Whole blood aggregation will be determined using a new-generation impedance aggregometer (Multiplate analyzer; Roche Diagnostics). In brief, when platelets expose receptors on their surface they get activated and stick on the Multiplate sensor wires enhancing the electrical resistance between them. An increase in impedance is expressed in arbitrary area under the curve (AUC) units. Platelet aggregation is determined in response to stimulation with arachidonic acid (ASPI (aspirin-sensitive patients identification) test designed to evaluate the effect to acetylsalicylic acid (ASA)) and adenosine diphosphate (ADP) (ADP test designed to evaluate the effect of thienopyridines, such as clopidogrel).