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.     

Utility of the PFA-100® for assessing bleeding disorders and monitoring therapy: a review of analytical variables, benefits and limitations

The PFA-100 (platelet function analyser) is a relatively new tool for the investigation of primary haemostasis. Recent studies have shown its utility in monitoring antiplatelet therapy (including aspirin) and as a screening tool for investigating possible von Willebrand disease (vWD) and various platelet disorders. More recently, the PFA-100 has been shown to be of value in monitoring DDAVP therapy in both vWD and platelet disorders. This paper reviews current findings, details the utility of the PFA-100 for some of these purposes, as well as reviewing analytical variables that may complicate the interpretation of results. The author highlights the benefits, as well as noting the limitations, of its use. Ultimately, the greatest strengths of the PFA-100 are its simplicity in use and excellent sensitivity to particular haemostatic 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, as the PFA-100 is not specific for, nor predictive of, any particular disorder. However, utilized appropriately, the PFA-100 can be considered a worthwhile addition to any haemostasis laboratory involved in the diagnosis or therapeutic-monitoring of bleeding disorders including vWD and platelet-dysfunctions. This review should be of value to both haemostasis scientists and clinical specialists.     

Can platelet function tests predict the clinical efficacy of aspirin?

"Aspirin resistance" and "aspirin nonresponsiveness" are terms used both to describe both the failure of aspirin to protect subgroups of individuals from severe vascular events and to evoke an appropriate inhibition of platelet function. Several studies utilizing a broad range of platelet function tests have shown that some subgroups of individuals exhibit a reduced or completely missing antiplatelet response to aspirin. The clinical significance of aspirin nonresponsiveness for the prediction of clinical endpoints remains, however, to be determined. Thus far, only three prospective clinical trials have demonstrated a possible relationship between aspirin nonresponsiveness and subsequent vascular events. Most platelet function tests used in respective clinical studies cannot be reliably performed in clinical routine and are not interchangeable for monitoring antiplatelet treatment. There is a need for a simple and reliable assay for predicting the clinical efficacy of antiplatelet therapy. Recent data demonstrate that none of the currently developed assays, including the PFA-100 system, are presently able to accomplish these objectives.     

Screening women with menorrhagia for underlying bleeding disorders: the utility of the platelet function analyser and bleeding time

Menorrhagia is a very common clinical problem among women of reproductive age and recent studies have suggested that underlying bleeding disorders, particularly von Willebrand's deficiency and platelet function defects, are prevalent in women presenting with menorrhagia. The objective of this study was to determine the utility of the platelet function analyser (PFA-100) and bleeding time (BT) as initial screening tests for underlying bleeding disorders in women with menorrhagia. In this study, 81 women with a physician diagnosis of menorrhagia underwent PFA-100 testing, BT and comprehensive haemostatic testing. The effectiveness of the PFA-100 and BT as screening tools in women with menorrhagia was assessed using results of haemostatic testing for von Willebrand's disease (VWD) and platelet dysfunction. In women presenting with menorrhagia, the PFA-100 had a sensitivity 80%, specificity 89%, positive predictive value (PPV) 33%, negative predictive value (NPV) 98% and efficiency 88% for VWD. For platelet aggregation defects, the PFA-100 closure time had a sensitivity 23%, specificity 92%, PPV of 75%, NPV of 52% and efficiency 55%. The data suggest that the PFA-100 may be useful in stratifying women with menorrhagia for further von Willebrand testing; however, neither the PFA-100 nor the BT tests are effective for purposes of classifying women for standard platelet aggregometry testing in women presenting with menorrhagia.     

