[Monitoring anticoagulation by fondaparinux: determination of anti factor Xa-level]. / Uberwachung der Antikoagulanzientherapie mit Fondaparinux. Bestimmung der Anti-Faktor-Xa-Spiegel.

In contrast to low molecular weight heparin and unfractionated heparin fondaparinux is a selective synthetic factor Xa inhibitor. It was approved for thromboprophylaxis after major orthopaedic surgery three years ago. Because of its excellent bioavailability after subcutaneous application once daily independent of body weight (within the range of 50-100 kg) monitoring of the substance is usually not necessary. Since its recent approval for therapeutic use in acute venous thrombosis monitoring might be useful in special clinical situations (e. g. suspected over- or underdosing, renal insufficiency or severe bleeding complications). Based on standard laboratory methods for the chromogenic determination of the anti-factor Xa activity of low molecular weight heparin several chromogenic substrate assays were published for monitoring. Only fondaparinux should be used for calibration and measured fondaparinux levels should be expressed in mug/ml of the fondaparinux calibrator. Because of its single synthetic chemical entity, a linear dose/response relationship over a wide concentration range is observed and measurements are of high precision.

Use of native or platelet count adjusted platelet rich plasma for platelet aggregation measurements.

BACKGROUND: It is still not clear whether native or platelet count adjusted platelet rich plasma (PRP) should be used for platelet aggregation measurements. AIM: To evaluate the necessity of using adjusted PRP in platelet function testing. METHODS: Platelet aggregation with native PRP and adjusted PRP (platelet count: 250/nl, obtained by diluting native PRP with platelet poor plasma) was performed on the Behring Coagulation Timer (BCT(R)) using ADP, collagen, and arachidonic acid as agonists. Healthy subjects, patients on antiplatelet treatment, and patients with thrombocytosis (platelet counts in PRP > 1250/nl) were investigated. RESULTS: No significant differences in the maximum aggregation response were seen when using either native or adjusted PRP from healthy subjects and patients on antiplatelet treatment. Nevertheless, some patients taking aspirin or clopidogrel showed reduced inhibition of ADP and arachidonic acid induced aggregation in adjusted PRP but not in native PRP. The maximum velocity of healthy subjects and patients on antiplatelet treatment varied significantly as a result of the degree of dilution of the adjusted PRP. Surprisingly, the BCT provided good results when measuring platelet aggregation of native PRP from patients with thrombocytosis, whereas commonly used aggregometers could not analyse platelet aggregation of native PRP in these patients. CONCLUSION: The time consuming process of PRP adjustment may not be necessary for platelet aggregation measurements. Moreover, using adjusted PRP for monitoring aspirin or clopidogrel treatment may falsify results. Therefore, it may be better to use native PRP for platelet aggregation measurements, even in patients with thrombocytosis.

Influence of blood collection techniques on platelet function.

For investigations of platelet function it is recommended that venipuncture should be performed using ordinary needle systems instead of butterfly cannulae systems. Platelets might be activated in the long plastic tubes of butterfly systems. The aim of this study was to investigate the dependency of platelet function results on blood sampling using different collection systems. Therefore, blood of 25 healthy volunteers was collected from both arms using at the same time on one side a 21-gauge needle and on the other side a 21-gauge butterfly cannula system. Both samples of each volunteer were analyzed on the PFA-100. Platelet aggregation was performed on the Behring Coagulation Timer (BCT) and the optical aggregometer PAP-4 using ADP, collagen and arachidonic acid to induce platelet aggregation in platelet-rich plasma. No significant prolongation of the closure times on the PFA-100 with the COL/EPI cartridge and the COL/ADP cartridge was observed when using butterfly cannulae. The results of optical aggregometry were not significantly different. The maximum aggregation response did not differ significantly for both collection systems. Aggregometry and the PFA-100 system are not affected by different blood collection systems. Therefore butterfly cannulae can be used for sample collection to investigate platelet function.