg and clopidogrel 75 mg therapy with the PFA-100 device in patients with peripheral arterial disease

A tool to identify vascular patients who receive antiplatelet therapy nd to distinguish between responders and non-responders to antiplatelet therapy could be of clinical importance. The present observational study was designed to investigate whether the PFA-100 device (Dade Behring) is suitable to detect long-term therapy of aspirin (100 mg/d) and/or clopidogrel (75 mg/d) in a cohort of patients with peripheral arterial disease (PAD). A total of 150 consecutive patients with PAD were studied; 34 patients were excluded from the study due to irregular intake of antiplatelet therapy or due to method limitations. Of the remaining 116 patients, 42 had no antiplatelet therapy, 47 had daily aspirin (100 mg) intake, 19 were administered clopidogrel 75 mg daily, and 10 received a medication with 100 mg aspirin plus clopidogrel 75 mg daily, all for at least 10 days. Nonparametric Kruskal-Wallis test with post hoc comparisons showed that collagen plus epinephrine (CEPI) closure times of the patient group receiving aspirin and the group receiving aspirin plus clopidogrel were similar (p>0.05). In contrast, both patient groups exhibited prolonged CEPI values compared to patients without antiplatelet therapy and patients taking clopidogrel (p<0.001). Finally, both patients without antiplatelet therapy and patients with clopidogrel did not show marked differences with respect to their CEPI values (p>0.05). However, Kruskal-Wallis test results revealed that collagen plus adenosine-5'-diphosphate closure times were not significantly different in all four patient groups (p=0.257). In conclusion, the PFA-100 device may be a suitable tool for monitoring aspirin 100 mg therapy, but it is not appropriate for the detection of clopidogrel administration in its current setup. Although it appears plausible that patients with PAD could benefit from monitoring platelet inhibition, clear evidence for this concept is still lacking.

Glycoprotein Ib polymorphisms influence platelet plug formation under high shear rates

Platelet polymorphisms (Kozak, VNTR and HPA-2) within glycoprotein (GP)Ib alpha may be associated with an increased risk of arterial thrombosis. However, the functional role of these polymorphisms has not been clarified. Their influence on platelet plug formation under high shear rates was, therefore, examined in 233 healthy individuals. Collagen-adrenaline-induced closure time was shorter in carriers of the C/D versus C/C VNTR allele and in homozygotes with the (-5)T/T versus (-5)C/T Kozak genotype as determined by novel polymerase chain reaction methods. The HPA-2 genotype had no effects, and the density of GPIb alpha molecules was not influenced by GPIb alpha genotypes.

Clinical application of the PFA-100

The PFA-100 (platelet function analyzer) is a relatively new tool for the investigation of primary hemostasis. Recent studies have shown its utility as a screening tool for investigating possible von Willebrand disorder (VWD) and various platelet disorders. More recently, the PFA-100 has been shown to be valuable in monitoring desmopressin acetate (DDAVP) therapy in both VWD and platelet disorders. The PFA-100 has also been evaluated in many other studies for its utility in assessing drug effects, for potential monitoring of antiplatelet medication (including aspirin), or for evaluation of overall primary hemostasis in various clinical disorders or during surgical procedures. This article reviews current findings and highlights the benefits and limitations of the clinical utility of the PFA-100. Ultimately, the greatest strengths of the PFA-100 are its simplicity of use and excellent sensitivity to particular hemostatic disturbances such as VWD, platelet disorders, and platelet-affecting medication. However, because it is thus a global test system, this also creates a significant limitation because the PFA-100 is not specific for, nor predictive of, any particular disorder. However, used appropriately, the PFA-100 can be considered a worthwhile addition to any hemostasis laboratory involved in the diagnosis or therapeutic monitoring of bleeding disorders and potentially of antiplatelet medication. This review should be valuable to both hemostasis scientists and clinical specialists.

The PFA-100: a potential rapid screening tool for the assessment of platelet dysfunction

The PFA-100 is a device that simulates high shear dependent platelet function in vitro and thus is particularly useful for screening for von Willebrand's disease (VWD). The aim of this study was to assess the overall potential of the PFA-100 as a primary clinical screening tool using the wide spectrum of clinical samples assessed for platelet function within our institution. The PFA-100 test was performed using both collagen/ADP (CADP) and collagen/epinephrine (CEPI) cartridges on samples from 337 patients with a wide variety of haemostatic defects. One hundred and eighty-two patients were defined as having normal platelet function based on classical laboratory tests and von Willebrand factor levels. The overall clinical sensitivity of the PFA-100 for platelet abnormalities (including VWD) was 81% for CADP and 86% for CEPI. The overall specificity was found to be 82% for CADP and 80% for CEPI. When utilizing both cartridges in combination (with both results either higher or lower than the upper cutoff of the normal ranges), the overall false positive and false negative rates were 12% and 6%, respectively. The PFA-100 proved to be sensitive in detecting classical defects by giving prolonged closure times in samples from patients with major platelet function defects (e.g. von Willebrand's disease, Glanzmann's thrombasthenia and Bernard Soulier syndrome). However, there were a small number of false negative results (6%) obtained with various milder platelet defects (e.g. Hermansky Pudlak syndrome, storage pool and release defects, type I VWD and macrothrombocytopenia). The PFA-100 test provides a useful rapid screening tool and should increase the efficiency and reduce the cost of the routine diagnosis of platelet dysfunction.