BACKGROUND: The aim of this study was to assess the effect of anticoagulation treatment on platelet aggregation. METHODS: The study group consisted of 24 patients on long-term warfarin therapy without any antiaggregation therapy. Platelet aggregation was measured using VerifyNow with arachidonic acid (AA) as an inducer in 23 patients and with light transmission aggregometry (LTA) in 19 patients using four different agonists. All patients had their international normalized ratio (INR) checked regularly. RESULTS: The mean INR value was 2.07 (SD 0.6). The average aggregation measured by VerifyNow was found to be 637.5 (SD 36.6) aspirin reaction units. The values of average aggregability in LTA were 73.3 % (SD 4.5 %), 73.2 % (SD 6 %) and 72.1 % (SD 4.8 %) in case of aggregation induced by AA, ADP, and collagen, respectively. Epinephrine-induced aggregability was 65.3 % (SD 14.7 %). Regression analysis between INR and values of collagen- or epinephrine-induced aggregability (r = 0.654 and 0.575) was found statistically signifi cant (p = 0.004 and 0.016); every increase in INR by 0,1 brings about an increase in collagen- and epinephrine-induced aggregation values by 1.5 and 4, respectively. CONCLUSION: Administration of warfarin does not produce a signifi cant reduction in platelet aggregation. On the contrary, prolonged INR evokes a mild increase in aggregation induced by collagen or epinephrine (Tab. 2, Fig. 3, Ref. 32).

Department of Biostatistics National Institute of Public Health Prague Czech Republic and Department of Epidemiology and Biostatistics 3rd Faculty of Medicine Charles University Prague Czech Republic

Department of Cardiology 3rd Faculty of Medicine Charles University and University Hospital Královské Vinohrady Prague Czech Republic

Department of Neurology 3rd Faculty of Medicine Charles University and University Hospital Královské Vinohrady Prague Czech Republic

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