Operating characteristics of serial hematocrit measurements in detecting major injury after trauma

Abstract

Study objectives: Assessment of occult blood loss is an essential part of the primary survey after trauma. Physical examination and imaging studies may reveal injuries that contribute to blood loss; hypovolemic shock may be difficult to diagnose in its early stages. Serial hematocrit measurements (ΔHct) are a routine part of the trauma evaluation for identifying volume loss. We designed this study to test the diagnostic performance of serial hematocrit in detecting blood loss after trauma. Methods: This retrospective review was the pilot project for an ongoing prospective study. Data were collected at Kings County Hospital Center, a Level I trauma center with annual census of 150,000 emergency department (ED) visits. Inclusion criteria were all trauma patients suspected of having major injury. Exclusion criteria were patients with known anemia, renal failure, or congestive heart failure or patients receiving blood transfusion within the first 24 hours of ED admission. Demographic information and data acquired from diagnostic procedures were recorded. Hct values at 0 hours (on patients' arrival), 4 hours, and 24 hours were recorded. Patients were classified into 1 of the 2 categories, either minor or major (Injury Severity Score >3) injury. Using receiver operating characteristic curves, the diagnostic performance of ΔHct in detecting major injury was tested. Results: The study subjects consisted of 84% men (n=402) and 16% women (n=76). The mean age was 33±15 years (range 13 to 90 years). Two hundred twenty-nine patients (48%) had major injury, and the remaining (n=249) patients had minor injury. The area under the curve for the initial Hct (0.522, 95% confidence interval [CI] 0.470 to 0.574) was not significantly different (<i>P</i>=.20) from that of the unity line, representing a test with no discriminating power. However, the area under the curves for ΔHct after 4 hours (0.675, 95% CI 0.585 to 0.765) and ΔHct after 24 hours (0.413, 95% CI 0.678 to 0.835) were significantly different (<i>P</i><.0001) from the unity line. There was no significant difference between the area under the curves for ΔHct at 4 hours and ΔHct at 24 hours (<i>P</i>=.07). Conclusion: ΔHct (either after 4 or 24 hours) is a valid diagnostic tool in detecting occult blood loss and could be used in conjunction with other diagnostic tests to identify patients with major injury.