Screening US for blunt abdominal trauma: objective predictors of false-negative findings and missed injuries

PURPOSE: To determine the risk for missed injury in patients with blunt abdominal trauma and negative findings at screening ultrasonography (US) and with coexistent hematuria or fracture of the sixth through 12th ribs, lumbar spine, or pelvis. MATERIALS AND METHODS: From a database of 4,000 patients screened with US for blunt abdominal trauma at a level 1 trauma center, the 3,679 patients with negative US findings were retrospectively classified by consensus of two authors into high-risk (n = 494) and low-risk (n = 3,185) groups based on the presence of hypothetical predictors of missed injury: hematuria (n = 96) or fracture of the sixth through 12th ribs (n = 216), lumbar spine (n = 105), or pelvis (n = 174). Outcome in each patient was determined by the same two authors consensually after retrospective review of the trauma registry and all radiologic, surgical, and autopsy reports. The risk for missed abdominal injury was determined for each patient risk group and for each hypothetical predictor. Risks were statistically compared by using the Pearson chi2, Fisher exact, or Fisher-Freeman-Halton exact test, depending on expected frequencies. RESULTS: High-risk patients were 24 times more likely to have abdominal injuries after negative US findings (30 6.1%] of 494) than were low-risk patients (eight 0.25%] of 3,185) (P <.001). Among high-risk patients, the absolute risks for missed abdominal injury associated with specific predictors were 15.6% (15 of 96 patients) for hematuria, 6.0% (13 of 216) for lower rib fractures, 7.6% (eight of 105) for lumbar spine fractures, and 5.2% (nine of 174) for pelvic fractures. Each of these risks was significantly higher for patients in the high-risk group than for those in the low-risk group (P <.001). CONCLUSION: Hematuria and fracture of the lower ribs, lumbar spine, or pelvis are objective predictors of missed abdominal injury in patients with blunt abdominal trauma and negative US findings, and such patients may benefit from additional screening with computed tomography.