Use of the PFA-100 in the assessment of primary, platelet-related hemostasis in a pediatric setting

Primary hemostasis in fetal growth restricted neonates studied via PFA-100 in cord blood samples

BACKGROUND

Platelet function of fetal growth restricted (FGR) neonates remains a field of debate. Platelet function analyzer (PFA-100) offers a quantitative in vitro assessment of primary, platelet-related hemostasis. Our aim was to examine platelet function using PFA-100 in FGR neonates and associate our results with perinatal parameters.

METHODS

PFA-100 was applied on 74 FGR neonates, 48 full-term (>37 weeks' gestation) and 26 preterm neonates (<37 weeks). The control group consisted of 118 healthy neonates. Two closure times (CTs) with COL/EPI and COL/ADP cartridges were determined on cord blood samples for each subject. Statistical analysis was performed by SAS 9.4. The statistical significance level was set at 0.05 and all tests were two-tailed.

RESULTS

COL/EPI CTs were prolonged in FGR (median 132 s, IQR 95-181 s) compared with control neonates (median 112.5 s, IQR 93-145 s), p = 0.04. Median COL/EPI CT for term and preterm FGR neonates was 126 s (IQR 90-157 s) and 137 s (IQR 104-203), respectively (p = 0.001), and COL/ADP CT was 70 s (IQR 62-80 s) for term and 75 s (IQR 68-82 s) for preterm FGR neonates (p = 0.08). Among FGR neonates, COL/EPI CT was related with delivery time (with preterm neonates exhibiting prolonged COL/EPI CTs), p = 0.05. No correlation was proved between both CTs and hematological parameters in FGR neonates.

CONCLUSION

FGR neonates showed impaired platelet function via PFA-100, with preterm FGR neonates confronting the greatest risk. Prolonged COL/EPI CTs in FGR neonates seemed to be independent of hematological parameters and could warn for closer evaluation during the first days of their lives.

Association Between Platelet Function and Disc Hemorrhage in Patients With Normal-Tension Glaucoma: A Prospective Cross-Sectional Study

PURPOSE

To evaluate the association between platelet function and disc hemorrhage in patients with normal-tension glaucoma.

DESIGN

Prospective, cross-sectional study.

METHODS

Study involved a total of 315 subjects, including patients with normal-tension glaucoma and disc hemorrhage (n = 120), patients with normal-tension glaucoma without disc hemorrhage (n = 75), and healthy individuals (control group, n = 120). A detailed eye examination including visual field testing, color disc photography, optical coherence tomography scanning, and measurement of collagen/epinephrine closure time using a platelet function analyzer were performed for all subjects.

RESULTS

The collagen/epinephrine closure time (s) as measured by the platelet function analyzer was approximately 14%-24% longer in the normal-tension glaucoma and disc hemorrhage group compared with the other groups (141.92 ± 53.44 [with normal-tension glaucoma and disc hemorrhage] vs 124.60 ± 46.72 [with normal-tension glaucoma without disc hemorrhage] vs 114.84 ± 34.84 [healthy individuals], 1-way analysis of variance test, P < .001). The activated partial thromboplastin time (s) value of the normal-tension glaucoma with disc hemorrhage group was also higher than the control group. Stepwise multiple logistic regression analysis revealed that only a longer collagen/epinephrine closure time (OR adjusted for age, sex, prothrombin time, activated partial thromboplastin time, diabetes mellitus, hypertension, hypotension, heart disease, hypothyroidism, migraine, stroke, hypercholesterolemia: 2.94; 95% CI: 1.40-6.17) was independently associated with disc hemorrhage. A similar trend was observed when platelet function was compared among the 3 groups with respect to age.

CONCLUSIONS

Our results suggest that platelet function is significantly associated with disc hemorrhage in patients with normal-tension glaucoma. Delayed absorption resulted from prolonged bleeding due to delayed platelet aggregation may have an effect on the detectability of disc hemorrhage in patients with normal-tension glaucoma.