The utility of the PFA-100 in the identification of von Willebrand disease: a concise review

The PFA-100 (platelet function analyser; Dade-Behring, Marburg, Germany) is a relatively new tool for the investigation of primary hemostasis. Recent studies have shown its utility as a screening tool for investigating various platelet disorders and possible von Willebrand disorder (vWD), both in the initial investigation and in subsequent therapeutic monitoring of desmopressin therapy. This article reviews current findings with respect to the identification of vWD, and highlights both the benefits and the limitations of its clinical utility. In brief, sensitivity to vWD types 2A, 2B, 2M, and 3 is > 98%, but overall sensitivity to vWD (types 1, 2A, 2B, 2M, and 3 combined) is ~85 to 90%. Ultimately, the high sensitivity of the PFA-100 to vWD and its simplicity of use provide its greatest strengths. However, because it is a global test system, and also sensitive to low hematocrit, low platelet counts, and platelet dysfunction (both congenital and acquired; e.g., secondary to medication such as aspirin) it must be recognized that the PFA-100 is neither specific for, nor predictive of, any particular disorder (inclusive of vWD). Nevertheless, used appropriately, the PFA-100 can be considered a worthwhile addition to the hemostasis laboratory involved in the diagnosis or therapeutic monitoring of vWD, and a normal PFA-100 result can be used with some confidence to exclude severe vWD.     

Response to aspirin and clopidogrel monitored with different platelet function methods

Laboratory non-response to aspirin or clopidogrel is defined as an inability to cause in vitro detectable platelet function inhibition. It would be beneficial to monitor response to aspirin or clopidogrel with widely available and routinely used platelet function methods, like the platelet function analyzer (PFA-100) or the fully automated coagulation analyzer BCT. The aim of this study was to assess the potential of the coagulation analyzer BCT and the platelet function analyzer PFA-100 in monitoring the response of aspirin and clopidogrel. A group of 125 consecutive patients with arterial occlusive disease treated either with aspirin 100 mg/day (82 patients) or clopidogrel 75 mg/day (43 patients) as only antiplatelet drug were investigated. For the first time platelet-enriched plasma (PRP), not adjusted to a fixed predetermined concentration of platelets, was used for aggregation studies and the effect of clopidogrel alone without combination of aspirin treatment on platelet function was investigated. Response to aspirin was observed in 85% (70/82) of patients using PFA-100, while performing the arachidonic acid-induced aggregation on the BCT showed an inhibitory effect to aspirin in 91% (75/82) of patients. Non-response to aspirin was assessed with both platelet function methods in 7% (6/82) of patients. Clopidogrel response was observed in 58% (25/43) of patients when performing ADP-induced aggregation on the BCT. On the PFA-100 the antiplatelet effect of clopidogrel could not be detected. In conclusion, measurement of platelet aggregation on the BCT using native platelet-enriched plasma allows the quantification of individual inhibitory effects to aspirin as well as to clopidogrel, while the PFA-100 seems only suitable to investigate the degree of platelet inhibition induced by aspirin but not by clopidogrel.     

Variability of non-response to aspirin in patients with peripheral arterial occlusive disease during long-term follow-up

Non-responsiveness to aspirin as detected by laboratory tests may identify patients at high risk for future vascular events. The aim of this prospective study was to evaluate whether non-responsiveness to aspirin is stable over time. Ninety-eight patients with stable peripheral arterial occlusive disease (PAOD) treated with 100 mg/d aspirin were followed over a median timeframe of 17 months. Platelet function tests were performed initially and at follow-up using arachidonic acid-induced light transmittance aggregometry (LTA) in native platelet-rich plasma with the Behring Coagulation Timer® and by measuring the collagen?epinephrine closure time (CT) on a Platelet Function Analyzer (PFA-100®). When determining platelet function using LTA, four patients (4.1%) had residual platelet function (i.e., MaxAggr ≥78%) despite aspirin treatment, whereas, according to the PFA-100® results, 12 patients (12.2%) were identified as non-responders (i.e., CT <192 s). Fifty-seven patients who were still under treatment with 100 mg/d aspirin at the time of follow-up provided a second blood sample. Further platelet function tests with the PFA-100® system identified a persistent non-responsiveness to aspirin over time in three patients (5.3%) whereas four (7.0%) and 15 (26.3%) patients had changes in response status when platelet function was assessed by LTA and on the PFA-100®, respectively. We conclude that true non-responsiveness to aspirin is a rare phenomenon in stable PAOD patients. Furthermore, we conclude that in a number of patients, aspirin non-responsiveness is not stable over time.     

