An apparatus for use with a patient having a vessel carrying blood to ascertain characteristics of the blood. The apparatus includes a display module and a probe having a distal extremity adapted to be inserted into the vessel of the patient and having a proximal extremity coupled to the display module. The probe includes a sensor in the distal extremity for providing an electrical signal to the display module when the probe is disposed in the blood. The probe can have an antithrombogenic surface treatment for inhibiting the adhesion of blood components to the probe when disposed in the blood.