The platelet-function analyzer (PFA-100 ® ) for evaluating primary hemostasis

The platelet-function analyzer, PFA-100, is a relatively novel method for rapid in vitro global evaluation of primary hemostasis. The aim of this review is to summarize the published studies reporting on the utility of the PFA-100 device as a screening tool for primary hemostasis. Data were identified by searches of the published literature, including PubMed, references from reviews and abstracts from the most important meetings on this topic. The literature data support the use of the PFA-100 as a useful screening tool for the investigation of von Willebrand's disease and other acquired and congenital intrinsic platelet function disorders. Moreover, it is useful for evaluating primary hemostasis before surgical procedures and for monitoring desmopressin therapy in both von Willebrand's disease and platelet function disorders. Given its high sensitivity, speed and simplicity of use, we conclude that the PFA-100 could replace the in vivo bleeding time as a screening test for primary hemostasis in routine clinical practice.

Influence of mechanical trauma of blood and hemolysis on PFA-100 testing

Although the appropriate quality of samples is essential for platelet function testing, information is lacking on interference from mechanical trauma of blood and hemolysis on PFA-100 analyzer. Citrated blood collected from nine healthy volunteers was divided into three aliquots. The first aliquot ('A') was processed without further manipulation, whereas the second and third were subjected to mechanical trauma by two ('aliquot B') or four passages ('aliquot C') through a very fine needle (30 gauge) to produce hemolysis and cell trauma mimicking poor sample collection. Samples were tested on PFA-100 and Advia 2120, and plasma then separated and tested for lactate dehydrogenase (LDH) and hemolysis index. Negligible hemolysis was present in aliquot A (hemolysis index 0.2 ± 0.1, cell-free hemoglobin 0-0.5 g/l), whereas an increasing amount was present in aliquots B (hemolysis index of 13.1 ± 1.8, cell-free hemoglobin 6.0-6.5 g/l) and C (hemolysis index 24.0 ± 1.1, cell-free hemoglobin 11.5-12.0 g/l). Increases in LDH, and concomitant reductions in platelet and red blood cell counts were observed in aliquots B and C. In hemolyzed aliquots B, four out of nine samples yielded 'flow obstruction' with both PFA-100 agonist cartridges, whereas the closure times were dramatically prolonged in the remaining five samples. In hemolyzed aliquots C, flow obstruction was recorded in six of nine samples for collagen and ADP and all samples for collagen and epinephrine, whereas closure times of collagen and ADP in the remaining three samples were dramatically prolonged. Mechanical trauma of blood causing hemolysis makes PFA-100 testing unreliable. When flow obstructions are observed, the potential presence of hemolysis should be investigated.

Effect of titanium maxillofacial implants and osteosynthesis materials on platelet function

The use of titanium implants and osteosynthesis materials in orthopaedics, neurosurgery, and maxillofacial surgery has increased considerably, and they may be retained in patients for a long time. We could find no papers in English that provided data about the effect of titanium osteosynthesis materials on platelet function, and the purpose of this study was to evaluate those effects. Platelet function was tested in 33 patients with titanium maxillofacial implants using the PFA-100 (platelet function analyser) system and these results were compared with those of a control group of 34 healthy volunteer blood donors. Platelet function was measured in each group using collagen/adrenaline and collagen/adenosine diphosphate (ADP) as agonists for assessment of platelet function. There were no significant differences between the groups (collagen/adrenaline p=0.1, collagen/ADP p=0.09). Titanium maxillofacial osteosynthesis materials have no significant effect on platelet function as assessed by the PFA-100 system.

Diagnostic performance of the platelet function analyzer (PFA-100®) for the detection of disorders of primary haemostasis in patients with a bleeding history–a systematic review and meta-analysis

