- Sample draws not to exceed 23 per subject for the duration of the study. This includes 18 comparative blood samples and 5 calibration samples to be draw as needed (e.g. recalibration). Including the partial thromboplastin time (PTT) draws described in section 3.1.2, there would be approximately 100 mL or 6.8 tablespoons drawn per subject. In the case where a closed blood drawing system is used consistently, this would be approximately 50 mL or 3.4 tablespoons per subject.