ObjectiveTo investigate protein complement 3a (C3a) and protein complement 3 (C3) plasma levels in trauma patients directly after the injury, in relation to the patients' outcome, the development of sepsis, or the injury severity, as determined by either the Polytrauma Score (PTS), the Injury Severity Score (ISS), or the Trauma and Injury Severity Score (TRISS).DesignProspective study.SettingSurgical intensive care unit in a university hospital.PatientsThirty-four patients with multiple trauma.InterventionsNone.Measurements and Main ResultsC3a and C3 concentrations, as well as the C3a/C3 ratio, were determined at the time of the accident (T0), at the emergency admission (T1), 8 hrs after the accident (T2), and every 8 hrs until day 3, every 12 hrs until day 6, and once daily on days 7 and 8. The C3a plasma concentrations and the C3a/C3 ratios of nonsurvivors were significantly greater at T0 or T1 as compared with those survivors (p = .008 or .033). Patients who developed sepsis had higher C3a plasma levels at the scene of accident than patients without complications. However, these differences did not reach statistical significance (p = .051), although a clear trend was apparent.Patients were grouped according to the severity of injury, as determined by either the PTS, ISS, or TRISS. We found significant differences in the both the mean C3a values and the C3a/C3 ratio among the different groups, during the first 8 hrs after the injury. In addition, a significant correlation was observed between the C3a concentration or the C3a/C3 ratio at T0 to T2 and either the ISS (r2= .49), PTS (r2= .22) or the TRISS (r2= .45), which was similar to correlations between injury severity scores themselves (r2= .36 to .58).ConclusionsComplement activation occurs immediately after the injury. The degree of activation is a hallmark for the outcome of a patient. Determination of C3a concentrations, at the scene of the accident, may prove helpful to assess the severity of the injury and to determine the prognosis. The amount of C3a and the C3a/C3 ratio may be useful as additional parameters to the existing trauma scoring systems, such as, PTS, ISS, and TRISS. (Crit Care Med 1997; 25:2015-2024)