Objective-To identify an appropriate diagnostic tool for the early diagnosis of Acute Traumatic Coagulopathy (ATC) and validate this modality through prediction of transfusion requirements in trauma hemorrhage. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Design-Prospective observational cohort study Setting-Level 1 trauma centre Europe PMC Funders Group Europe PMC Funders Author Manuscripts Europe PMC Funders Author ManuscriptsPatients-Adult trauma patients who met the local criteria for full trauma team activation. Exclusion criteria included emergency department (ED) arrival >2 hours after injury, >2000ml of intravenous fluid before ED arrival or transfer from another hospital. Interventions-NoneMeasurements-Blood was collected on arrival in ED and analysed with laboratory prothrombin time (PT), point of care (PoC) PT and rotational thromboelastometry (ROTEM). Prothrombin ratio (PTr) was calculated and ATC defined as laboratory PTr>1.2. Transfusion requirements were recorded for the first 12 hours following admission.Main Results-300 patients were included in the study. Laboratory PT results were available at median 78 (62-103) minutes. PoC PTr had reduced agreement with laboratory PTr in patients with ATC, with 29% false negative results. In ATC the ROTEM Clot Amplitude at 5 minutes (CA5) was diminished by 42% and this persisted throughout clot maturation. ROTEM clotting time was not significantly prolonged. A CA5 threshold ≤35mm had a detection rate of 77% for ATC with a false positive rate of 13%. Patients with CA5 ≤35mm were more likely to receive red cell (46% vs 17%, p<0.001) and plasma (37% vs 11%, p<0.001) transfusions. The CA5 could identify patients who would require massive transfusion (detection rate of 71%, vs 43% for PTr >1.2, p<0.001).Conclusions-In trauma hemorrhage PTr is not rapidly available from the laboratory and PoC devices can be inaccurate. ATC is functionally characterised by a reduction in clot strength. With a threshold of CA5 ≤35mm ROTEM can identify ATC at 5 minutes and predict the need for massive transfusion.