Thromboelastography uses cups and pins made of Cyrolite® plastic to analyze the rate of fibrin clot formation in blood samples. In this study, TEG cups and pins were modified by 4 distinct coating types using plasma-enhanced chemical vapor deposition (PECVD): carboxylated, amine-rich, hydrophobic, SiO2, and analyzed for surface chemistry and wettability. We tested the hypothesis that the coagulation kinetics of recalcified citrated blood plasma is controlled by surface chemistry, in the absence of clot activator. Only carboxylated surfaces became negatively charged upon wetting, and accelerated clot formation in a highly reproducible manner, whereas Cyrolite® and the other coatings had delayed and unpredictable clotting times. These data are consistent with a model whereby carboxylated surfaces selectively adsorb and activate factor XII while repelling other more abundant anionic blood proteins, resulting in reproducible clot kinetics.