Profile and prevalence of aspirin resistance in patients with cardiovascular disease

We determined the prevalence and clinical predictors of aspirin resistance by prospectively studying 325 patients with stable cardiovascular disease who were receiving aspirin (325 mg/day for > or =7 days) but no other antiplatelet agents. We also compared the detection of aspirin resistance with optical platelet aggregation, a widely accepted method, with a newer, more rapid method, the platelet function analyzer (PFA)-100, a whole blood test that measures platelet adhesion and aggregation ex vivo. Blood samples were analyzed in a blinded fashion for aspirin resistance by optical aggregation using adenosine diphosphate (ADP) and arachidonic acid, and by PFA-100 using collagen and/or epinephrine and collagen and/or ADP cartridges to measure aperture closure time. Aspirin resistance was defined as a mean aggregation of > or =70% with 10 microM ADP and a mean aggregation of > or =20% with 0.5 mg/ml arachidonic acid. Aspirin semiresponders were defined as meeting one, but not both of the above criteria. Aspirin resistance by PFA-100 was defined as having a normal collagen and/or epinephrine closure time (< or =193 seconds). By optical aggregation, 5.5% of the patients were aspirin resistant and 23.8% were aspirin semiresponders. By PFA-100, 9.5% of patients were aspirin resistant. Of the 18 patients who were aspirin resistant by aggregation, 4 were also aspirin resistant by PFA-100. Patients who were either aspirin resistant or aspirin semiresponders were more likely to be women (34.4% vs 17.3%, p = 0.001) and less likely to be smokers (0% vs 8.3%, p = 0.004) compared with aspirin-sensitive patients. There was a trend toward increased age in patients with aspirin resistance or aspirin semiresponders (65.7 vs 61.3 years, p = 0.06). There were no differences in aspirin sensitivity by race, diabetes, platelet count, renal disease, or liver disease.     

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.The main supporter of the study was Turkish Society of Cardiology (Istanbul, Turkey). The society is a non-profit association and a member of the European Society of Cardiology. The PFA-100 device and test cartridges are bought with the grant of this non-profit association.     

Can the Platelet Function Analyzer (PFA)-100 test substitute for the template bleeding time in routine clinical practice?

The bleeding time (BT) is widely used in clinical medicine as a screening test of platelet function, although its deficiencies in such a role are well recognized. The Platelet Function Analyzer (PFA)-100 measures the ability of platelets activated in a high-shear environment to occlude an aperture in a membrane treated with collagen and epinephrine (CEPI) or collagen and ADP (CADP). The time taken for flow across the membrane to stop (closure time) is recorded. This study compared the PFA-100 with the BT as a screening test of platelet dysfunction in 113 hospital inpatients. The PFA-100 test was performed initially using the CEPI cartridge; CADP tests were performed on those with abnormal (> 163 s) CEPI closure times. Whole blood platelet aggregation studies and chart review were performed on patients in whom the BT and PFA-100 results did not agree.Abnormal bleeding times and PFA-100 results were obtained in 20.4% and 35.4% of patients, respectively. The results of BT and PFA-100 agreed in 74.3% of patients. Of the 29 patients in whom the BT and PFA-100 results were discordant, whole blood platelet aggregation studies supported the PFA-100 result in 25 (86.2%). The PFA-100 was more sensitive to aspirin-induced platelet dysfunction and was more rapidly and cheaply performed than the BT. Since the PFA-100 test reflects platelet function better than the BT, we conclude that this test could replace the BT as a first-line screening test for platelet dysfunction in clinical practice.     

