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 function in whole-blood donors is impaired: the effects of painkillers

BACKGROUND: Aspirin (ASA) or non-aspirin-like nonsteroidal anti-inflammatory drugs (NSAIDs) influence platelet (PLT) function by inhibiting cyclooxygenase enzymes. In this study, the aim was to address the use of ASA or NSAIDs before donation and the effect on PLT function. STUDY DESIGN AND METHODS: Donors were asked questions about recent use of ASA or NSAIDs. Furthermore, PLT function was evaluated by measurement of the closure time (CT) in a PLT function analyzer (PFA-100, Dade Behring) and by aggregometry (response to ADP or arachidonic acid [AA]). RESULTS: Of 100 questioned donors, 22 percent had used ASA (n = 4), NSAIDs (n = 6), or paracetamol (n = 12) before donation. Upon assessment of the PLT function in the PFA-100, 27 donors showed values of greater than 180 seconds, indicative of impaired PLT function. Of these, only 7 had used pain killers before donation. Furthermore, 15 of 22 users had normal CTs. Aggregation after stimulation with AA was absent in 33 PLT-rich samples. Again only 8 had reported use of ASA (3), NSAIDs (1), or paracetamol (4). Of the 22 users, 14 had normal AA aggregation responses. All donor samples showed ADP-induced aggregation, indicating PLT integrity. There was no difference between the group of donors who reported the intake of ASA or NSAIDs and the group of donors who did not with respect to the tested PLT function assays. CONCLUSION: It is concluded that there is a considerable group of donors that use PLT-influencing medication before donation. A relation between the reported use and impaired PLT function in blood donors could not be established, however. Impaired PLT function as tested may have other causes than intake of ASA or NSAIDs.     

Effect of preanalytical time-delay on platelet function as measured by multiplate,PFA-100 and VerifyNow

Background and aims: Platelet function testing may help to identify poor responders to antiplatelet drugs. The aim of this study was to compare three commonly used platelet function tests with special focus on the pre-analytical influence of time-delay on the tested parameters.

Methods: We assessed ADP-induced platelet function by the Multiplate, Platelet Function Analyzer-100 (PFA-100) and VerifyNow in nine healthy volunteers and 36 patients receiving clopidogrel or prasugrel 1 and 3?hours after sampling. Results: The PFA-100 demonstrated non-closure time in 23 patients. A more graded response could be detected with the two other devices. Aggregation in whole blood (Multiplate) decreased after 3?hours compared to 1?hour in all subjects (p?p?Conclusion: Responses to ADP are time-dependent after blood sampling for the Multiplate in all subjects and for the VerifyNow in patients on antiplatelet drugs. For both devices, platelet aggregation was reduced 3?hours after sampling which may affect data interpretation and clinical consequences.     

用血小板功能分析仪和血栓弹力图分析仪监测阿司匹林疗效的临床研究

目的 对比老年心血管患者口服阿司匹林后血小板功能抑制情况,以寻求更方便、可靠的监测心血管疾病的方法.方法 每位患者采用血小板聚集仪、血栓弹力图分析仪(TEG)检测血小板聚集率,同时用血小板功能分析仪(PFA-100)检测微孔堵塞时间.结果 血小板聚集仪与PFA-100比较测得的抵抗率差异有统计学意义(P<0.005).而与TEG比较差异无统计学意义(0.10相似文献   

The PFA-100: analysis and interpretation of a platelet function measurement

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.     

Resistance to acetylsalicylic acid and clopidogrel: current status

Resistance to acetylsalicylic acid (ASA) or clopidogrel is understood from the clinical point of view as failure of the drugs to prevent recurrent vascular occlusions. Non-response to ASA and clopidogrel is defined from the laboratory aspect as an inability to cause in vitro detectable platelet function inhibition. It would be beneficial to monitor non-response to ASA or clopidogrel with platelet function methods, which detect the specific effect of these drugs, and thus prevent clinical events caused by failure of therapy. Non-response to ASA and clopidogrel are detected with different platelet function methods, which are not always clinically standardized and are assessing only the global platelet function and not the specific drug effect. Although various studies reporting 5 to 59% non-response for both drugs, support a clinical relevance of ASA and clopidogrel non-response, well-designed clinical prospective trials are required to identify patients with antiplatelet drug resistance. Furthermore, mechanisms explaining this phenomenon of drug resistance are still unknown.     

