Trauma patients who sustain severe tissue injury and hemorrhage often receive fibrinogen repletion to avert coagulopathy and achieve hemostasis. However, fibrinogen supplementation has not shown a benefit in trauma patients with coagulopathy. The von Clauss functional fibrinogen assay is the primary indication for fibrinogen transfusion. This assay, however, infers fibrinogen levels optically via in vitro clot formation time and does not directly measure the quantity or quality of plasma fibrinogen. We hypothesized that the Clauss fibrinogen activity assay does not accurately reflect true fibrinogen levels in severely injured patients. Here, we demonstrate normal baseline plasma fibrinogen levels as measured by mass spectrometry despite coagulopathic Clauss values in severely injured patients. This discrepancy is most significant in patients with coagulopathy (international normalized ratio of >1.3) or with high shock, and persists even after fibrinogen repletion. These data highlight the need to reevaluate clinical testing of fibrinogen activity and transfusion criteria for the critically injured, and indicate that correcting shock and the oxidative, inflammatory milieu of trauma may be more effective at improving fibrinogen activity. This trial was registered at www.ClinicalTrials.gov as #NCT01838863.