The Platelet Function Analyzer (PFA-100) is increasingly being used in the workup of patients with a bleeding diathesis. A profound knowledge of the possible diagnostic performance of this test is essential in order to make sound clinical decisions based on its results. It was the aim of this study to systematically review the published literature and provide valid estimates of the diagnostic performance of the PFA-100 for detecting disorders of primary haemostasis in newly presenting patients with a bleeding diathesis. A comprehensive literature search was performed for studies published between January 1994 and February 2006. Studies were eligible for the systematic review if they provided data supposed to be applicable to the determination of the diagnostic performance of the PFA-100. Furthermore, they were included in a meta-analysis if study reporting allowed calculation of sensitivity and specificity and if study quality ensured minimized biases of these estimates for the described clinical setting. Pooled weighted sensitivity, specificity and diagnostic odds ratio were calculated applying random effects modelling and constructing summary operator characteristic curves. This was done separately for the available test modifications using either collagen/epinephrine (PFA-EPI) or collagen/adenosine-diphosphate (PFA-ADP) for platelet activation. Thirty-six articles were included in the systematic review. Six studies met our eligibility criteria for a meta-analysis. The major reason for exclusion from the meta-analysis was a case-control design. A total of 1486 and 1259 patients were included in the meta-analysis of the diagnostic performance of the PFA-EPI and PFA-ADP, respectively. Pooled weighted sensitivity and specificity of the PFA-EPI/PFA-ADP in detecting a disorder of primary haemostasis were: 82.5/66.9% (95%-confidence interval (95%-CI): 76.0-88.9%/57.9-75.9%), and 88.7/85.5% (95%-CI: 84.3-93.1%/82.0-89.1%). 83/75% of patients with a positive PFA-EPI/PFA-ADP result do have a disorder of primary haemostasis whereas 88/79% with a negative PFA-EPI/PFA-ADP result do not. The PFA-EPI appeared to have a higher sensitivity and better predictive values than the PFA-ADP in detecting disorders of primary haemostasis, although a rigorous gold standard definition for a disorder of primary haemostasis, particularly for platelet disorders, was not applied in most studies. The majority of the studies lacked important requirements for quality and reporting, precluding a more precise and definitive characterization of the clinical utility of the PFA-100. This emphasizes the need for an evidence-based critical appraisal of diagnostic studies in haemostasis research in order to promote the conducting of studies that produce clinically relevant results.

Aspirin resistance in children with heart disease at risk for thromboembolism: prevalence and possible mechanisms

Aspirin is used to prevent thromboembolism in children with heart disease without evidence supporting its efficacy. Studies in adults report a 5%-51% prevalence of aspirin resistance, yet the mechanisms involved are poorly understood. Our aims were to determine its prevalence in these children and to explore its possible mechanisms. One hundred twenty-three cardiac patients routinely receiving aspirin were prospectively enrolled. Platelet function was measured by Platelet Function Analyzer (PFA)-100 using epinephrine and adenosine diphosphate (ADP) agonists. Aspirin resistance was defined as failure to prolong the epinephrine closure time following aspirin administration. Urine levels of 11-dehydro-thromboxane B(2) (11-dTXB(2)) were measured to determine inhibition of the cyclo-oxygenase pathway. The prevalence of aspirin resistance was 26%. Median ADP closure time was shorter for aspirin-resistant (79.60-115 s) than for aspirin-sensitive (100.60-240 s) patients (p < 0.01). 11-dTXB(2) levels did not correlate with aspirin resistance. Aspirin-resistant patients had higher 11-dTXB(2) levels before (7297 vs. 4160 pg/mg creatinine; p < 0.01) and after (2153 vs. 1412 pg/mg; p = 0.03) aspirin, with a similar percentage decrease in thromboxane (70.5% vs. 66.1%; p = 0.43). Our findings suggest that resistance is not entirely due to lack of inhibition of platelet thromboxane production. Alternative sources of thromboxane and thromboxane-independent mechanisms, such as ADP-induced platelet activation, may contribute to aspirin resistance.

Risk of excessive bleeding associated with marginally low von Willebrand factor and mild platelet dysfunction

BACKGROUND

Bleeding symptoms are so commonly reported that it is not known whether they associate causally or coincidentally with mild but measurable primary hemostatic defects. OBJECTIVES/PATIENTS/METHODS: In order to evaluate if the mild primary hemostatic defects are truly causative of increased bleeding symptoms, we surveyed a population of healthy teenagers for bleeding symptoms. Using a case-control approach, we then estimated the risk of excessive bleeding associated with low von Willebrand factor (defined as VWF below the 5th percentile of a normal reference population), and with mild platelet dysfunction [PD, defined as concurrent reduced platelet aggregation responses to two agonists (adenosine diphosphate and epinephrine)].