Elinogrel, an orally and intravenously available ADP-receptor antagonist. How does elinogrel affect a personalized antiplatelet therapy?

The antiplatelet therapy with aspirin and the ADP-receptor blocker clopidogrel is currently the standard medication after coronary intervention or after acute coronary syndrome to prevent recurrent ischemic events and reduce mortality. However, high interindividual response variability to antiplatelet treatment is described in up to 44% of treated patients. A poor response to clopidogrel is caused by multifactorial mechanisms. Individual risk assessment including platelet function testing (PFT) can help to identify high risk patients, although recent randomized trials to investigate effects of PFT-guided therapy have failed to detect an impact on prognostic outcome. Poor response to standard antiplatelet agents can be overcome by switching to alternate substances. Elinogrel is a novel competitive, reversible ADP-receptor antagonist available in oral and intravenous formulation. Additional treatment with elinogrel showed advantages over clopidogrel, including more rapid, less variable, and more complete inhibition of platelet function without significantly increased bleeding complications. This review gives an overview over the investigational drug elinogrel for use in a personalized antiplatelet approach.     

The platelet function analyzer (PFA-100): an update on its clinical use

The platelet function analyzer, PFA-100, is a relatively new method which has been developed as a quantitative, simple and rapid in vitro tool of assessing 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 document that the PFA-100 is a useful screening tool for the investigation of von Willebrand's disease and various acquired and congenital intrinsic platelet function disorders. It is also useful for evaluating primary hemostasis before surgical procedures and for monitoring desmopressin therapy in patients with type 1 von Willebrand's disease. Moreover, recent studies have shown its potential for therapeutic monitoring of the effectiveness of anti-platelet medications in cardiovascular disease management. 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.     

Modified platelet aggregation test in patients on ASA and/or clopidogrel

Therapy with acetylsalicylic acid (ASA) and/or clopidogrel is used to achieve prophylactic inhibition of platelet aggregation in patients with arterial thrombosis. We examined if aggregometry can be used to see the effect of antiplatelet drugs (ASA 30, 50, 100, 300 mg/d, clopidogrel 75 mg/d or ASA 100 + clopidogrel 75 mg/d). A modified platelet aggregation test was used to investigate maximum aggregation in response to ADP, collagen, adrenalin and arachidonic acid. Reference values were established based on healthy individuals. We devised a simple scoring system for detection of inadequate platelet inhibition. Compared with the control group, we detected a significant delay of maximum aggregation in response to all agonists in patients on ASA and combination therapy ASA + clopidogrel. Patients on clopidogrel alone were found to have prolonged aggregation when induced with ADP, collagen and arachidonic acid. The failure rate to achieve adequate platelet inhibition on 100 mg/d ASA, 75 mg/d clopidogrel or combination therapy was 27%, 26% and 7%, respectively. Our results demonstrate that platelet inhibition in aggregometry is inadequate in many patients with arterial thrombosis.     

Platelet function testing to assess effectiveness of platelet transfusion therapy.