Antiplatelet drugs: mechanisms and risks of bleeding following cardiac operations

Preoperative antiplatelet drug use is common in patients undergoing coronary artery bypass grafting (CABG). The impact of these drugs on bleeding and blood transfusion varies. We hypothesize that review of available evidence regarding drug-related bleeding risk, underlying mechanisms of platelet dysfunction, and variations in patient response to antiplatelet drugs will aid surgeons as they assess preoperative risk and attempt to limit perioperative bleeding. The purpose of this review is to (1) examine the role that antiplatelet drugs play in excessive postoperative blood transfusion, (2) identify possible mechanisms to explain patient response to antiplatelet drugs, and (3) formulate a strategy to limit excessive blood product usage in these patients. We reviewed available published evidence regarding bleeding risk in patients taking preoperative antiplatelet drugs. In addition, we summarized our previous research into mechanisms of antiplatelet drug-related platelet dysfunction. Aspirin users have a slight but significant increase in blood product usage after CABG (0.5 U of nonautologous blood per treated patient). Platelet adenosine diphosphate (ADP) receptor inhibitors are more potent antiplatelet drugs than aspirin but have a half-life similar to aspirin, around 5 to 10 days. The American Heart Association/American College of Cardiology and the Society of Thoracic Surgeons guidelines recommend discontinuation, if possible, of ADP inhibitors 5 to 7 days before operation because of excessive bleeding risk, whereas aspirin should be continued during the entire perioperative period in most patients. Individual variability in response to aspirin and other antiplatelet drugs is common with both hyper- and hyporesponsiveness seen in 5 to 25% of patients. Use of preoperative antiplatelet drugs is a risk factor for increased perioperative bleeding and blood transfusion. Point-of-care tests can identify patients at high risk for perioperative bleeding and blood transfusion, although these tests have limitations. Available evidence suggests that multiple blood conservation techniques benefit high-risk patients taking antiplatelet drugs before operation. Guidelines for patients who take aspirin and/or thienopyridines before cardiac procedures include some or all of the following: (1) preoperative identification of high-risk patients using point-of-care testing; (2) withdrawal of aspirin or other antiplatelet drugs for a few days and delay of operation in patients at high risk for bleeding if clinical circumstances permit; (3) selective perioperative use of evidence-based blood conservation interventions (e.g., short-course erythropoietin, off-pump procedures, and use of intraoperative blood conservation techniques), especially in high-risk patients; and (4) platelet transfusions if clinical bleeding occurs.     

Hypersensitivity of platelets to adenosine diphosphate in patients with stable cardiovascular disease predicts major adverse events despite antiplatelet therapy

Enhanced platelet activity correlates with early markers of myocardial damage in patients with cardiovascular disease. However, the extent to which enhanced platelet function signals subsequent adverse clinical outcomes in patients with cardiovascular disease is unknown. Blood from patients with stable cardiovascular disease receiving aspirin (325 mg/day) as the only antiplatelet therapy was tested for closure time (CT) with the Dade PFA-100 Platelet Function Analyzer system collagen/adenosine diphosphate (ADP) [CADP] cartridge and platelet aggregometry using 10 microM ADP. This study intentionally focused on those patients defined as aspirin sensitive by previously established criteria of arachidonic acid- and ADP-induced platelet aggregometry, and separately by collagen/epinephrine (CEPI) CT using the PFA-100. Follow up averaged 22 months for the adverse clinical events of death, myocardial infarction or cerebrovascular accident. For aspirin sensitivity determined by aggregometry, patients with CADP CT < 90 seconds (125/296 = 42.2%) had a composite endpoint rate of 19.2% (24/125), while those with CADP CT 90 seconds (171/296 = 57.8%) had an endpoint rate of 5.3% (9/171). Patients with CADP CT <90 seconds had a relative risk (RR) of 3.65 (95% CI.: 1.76-7.57) for recurrent events and 6.56 (95% CI.: 1.93-22.35) for death compared to patients with CADP CT 90s. Nearly identical results were obtained when patients were categorized as aspirin sensitive by CEPI CT. Platelet aggregometry with 10 microM ADP yielded no significant RR for the selected outcomes. Platelet function testing using the PFA-100 system appears to identify a subgroup of stable cardiovascular disease patients with increased risk of major adverse events that is associated with hypersensitivity to ADP, regardless of apparently effective aspirin therapy.     