RESULTS

Excessive bleeding was present in 63 out of 809 teenagers (7.8%). Among the 49 cases who were tested for VWF, low values by three measures were more commonly present than in 166 controls, specifically, ristocetin cofactor (RCo) activity [20.4% vs. 5.4%, odds ratio (OR) 4.5], collagen binding (14.3% vs. 4.2%, OR 3.8), and antigen level (20.4% vs. 6.0%, OR 4.0). The low RCo values ranged from 35 to 45 U dL(-1) except for a single case with 26 U dL(-1). Of the 47 teenagers with excessive bleeding who underwent platelet aggregation studies, reduced responses were more common than in controls (12.8% vs. 4.4%, OR 3.2). Twenty-nine per cent of cases with excessive bleeding had either low RCo or PD.

CONCLUSION

Almost one in three teenagers who report excessive bleeding is likely to have a measurable hemostatic disturbance manifested either by marginally low VWF (by three measures) or by mild PD.

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.

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 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.

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.

Heparinase-Modified Thrombelastography in Term and Preterm Neonates

Thrombelastography (TEG) appears to be a promising test to assess coagulation in infants and children. TEG enables a rapid assessment of hemostatic function with only 300 microL of whole blood and provides information about plasmatic coagulation, platelet function, and fibrinolysis. In this study, we used TEG to assess the coagulation system of preterm and term neonates to determine the effects of their deficient coagulation factor levels on global hemostatic function. Heparinase-modified TEG, platelet and red blood cell count, plasma fibrinogen, and prothrombin time were assessed in four groups of clinically stable infants: severely preterm (gestational age [GA], 27-31 wk), moderately preterm (GA, 32-36 wk), term (GA, 36-40 wk), and former preterm (corrected GA, 34-40 wk). Healthy adult volunteers served as a control group. When compared with the adult group, thromboelastography revealed no defects in coagulation from groups of clinically stable infants, documenting the functional integrity of coagulation despite, in part, decreased conventional coagulation variables. Because clinically stable preterm and term infants show a relatively small incidence of bleeding, despite prolonged conventional coagulation tests, TEG may better reflect the hemostatic potential of these patients compared with conventional coagulation tests.

IMPLICATIONS

This study assessed the coagulation of preterm and term infants by thrombelastography and found functional integrity of coagulation despite, in part, decreased conventional coagulation variables.

Antiplatelet Effect of Aspirin in Patients With Cerebrovascular Disease

BACKGROUND AND PURPOSE

Aspirin is used commonly to prevent ischemic strokes and other vascular events. Although aspirin is considered safe and effective, it has limited efficacy with a relative risk reduction of 20% to 25% for ischemic stroke. We sought to determine if aspirin as currently used is having its desired antiplatelet effects.

METHODS

We ascertained patients with cerebrovascular disease who were taking only aspirin as an antiplatelet agent. Platelet function was evaluated using a platelet function analyzer (PFA-100). PFA test results were correlated with aspirin dose, formulation, and basic demographic factors.

RESULTS

We ascertained 129 patients, of whom 32% were taking an enteric-coated aspirin preparation and 32% were taking low-dose (< or =162 mg/d) aspirin. For the entire cohort, 37% of patients had normal PFA-100 results, indicating normal platelet function. For the patients taking low-dose aspirin, 56% had normal PFAs compared with 28% of those taking > or =325 mg/d of aspirin, while 65% of patients taking enteric-coated aspirin had normal PFAs compared with 25% taking an uncoated preparation (P<0.01 for both comparisons). Similar results were obtained if PFA results were analyzed using mean closure times (low-dose aspirin, 183 sec; high-dose aspirin, 233 sec; enteric-coated, 173 sec; uncoated, 235 sec; P<0.01 for comparisons). Older patients and women were less likely to have a therapeutic response to aspirin, independent of aspirin dose or formulation.

CONCLUSIONS

A significant proportion of patients taking low-dose aspirin or enteric-coated aspirin have normal platelet function as measured by the PFA-100 test. If these results correlate with clinical events, they have broad implications in determining how aspirin is used and monitored.

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.

Comparison of PFA-100 and bleeding time testing in pediatric patients with suspected hemorrhagic problems

BACKGROUND

The bleeding time test is difficult to perform, standardize, and interpret in children. In this study the authors evaluated the sensitivity, specificity, and efficiency of the bleeding time test and the PFA-100 in a series of children referred for possible bleeding problems.