BACKGROUND: Posttransfusion corrected count increments (CCI) following administration of platelets is the standard method for assessing effectiveness of platelet transfusion therapy. However, improvement in platelet count following transfusion may not necessarily indicate improvement in platelet function or restoration of primary hemostatic capacity. To address this possibility, we investigated the effectiveness of platelet transfusion based on results of the Platelet Function Analyzer (PFA-100) and post-transfusion CCI. INVESTIGATION DESIGN AND METHODS: Platelet transfusion requests with different indications received at the blood bank were evaluated for inclusion in the investigation. Pre-transfusion, the following laboratory tests were performed: (1) PFA-100 assays (blood collected in 3.2% buffered sodium citrate) performed with CEPI and CADP test cartridges; (2) complete blood count (in EDTA) and platelet count; and (3) routine coagulation profile including PT, PTT, fibrinogen and D-Dimer. Only patients with normal coagulation profiles were included. The same set of tests were performed on a new blood sample collected 10-60 min post-transfusion. Chart review and clinical evaluation for response to platelet therapy were performed on each occasion of transfusion. RESULTS: Thirty-one patients, five of whom were transfused on more than one occasion were evaluated. Thirty-five transfusion incidents were included. Posttransfusion outcomes were divided into two groups--those that resulted in shortening (>40 s) or normalization of the closure time (Group A) and those that had no change or greater prolongation of the closure time (Group B) when compared to the pre-transfusion value. Seventeen and eighteen transfusion episodes were categorized as Groups A and B, respectively. In Group A with improved PFA testing, nine patients had bleeding as indication for transfusion and six of these had concomitant improvement in their clinical picture as confirmed by control of hemorrhage. In contrast in Group B with no improvement in PFA testing, seven patients had bleeding as indication for transfusion and none showed cessation of hemorrhagic symptoms. These findings were statistically significant (p=0.0114). Similar evaluation using the post-transfusion CCI showed no correlation to bleeding symptoms in these patients (p-=0.500). CONCLUSIONS: In this evaluation, platelet function testing using the PFA-100 provided a better indication of transfusion outcome than did the post-transfusion CCI. Using this approach, PFA-100 may be an effective aid for supporting platelet transfusion decisions and may further aid in improving management of the hospital blood bank platelet inventory.     

Indices of platelet activation and the stability of coronary artery disease

AIM: To determine whether indices of platelet activation are associated with the stability of coronary artery disease (CAD). METHODS: Platelet function was examined in 677 consecutive aspirin-treated patients presenting for cardiac catheterization. Patients were grouped into recent myocardial infarction (MI), no MI but angiographically documented CAD (non-MI CAD) and no angiographically detectible CAD (no CAD), as well as additional subgroups. RESULTS: Compared with non-MI CAD or no CAD patients, more patients with recent MI had a shortened platelet function analyzer (PFA)-100 collagen-epinephrine closure time (CT) and increased circulating monocyte-platelet aggregates, neutrophil-platelet aggregates, activated platelet surface GPIIb-IIIa and plasma soluble CD40 ligand (sCD40L). More patients with non-MI CAD had shortened PFA-100 CTs and increased monocyte-platelet aggregates compared with patients with no CAD. Platelet surface P-selectin did not differ among the groups. Subgroup analysis revealed that decreasing PFA-100 CT correlated with the stability of CAD. CONCLUSIONS: Indices of platelet activation, especially the PFA-100 CT, are associated with the stability of CAD, and may reflect plaque instability, an ongoing thrombotic state and/or reduced responsiveness to aspirin.     

Aspirin therapy for inhibition of platelet reactivity in the presence of erythrocytes in patients with vascular disease

Inhibition of erythrocyte (RBC) promotion of platelet reactivity could improve the antiplatelet effect of aspirin (ASA). We tested different ASA regimens for optimal inhibition of platelets and the effects of RBC in patients with a history of vascular diseases. Collagen-induced platelet activation (14C-5HT, TXA2 release) and platelet recruitment (proaggregatory activity of cell-free releasates from activated platelets) were measured in PRP, platelet-RBC (Hct 40%), and whole blood (WB) in 206 patients initially on 200-300-mg ASA/day. Their regimen was modified to biweekly 500 mg (loading dose, L) plus daily or twice-daily low-dose ASA (50 or 100 mg). TXA2 was inhibited with all regimens. Percentage of patients with suboptimal inhibition of platelet recruitment in WB was 200-300 ASA/day (41%), L-50/day (87%), L-100/day (58%), L-50/twice-daily (39%), and L-100/twice-daily (20%; P < 0.05 vs other regimens). 14C-5HT release was inhibited to the greatest extent with L-100/twice-daily in PRP + RBC or WB (P < 0.05 vs other regimens) due to greater inhibition of the RBC prothrombotic effect. Compared with other ASA regimens, L-100 twice-daily (equivalent to 221-mg ASA/day in the 14-day cycle), reduced by >50% the proportion of patients with suboptimal inhibition of platelet recruitment in WB and inhibited 14C-5HT release to the greatest extent.     