An in vitro model for the detection of reduced platelet sensitivity to acetylsalicylic acid.

A discovery of 'aspirin resistance' has prompted the search for fast and reliable methods for the monitoring of antiplatelet efficacy of acetylsalicylic acid (ASA). Our aims were: (1) to evaluate the in vitro model-based method for detecting a reduced platelets' sensitivity to ASA using a point-of-care platelet function analyser PFA-100; and (2) to propose a simple method of data analysis that might be successfully employed to discriminate between 'good' and 'poor' responders to aspirin. Whole blood platelets from healthy volunteers were incubated in vitro with 30 microg/ml ASA (the in vitro method under evaluation) or analysed following a 10-day intake of an average 150 mg ASA (Aspirin Protect) daily (the reference ex vivo method). According to polynomial regression analysis of the bimodally distributed data, the donors with lower ('ASA poor responders') or higher platelet sensitivity to ASA ('ASA good responders') were discriminated at 58.6% of platelet function inhibition. Despite the similar proportions of 'ASA poor responders' (44 versus 41% using the ex vivo and in vitro tests, respectively), 30% of discordant classifications point to a rather unsatisfactory convergence between both methods. Due to a considerable discordance between the in vitro and ex vivo tests of ASA efficacy performed with the use of the PFA-100 system, the former cannot be reliably and interchangeably used for the monitoring of aspirin therapy and the selection of an effective therapeutic ASA dose. A novel approach to data analysis of the distribution of platelet inhibition rates facilitates an evaluation of cut-off points required to discriminate between 'poor responders' and 'good responders' to aspirin.     

Hypersensitivity of platelets to adenosine diphosphate in patients with stable cardiovascular disease predicts major adverse events despite antiplatelet therapy

Enhanced platelet activity correlates with early markers of myocardial damage in patients with cardiovascular disease. However, the extent to which enhanced platelet function signals subsequent adverse clinical outcomes in patients with cardiovascular disease is unknown. Blood from patients with stable cardiovascular disease receiving aspirin (325 mg/day) as the only antiplatelet therapy was tested for closure time (CT) with the Dade® PFA-100® Platelet Function Analyzer system collagen/adenosine diphosphate (ADP) [CADP] cartridge and platelet aggregometry using 10 µM ADP. This study intentionally focused on those patients defined as aspirin sensitive by previously established criteria of arachidonic acid- and ADP-induced platelet aggregometry, and separately by collagen/epinephrine (CEPI) CT using the PFA-100®. Follow up averaged 22 months for the adverse clinical events of death, myocardial infarction or cerebrovascular accident. For aspirin sensitivity determined by aggregometry, patients with CADP CT < 90 seconds (125/296 = 42.2%) had a composite endpoint rate of 19.2% (24/125), while those with CADP CT ≥90 seconds (171/296 = 57.8%) had an endpoint rate of 5.3% (9/171). Patients with CADP CT <90 seconds had a relative risk (RR) of 3.65 (95% CI.: 1.76–7.57) for recurrent events and 6.56 (95% CI.: 1.93–22.35) for death compared to patients with CADP CT ≥ 90s. Nearly identical results were obtained when patients were categorized as aspirin sensitive by CEPI CT. Platelet aggregometry with 10 µM ADP yielded no significant RR for the selected outcomes. Platelet function testing using the PFA-100® system appears to identify a subgroup of stable cardiovascular disease patients with increased risk of major adverse events that is associated with hypersensitivity to ADP, regardless of apparently effective aspirin therapy.     

Point-of-care platelet function tests: detection of platelet inhibition induced by nonopioid analgesic drugs.