PATIENTS AND METHODS

Between February 2000 and August 2001 patients aged more than 6 months and less than 18 years of age who were referred to the authors' institution for a hemostatic evaluation were included in the study if residual blood was available for testing on the PFA-100 instrument. Fifty-two children had platelet count, prothrombin time, partial thromboplastin time, bleeding time, and PFA-100 testing performed as well as an evaluation by a hematologist. For PFA-100 testing, 52 patients had Col/Epi measurements; adequate sample remained for Col/ADP testing on 32. Additional testing for diagnostic purposes was at the discretion of the treating physician.

RESULTS

Use of the Col/Epi cartridge in the PFA-100 instrument offered 100% sensitivity and 97% specificity for detection of qualitative platelet abnormalities, compared with 37% and 88%, respectively, for bleeding time testing. For pediatric patients with von Willebrand disease, the sensitivity was 100% using the Col/Epi cartridge, compared with 17% for the bleeding time test. The sensitivities for combined qualitative platelet defects and von Willebrand disease using the Col/Epi or Col/ADP cartridges were 100% and 87%, respectively, compared with 37% for the bleeding time test.

CONCLUSIONS

The PFA-100 is a more efficient test; it can replace the bleeding time test as a component of the laboratory evaluation of children with a potential bleeding problem.

Batroxobin-induced clots exhibit delayed and reduced platelet contractile force in some patients with clotting factor deficiencies

Thrombin causes platelet activation via multiple pathways, and deficient thrombin generation reduces platelet contractile force (PCF) during clot retraction. We hypothesized that PCF in blood samples from clotting factor-deficient patients would be diminished due to delayed or deficient thrombin generation. Blood samples from patients with fibrinogen, and factor V, VII, VIII, IX, X, XI and XIII deficiencies were compared to samples from normal controls. PCF in patient blood clotted with thrombin (1 NIH UmL(-1)) was compared to PCF in clots formed with batroxobin (0.25 micro g mL(-1)). PCF in the former should be normal, but PCF in the latter is dependent on thrombin generation within the sample and might be deficient. In factor VII-(n = 2, P < 0.05), factor VIII-(n = 6, P < 0.005) and factor XI-(n = 2, P < 0.05) deficient platelet-rich plasmas, PCF in batroxobin-induced clots was significantly lower than in thrombin-induced clots. In factor IX deficiency (n = 2), one patient had a dramatic reduction in PCF while a second patient had increased PCF. PCF was insignificantly (P = 0.346) reduced in two patients with factor X deficiency, and was normal in one patient with factor V deficiency. The factor X result is consistent with work in model systems, which indicates that as little as 1-3% factor X activity is sufficient to restore thrombin generation to normal. The factor V result probably indicates that the deficiency is incomplete. PCF in thrombin-induced clots was normal in all of these patients. Low fibrinogen and factor XIII deficiency reduced PCF in both thrombin- and batroxobin-induced clots. These results indicate that PCF is reduced, probably due to delayed thrombin generation, in some factor-deficient platelet-rich plasma samples.

Influence of platelet count, acetylsalicylic acid, von Willebrand's disease, coagulopathies, and haematocrit on results obtained using a platelet function analyser in dogs

The platelet function analyser PFA-100 aspirates blood in vitro from a sample reservoir in disposable test cartridges through a microscopic aperture cut into a biologically active membrane at the end of a capillary. In different cartridges the membrane is coated with collagen and adenosine diphosphate (ADP) or collagen and epinephrine (adrenaline) inducing a platelet plug and closure of the aperture. The closure time and total volume of blood flow through the capillary until closure of its aperture were measured. The correlation between platelet count in samples of thrombocytopenic dogs and results of the collagen/ADP cartridge (closure time: r(S)=-0.579; total volume: r(S)=-0.549) was closer than between platelet count and capillary bleeding time. No significant correlation was observed between platelet count and the results obtained with the collagen/epinephrine cartridge. In addition, a higher sensitivity was obtained for the collagen/ADP cartridge. Injection of acetylsalicylic acid into healthy dogs significantly increased closure time and total volume of both types of cartridges (P<0.01). Two dogs with von Willebrand's disease had abnormal values. In contrast, coagulopathies did not significantly influence the results of the platelet function analyser (P>0.05). Despite adequate sensitivity of measurements using the collagen/ADP cartridge to assess quantitative and qualitative platelet disorders in dogs, the influence of haematocrit (P<0.0001) will limit the clinical application of the analyser.

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.