Platelet reactivity index by Grotemeyer

In 1974 Wu and Hoak described a method for determining circulating platelet aggregates. This method was modified by Grotemeyer in 1983. The platelet reactivity index (PR) is based on the ratio of platelet aggregates in blood samples obtained in different buffer solutions. Platelet aggregates are resolved, when blood is sampled in EDTA-buffer, but remain fixed when EDTA-formalin-buffer is used. Generally, the PR is preferred, because in vitro manipulations of platelets are not necessary, and the results are estimated automatically. PR values above 1.05 are suspicious for elevated platelet aggregation. PR values above 1.2 indicate pathological changes in platelet aggregation. The PR is inexpensive (4.0 D ) and rapid to perform. PR-values were used successfully to identify non-responders to secondary prophylaxis with acetylsalicylic acid (ASA), i. e. patients suffering from stroke (33%) and after cardiac ischaemia (18%). Furthermore, elevated PR-values correlated significantly with the incidence of arterial thromboembolic complications. The PR correlated well in a own prospective study (drug monitoring) with values received from the retention test Homburg (RT-H) and the platelet function analyser (PFA-100). These data indicate that the values of the PR seems to be highly predictive for the evaluation of the ASA therapy. However, the PR is not suitable for the determination of ASA overdosage.     

Platelet reactivity tests for assessing antiplatelet drug response: what the clinician needs to know

Antiplatelet therapy is a cornerstone in the treatment of cardiovascular disease to prevent ischemic events. Various tests have become clinically available to measure platelet function after antiplatelet treatment. A wide interpatient variability in the magnitude of platelet inhibition has been demonstrated in numerous studies, especially in response to clopidogrel. Several reasons including clinical, pharmacological and genetic factors have been identified. High on-clopidogrel platelet reactivity has been linked to adverse clinical outcome, in particular to stent thrombosis after percutaneous coronary interventions. New antiplatelet drugs including prasugrel and ticagrelor have been advocated to overcome the limitations of clopidogrel. Several studies addressed the concept of tailored antiplatelet treatment according to the results of platelet function testing. Within this review, we summarize the current status of personalized antiplatelet therapy for cardiovascular disease.     

Preoperative identification of patients with impaired (primary) haemostasis. A practical concept

The findings of a large prospective study designed to identify primary and/or secondary haemostatic disorders before surgical interventions are presented. A total of 5649 unselected adult patients were enrolled to identify impaired haemostasis before surgical interventions. Each patient was asked to answer a standardized questionnaire concerning bleeding history. Activated partial thromboplastin time (aPTT), prothrombin time (PT), and platelet counts (PC) including PFA-100 (platelet function analyzer): collagen-epinephrine (C/E), and collagen-ADP (C/ADP) were routinely done in all patients. Additional tests, bleeding time (BT), von Willebrand factor (VWF:Ag, VWF:Rcof) and a further haemostaseological diagnostic was performed only in patients with a positive bleeding history and/or evidence of impaired haemostasis; e.g., drug ingestion. The bleeding history was negative in 5021 patients (88.8%) but positive in the remaining 628 (11.2%). Impaired haemostasis could be verified only in 256 (40.8%) of these patients. The vast majority was identified with PFA-100: C/E (n = 250; 97.7%). The sensitivity of the PFA-100: collagen-epinephrine was the highest (90.8%) in comparison to the other screening tests (BT, aPTT, PT, VWF : Ag). The positive predictive value (to detection of impaired haemostasis) of the PFA-100: collagen-epinephrine with the standardized questionnaire was high (82%), but the negative predictive value was higher (93%). The use of a standardized questionnaire and, if indicated, the PFA-100: C/E and/or other specific tests not only ensure the detection of impaired haemostasis in almost every case but also a significant reduction of the costs. Based on these data, national regards are formulated or under construction.     