Detection of platelet inhibition is of clinical relevance in the preinterventional risk-benefit assessment in chronic low-back-pain patients scheduled for invasive pain therapy. We evaluated the sensitivity of various point-of-care platelet function tests for the detection of platelet inhibition induced by nonopioid analgesic drugs. After Institutional Review Board approval and informed consent, citrated whole blood from 40 patients with chronic unspecific low back pain was investigated before and 30 min after intravenous infusion of the study medication consisting of diclofenac 75 mg (plus orphenadrin 30 mg; Neodolpasse; Fresenius Kabi Austria GmbH, Austria), parecoxib 40 mg (Dynastat; Pharmacia Europe EEIG, UK), paracetamol 1 g (Perfalgan; Bieffe Medital S.P.A., Italy), or normal saline in a randomized, cross-over, double-blinded, placebo-controlled study. Platelet function was assessed using the PFA-100 platelet function analyzer and thromboelastometry, as well as impedance aggregometry (in the last 17 patients recruited after it became commercially available). Sensitivity for detecting diclofenac-induced platelet inhibition was 85% for the PFA-100 using epinephrine as agonist and 94% for arachidonic acid-induced impedance aggregometry. ADP-induced platelet function tests, as well as cytochalasin D-modified thromboelastometry were unreliable. All tests had a low incidence of false-positive test results after normal saline. Paracetamol and parecoxib had no significant platelet inhibiting effect. The PFA-100 using epinephrine as agonist and arachidonic acid-induced impedance aggregometry are recommended for the detection of cyclooxygenase-I-inhibiting effects of nonsteroidal anti-inflammatory drugs such as diclofenac. Our findings confirm that a single rescue dose of paracetamol and parecoxib has no antiplatelet effect.     

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.     

Evaluation of platelet function in aspirin treated patients with CAD

Background: Studies of platelet function in the acute coronary syndrome (ACS) have revealed both increased and unchanged platelet activation. To obtain a better understanding of platelet function in coronary artery disease in the setting of aspirin therapy, we performed platelet functional testing in patients with ACS and compared results to patients without CAD. Methods: We measured platelet aggregation and activation in response to ADP and epinephrine in 80 age and gender matched hospitalized patients (40 with ACS, 40 with non-cardiac chest pain (NCCP)). All subjects received ASA (81–325 mg). Platelet aggregation was performed using standard light transmission in platelet-rich plasma and activation was measured via flow cytometric analyses. We also studied platelet function under high shear rates measured by the platelet function analyzer (PFA-100). Results: ASA effect was found to be present in all subjects by blunted platelet aggregation in response to arachidonic acid. Patients with ACS showed significantly higher levels of platelet aggregation to epinephrine compared to patients with NCCP (p = 0.001). Other measures of platelet function including ADP aggregation, Pselectin, activated glycoprotein IIb/IIIa expression, and PFA-100 were unchanged between the two groups. Conclusions: We have found conflicting results of platelet functional testing in ACS. Specifically aspirin therapy in patients with ACS is effective in suppressing the platelet release response and is effective in the partial suppression of platelet aggregation; however, it appears that ACS patients have increased platelet aggregation to adrenergic stimuli when compared to age and gender matched controls without CAD despite the use of aspirin. In some studies, because ACS patients have an accentuated response to adrenergic stimuli this might be interpreted as aspirin resistance. Our study suggests that depending on the assay used to determine aspirin resistance, not all patients with this label are resistant to the biological effects of aspirin but they may have higher than normal baseline platelet sensitivity to adrenergic stimuli. This research was supported by grant K23HL67066 from the National Institute of Health.     

Laboratory monitoring of therapy in von Willebrand disease: efficacy of the PFA-100 and von Willebrand factor:collagen-binding activity as coupled strategies

Clinical management of von Willebrand disease (or von Willebrand disorder [vWD]) often involves factor replacement or desmopressin acetate (DDAVP) therapy to control (potential) bleeding. Laboratory monitoring involves testing patient samples prior to therapy and at discreet time points after therapy. Classical testing generally comprises assays for factor VIII:coagulant activity, von Willebrand factor (vWF):antigen and vWF:ristocetin cofactor activity. The PFA-100 (platelet function analyser) is a relatively new tool for the investigation of primary hemostasis, and studies have shown its potential utility in identifying both vWD and platelet disorders, and in monitoring DDAVP therapy in these patients. However, the PFA-100 has limited utility in monitoring factor replacement therapy. The collagen-binding activity (vWF:CB) assay is a relatively new functional vWF assay and studies have also shown its utility in identifying vWD, and in monitoring both DDAVP and factor replacement therapy in these patients. This review assesses the laboratory monitoring of therapy for vWD with a special focus on the combined potential utility of the PFA-100 and a vWF:CB assay sensitive for the presence or absence of large vWF multimers. This review should be of value to both hemostasis scientists and clinical specialists.     

