BackgroundActivated monocytes are able to express tissue factor (TF), a potent procoagulant. The effect of injury on monocyte TF expression is not known. We have found that patients with head injury (HI) have increased antithrombin activity and decreased platelet function compared with non–head-injured trauma patients. Our objective was to determine whether injury increases TF expression by monocytes and whether this increased TF expression is attenuated in patients with HI.MethodsWe prospectively enrolled 37 trauma patients (meeting the entry criterion of an Injury Severity Score [ISS] ≥ 9) and 11 healthy control subjects. We sampled blood on arrival and then at 24, 48, and 72 hours. We performed flow cytometry with antibody markers for monocytes (CD14), platelets (CD42a), and TF. We compared results of patients with HI (Glasgow Coma Scale score ≤ 9 and Abbreviated Injury Scale Head/Neck score ≥ 3) with patients without HI and with controls.ResultsPatients had a mean ISS of 23.9 ± 2.3 (± SEM), mean age of 45 ± 3 years, and mean length of stay of 17.9 ± 3.2 days. Seventy-six percent were men, and 97% had blunt trauma. The overall mortality rate was 11%. Trauma patients had greater monocyte TF expression than controls for all time periods (p< 0.05). Trauma patients with HI had elevated monocyte TF expression compared with controls for the initial and 24-hour time periods, but they subsequently had more rapid return of monocyte TF expression to baseline (despite a higher ISS) than trauma patients without HI. Trauma patients both with and without HI had increased platelet-monocyte binding at each time versus controls.ConclusionTrauma induces TF expression on monocytes. Patients with HI have attenuation of this expression by 24 hours after injury. The attenuation of TF expression by monocytes in HI parallels the increase in AT and the decrease in platelet function seen after HI. The correlation of TF expression with platelet-monocyte binding suggests that platelet binding may lead to monocyte activation.