Monitoring of clopidogrel action: comparison of methods

BACKGROUND: Clopidogrel is a potent drug for prevention of adverse effects during and after coronary intervention. Increasing experience indicates that a significant proportion of patients do not respond adequately to clopidogrel. Because failure of antiplatelet therapy can have severe consequences, there is need for a reliable assay to quantify the effectiveness of clopidogrel treatment. METHODS: Of 24 healthy volunteers admitted to the study, 18 were treated for 1 week with clopidogrel (300-mg loading dose and 75-mg maintenance dose), and 6 with placebo. Platelet function was monitored by 2 assays, based on flow cytometry and enzyme immunoassay, that measure the phosphorylation status of vasodilator-stimulated phosphoprotein (VASP) and by aggregometry, flow cytometry of P-selectin, and the platelet function analyzer at baseline, on days 1-5, and on day 9 of treatment. RESULTS: Aggregometry and VASP phosphorylation revealed a loss of platelet response to ADP within 12 h after clopidogrel intake. The phosphorylation status of VASP correlated with the inhibition of platelet aggregation. In contrast, neither P-selectin expression nor PFA-100 closure time was a clear indicator of clopidogrel effects on platelets. CONCLUSIONS: VASP phosphorylation assays are reliable for quantifying clopidogrel effects. Because the VASP assay directly measures the function of the clopidogrel target, the P2Y12 receptor, the assay is selective for clopidogrel effects rather than effects of other platelet inhibitors commonly in use.     

New Antiplatelet Agents and the Role of Platelet Function Testing in Acute Coronary Syndromes

Background

Dual antiplatelet therapy is a guideline mandated for patients with acute coronary syndromes (ACS). Despite its use, thrombotic events continue to occur both early and late. Platelet function testing has been used to define the in vitro effects of new antiplatelet agents, and it has been suggested that it be used to choose therapy. The role of platelet function testing, particularly with newer antiplatelet agents, remains unclear.

Objective

We review the rationale for platelet function testing and its application in monitoring patients on antiplatelet therapy. We also review recent clinical trials of newer antiplatelet agents. On the basis of this review, we reach conclusions on the current role of antiplatelet function testing in monitoring modern antiplatelet therapy and the role of the new antiplatelet agents in the treatment of ACS.

Methods

We reviewed recent publications on platelet function testing and clinical trials of newer antiplatelet therapies compared with clopidogrel.

Results

Platelet function testing is complex, but there is now a bedside test, VerifyNow. High platelet reactivity has been associated with worse cardiovascular outcomes in patients undergoing percutaneous coronary intervention. Recent clinical trials have not found any advantage in outcomes in patients who have their therapy adjusted by monitoring their platelet function. Newer agents, prasugrel, ticagrelor, and cangrelor, produce more rapid, complete, less variable effects on platelet function than clopidogrel. Prasugrel was found to improve outcomes compared with clopidogrel in patients with ACS undergoing percutaneous intervention. Ticagrelor is beneficial in all patients with ACS and reduces cardiovascular mortality compared with clopidogrel. Cangrelor improves outcomes in patients undergoing stenting. Recent studies to assess the role of platelet function monitoring of the effects of clopidogrel and modifying treatments have not been successful.

Conclusion

Recent clinical trials have indicated that newer antiplatelet agents have advantages over clopidogrel in the treatment of ACS. Platelet function testing gives us a guide to the timing, efficacy, and variability of therapy and can correlate with poor patient outcomes; however, the use of antiplatelet function testing to tailor therapy does not seem appropriate.     

Monitoring of antiplatelet therapy. Current limitations, challenges, and perspectives

Screening of platelet function can be performed by point-of-care testing followed by platelet aggregometry in response to agonists such as collagen, adenosine diphosphate, epinephrine, and arachidonic acid. Despite in use for decades, this technique is not well standardized. Monitoring of antiplatelet therapy is increasingly applied in patients at high risk for re-thrombosis or bleeding. To assess pharmacological inhibition of platelet function, agonist-induced platelet aggregation, thromboxane B2 (TxB2) and vasodilator-stimulated protein phosphorylation (VASP) are being measured. While serum TxB2 levels of < 2 ng/ml reflect aspirin-induced inhibition of cyclo-oxygenase-1 activity with high sensitivity, VASP exhibits a wide variability upon treatment with clopidogrel or prasugrel. Multiple studies reveal an association between high residual platelet reactivity and adverse cardiovascular events in patients on antiplatelet therapy. However, despite the plethora of platelet function assays currently under investigation, their use in daily practice cannot be recommended. This is due to several reasons: (i) there is no consensus on the method and a respective cut-off value associated with clinical adverse outcome, and (ii) data demonstrating any benefit of tailored antiplatelet therapy and its monitoring (based on assessment of platelet functions) are still limited. Thus, appropriate identification of 'resistant' or 'poor responders' to antiplatelet agents remains challenging in clinical practice.     