Antithrombotic agents: implications in dentistry

Thrombosis and the complicating emboli that can result are important causes of illness and death. Thrombosis is of greater overall clinical importance in terms of morbidity and mortality than all of the hemorrhagic disorders combined. Agents such as heparin, low-molecular weight heparin, warfarin, aspirin, ticlopidine, clopidogrel, and tirofiban are used to prevent venous or arterial thrombosis. Patients taking these antithrombotic agents may be at risk for excessive bleeding after invasive dental procedures. The current antithrombotic agents used in medicine are reviewed, and the dental management of patients taking these agents is discussed.

In vitro bleeding test with PFA-100-aspects of controlling individual acetylsalicylic acid induced platelet inhibition in patients with cardiovascular disease

OBJECTIVES

This study investigated the usefulness and practicability of a platelet function analyzer (PFA-100(TM), DADE-Behring, Germany) to determine individual platelet inhibition in patients treated with acetylsalicylic acid (ASA).

BACKGROUND

Patients with coronary artery disease (CAD) routinely and during angioplasty (PTCA) receive standard doses of ASA to avoid acute coronary syndromes and abrupt vessel closures without information of the individual efficacy of platelet inhibition.

METHODS

With the PFA-100(TM) a standardized bleeding time is measured. Whole-blood anticoagulated with 3.2% sodium citrate is aspirated through a capillary ( solidus in circle 200 microm) and through an aperture ( solidus in circle 147 microm). The time until occlusion of the aperture (closure time, CT) by a stable platelet plug induced by shear stress, collagen and epinephrine (COLL/EPI-CT) or shear stress, collagen and adenosine 5'-diphosphate (COLL/ADP-CT) is determined. To examine the usefulness of the PFA-100(TM) as a rapid bedside test and the individual effect of ASA, closure time was measured in healthy individuals (n=17), in patients with stable CAD (n=19) and in patients undergoing PTCA (n=8).

RESULTS

Patients with stable CAD and regular medication with 100 mg ASA per day for at least 3 month showed shorter COLL/ADP-CT in comparison to healthy individuals who took only one single dose of 100 mg ASA. Of the patients with CAD 63% had a COLL/EPI-CT within normal range suggesting a low or no response to ASA. Also only 50% of the patients undergoing PTCA reached the expected COLL/EPI-CT>300 s after an additive single dose of 500 mg ASA intravenously. Neither heparin, phenprocoumon, sex nor different blood sampling methods seem to influence the measurements relevantly.

CONCLUSIONS

This pilot study indicates that with the PFA-100(TM) test device a simple and quick measurement of an in vitro bleeding time is possible. It is able to detect an increase in the bleeding time after a single dose of ASA 100 mg in healthy subjects, reflecting a sensitive detection of ASA induced changes in platelet inhibition respective activation. Differences in the individual response to ASA could be observed in healthy subjects, patients with stable CAD and patients undergoing PTCA. Further studies should validate the PFA-100(TM) with standard methods to determine ASA response in patients with cardiovascular disease and investigate implications for treatment and outcome in this patient group.

Appropriate laboratory assessment as a critical facet in the proper diagnosis and classification of von Willebrand disorder

The correct diagnosis and classification of von Willebrand disease or disorder (VWD) is crucial because the presenting biological activity of von Willebrand factor (VWF) determines both the haemorrhagic risk and the subsequent clinical management. A variety of laboratory assays may be employed, not necessarily restricted to assessments of VWF. Because of assay limitations and von Willebrand disease heterogeneity, no single test procedure is sufficiently 'robust' to permit the detection of all VWD variants. Classically, the test panel might include any combination of: (a) skin bleeding time, (b) von Willebrand factor antigen assay, (c) factor VIII C level, (d) assessment of 'functional' von Willebrand factor (collagen-binding activity or ristocetin co-factor assay), (e) ristocetin-induced platelet aggregation, and (f) multimer analysis. There have also been many new diagnostic developments that have begun to influence the diagnostic process. These include the automation of existing assay procedures, new automated platelet function analyzers such as the PFA-100, and specific von Willebrand factor-factor VIII-binding assays. This chapter focuses on the recommended laboratory process for the investigation of VWD. The selection of an appropriate combination test panel and testing sequence is crucial for the proper diagnosis and classification of congenital von Willebrand disease.

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.