A comparison of six major platelet function tests to determine the prevalence of aspirin resistance in patients with stable coronary artery disease.

AIMS: We sought to compare the results obtained from six major platelet function tests in the assessment of the prevalence of aspirin resistance in patients with stable coronary artery disease. METHODS AND RESULTS: 201 patients with stable coronary artery disease receiving daily aspirin therapy (> or =80 mg) were recruited. Platelet aggregation was measured by: (i) light transmission aggregometry (LTA) after stimulation with 1.6 mM of arachidonic acid (AA), (ii) LTA after adenosine diphosphate (ADP) (5, 10, and 20 microM) stimulation, (iii) whole blood aggregometry, (iv) PFA-100, (v) VerifyNow Aspirin; urinary 11-dehydro-thromboxane B(2) concentrations were also measured. Eight patients (4%, 95% CI 0.01-0.07) were deemed resistant to aspirin by LTA and AA. The prevalence of aspirin resistance varied according to the assay used: 10.3-51.7% for LTA using ADP as the agonist, 18.0% for whole blood aggregometry, 59.5% for PFA-100, 6.7% for VerifyNow Aspirin, and finally, 22.9% by measuring urinary 11-dehydro-thromboxane B(2) concentrations. Results from these tests showed poor correlation and agreement between themselves. CONCLUSION: Platelet function tests are not equally effective in measuring aspirin's antiplatelet effect and correlate poorly amongst themselves. The clinical usefulness of the different assays to classify correctly patients as aspirin resistant remains undetermined.     

Residual platelet reactivity is associated with clinical and laboratory characteristics in patients with ischemic heart disease undergoing PCI on dual antiplatelet therapy

A residual platelet reactivity (RPR) on antiplatelet therapy in patients with ischemic heart disease (IHD) has been reported to be associated with adverse clinical events by some Authors. However, scarce data are present on the clinical parameters associated with this phenomenon. No study, at our knowledge, was designed with the specific aim to examine the relationship between clinical characteristics and RPR. We sought to evaluate the clinical and laboratory characteristics associated with RPR in patients with IHD undergoing coronary revascularization on dual (aspirin plus clopidogrel) antiplatelet therapy. We included in the study 868 patients undergoing a coronary angiography: 386 with acute coronary syndromes undergoing a primary coronary revascularization and 482 IHD patients scheduled to undergo an elective coronary angiography. We measured platelet function by both platelet aggregation with two agonists [0.5 mg/mL arachidonic acid (AA); 2 and 10 microM adenosine 5'-diphosphate (ADP)] and a point-of-care assay (PFA-100) on venous blood samples collected within 24 h from the end of the procedure. In patients with acute coronary syndromes and elective PCI diabetes is independently associated with RPR [group A: OR=2.9 (1.5-5.7) by 10 microM ADP, OR=5.3 (1.1-27.8) by PFA-100; group B: OR=4.0 (1.6-10.0) by 10 microM ADP]; reduced left ventricular systolic function [OR=3.7 (2.2-6.5) by AA-PA, OR=2.7 (1.6-4.6) by PFA-100], chronic use of aspirin [OR=0.2 (0.1-0.4) by AA-PA, OR=0.3 (0.2-0.5) by PFA-100] and loading dose of clopidogrel [OR=0.2 (0.06-0.5) by 10 microM ADP] were independent variables significantly associated with RPR in patients undergoing elective PCI. In addition, inflammatory status was found to be significantly associated with RPR in both groups of patients. These results provide indications for the selection of patients for whom the evaluation of platelet reactivity could be useful.