Preservation of in vitro function of platelets stored in the presence of a synthetic dual inhibitor of factor Xa and thrombin

BACKGROUND: The use of citrate anticoagulant limits the clinical significance of platelet function tests. Thrombin inhibitors cannot prevent thrombin-induced platelet activation completely. We examined the influence of benzylsulfonyl-d-Arg-Pro-4-amidinobenzylamide (BAPA), a dual inhibitor of Factor Xa (FXa) and thrombin, on platelet responsiveness to agonists when measured between 2 and 24 h after venipuncture. METHODS: Blood samples from 36 individuals were anticoagulated with citrate and BAPA, respectively. Turbidimetric platelet aggregometry (TPA) and impedance platelet aggregometry (IPA), a whole blood platelet counting assay for measuring platelet aggregation (PCA), and Platelet Function Anlayzer-100 (PFA-100 closure times (CTs) were determined after whole blood storage between 2 and 24 h after venipuncture. Native whole blood was studied over 48 h to determine the inhibition of thrombin generation by BAPA, hirudin and melagatran. RESULTS: BAPA inhibited thrombin generation completely for 48 h, while hirudin and melagatran did not. The use of citrate resulted in significantly reduced TPA induced by arachidonic acid (AA) or adenosine 5'-diphosphate (ADP), and significantly reduced IPA regardless of agonist when measured 10 and 24 h after blood collection. PCA ratios in citrated blood also dropped significantly 10 and 24 h after venipuncture. The length of storage of BAPA-anticoagulated blood samples over 24 h had no significant influence on any platelet response. The reproducibility of platelet function assay results obtained from BAPA-anticoagulated samples was significantly better than corresponding data from citrated blood. CONCLUSION: TPA, IPA, PCA or PFA-100 CTs remain stable for 24 h when whole blood is anticoagulated with a dual inhibitor of FXa and thrombin. This would greatly simplify the shipment of samples for platelet function testing.     

Limited usefulness of the PFA-100 for the monitoring of ADP receptor antagonists--in vitro experience.

We have evaluated the usefulness of the PFA-100 system (collagen/ADP and collagen/epinephrine cartridges) to assess the in vitro effects of a few platelet function inhibitors: Aspisol (60 microg/ml), 4-[4-[4-(aminoiminomethyl]-1-piperazinyl]-1-piperidineactetic acid, hydrochloride trihydrate (GR144053F, fibrinogen receptor antagonist, 100 nM), adenosine-3',5'-diphosphate (A3P5P, P2Y1 ADP receptor antagonist, 500 microM) and Bis[(adenosine-5'-O-phosphorodithioyl)methylene]-phosphinic acid (APTMPA, P2Y12 ADP receptor antagonist, 500 microM) on platelet function, as compared with the other commonly used diagnostic technique, a whole blood electrical aggregometry (20 microM ADP or 0.5 mM arachidonic acid). The in vitro studies were carried out on a group of 38 subjects. Whereas all the examined platelet antagonists and inhibitors almost completely blocked the 20 microM ADP- or 0.5 mM arachidonic acid-induced (in the case of acetylsalicylic acid) whole blood aggregation, only two inhibitors (Aspisol and GR144053F) remained effective in a significant prolongation of the PFA-100 occlusion time. Otherwise, using the PFA-100 system we were not able to detect the inhibitory actions of ADP receptor antagonists- P2Y1 and P2Y12. Our findings point to a limited usefulness of the PFA-100 system for the monitoring of the effectiveness of ADP receptor antagonists. The outcomes of this study show that platelet aggregometry in whole blood is characterised by the highest sensitivity in the monitoring of the investigated blood platelet inhibitors.