ProC Global is a new global clotting assay designed to evaluate the functionality of the protein C anticoagulant pathway. It is based on the ability of endogenous activated protein C, generated by activation of protein C by Protac, to prolong an activated partial thromboplastin time, and the results are expressed in protein C activation time normalized ratio (PCAT-NR), after normalization. This multicenter trial involving five European laboratories was designed in order to determine the ability of the ProC Global assay to distinguish patients with and without abnormalities of the protein C pathway. The PCAT-NR was significantly lower in the patients with a thrombotic history not on oral anticoagulant treatment (n = 627) than in the healthy controls (n = 148), even after exclusion from both groups of the patients with abnormality of the protein C pathway. Using receiver operator characteristics analysis, the cut-off level of PCAT-NR = 0.80 was found to provide the best sensitivity-specificity ratio. All the carriers of the factor V Leiden mutation (n = 73), as well as all the patients with activated protein C resistance (n = 42), had a PCAT-NR below 0.80. The ProC Global assay performed well in patients with combined defects (97.0%, n = 33) or protein C deficiency (91.3%, n = 46), but it failed to detect all of them, and one patient with combined defects as well as four patients with a low protein C level had a PCAT-NR above the cut-off level. The sensitivity of the assay for protein S deficiency (n = 58) was weak (only 69.0%) and, surprisingly, more than 40% of the 375 patients without any of these abnormalities of the protein C pathway had a PCAT-NR below the cut-off level.