OBJECTIVE Work toward a better definition of disseminated intravascular coagulation (DIC) by characterizing the difference between compensated and uncompensated responses of the hemostatic system to inflammatory stress in baboons and human subjects using global coagulation and molecular marker assays of hemostatic, inflammatory, and endothelial perturbation. DESIGN We conducted prospective evaluation of the response of baboons to increasing concentrations of intravenous Escherichia coli, human subjects to intravenous endotoxin, and baboons to intraperitoneal E. coli. SETTING Animal laboratory and medical intensive care facilities, University of Oklahoma Medical School laboratories. SUBJECTS Cynocephalus baboons; normal healthy male human subjects (age, 24-37 yrs). MEASUREMENTS AND MAIN RESULTS Global coagulation assays, white blood cell counts, and molecular marker assays (ELISA) of components of the inflammatory and hemostatic systems, neutrophil release products, and endothelial injury. A fall in both fibrinogen concentration and platelet counts indicated a decompensated hemostatic response to inflammatory stress (ie, overt DIC). These responses were observed 2-6 hrs after intravenous infusion of 10(9) and 10(10) colony-forming units (CFU)/g of E. coli and after implantation of 10(11) CFU/g of E. coli into the peritoneal cavity. However, 6 hrs after E. coli challenge, these tests were much less reliable as markers of overt DIC because the fibrinogen underwent an acute phase response and the platelet count fell and remained depressed for 48 hrs in the face of a coagulopathic response that was already beginning to resolve, as reflected by a rising fibrinogen concentration. This lack of reliability was particularly evident in the E. coli peritonitis studies, in which one third of the animals recovered, one third remained sick for up to 14 days, and one third died. In contrast, fibrin degradation products and the molecular markers thrombin/antithrombin, soluble fibrin monomer, protein C, and activated protein C/inhibitor complexes responded consistently in a dose-dependent manner regardless of the length of time after challenge. These variables exhibited this dose response to 106 and 108 CFU/g of E. coli in absence of a fall in fibrinogen concentration. This was defined as a compensated hemostatic response to inflammatory stress (ie, non-overt DIC). The values of these variables correlated closely with rising concentrations of markers of neutrophil activation (elastase/alpha 1 antitrypsin) and endothelial injury (soluble thrombomodulin). This was particularly evident in the human response to endotoxin, in which there was abundant evidence of hemostatic marker response in absence of a fall in platelet or fibrinogen concentration, both immediately after endotoxin infusion (first stage, 0-8 hrs after endotoxin) and later (second stage, 12-48 hrs after endotoxin). CONCLUSION Global coagulation tests are most useful in detecting overt consumptive coagulopathy (overt DIC) near the time of challenge or injury (1 to 6 hrs). Molecular markers can detect and grade the degree of hemostatic stress of a non-overt consumptive coagulopathy (nonovert DIC). These markers correlate with degree of endothelial cell injury and reveal a reperfusion injury stage (second stage) in the human endotoxin model of compensated hemostatic stress after all clinical symptoms have subsided and the subjects have returned to work.