Differentiate pleural effusion from hemothorax after blunt chest trauma; comparison of computed tomography attenuation values

Background/Purpose: Hemothorax should be suspected in any patient with blunt chest trauma. However, not every fluid detected by ultrasound or computed tomography (CT) is a hemothorax, especially in elderlies and multi-morbid patients. To avoid unnecessary emergent tube thoracostomy, we have to make the differentiation in a time fashion. Purpose: To assess the applicability of the CT attenuation value in differentiating hemothorax from pleural effusions. Methods: In this retrospective study, we identified patients who underwent chest CT during a 39-month period. Patients with definitive diagnoses of hemothorax, pleural effusion, and empyema were enrolled. We selected the 3 non-enhanced CT scan slices containing the largest amount of fluid to measure the Hounsfield unit (HU) values of the pleural fluid, and those of the aortic blood. The HU value ratios of the pleural fluid over aortic blood (P/A) were calculated. We compared the HU and P/A values between the patient groups. Receiver operating characteristic (ROC) curves were constructed to determine the validity and cutoff values. Results: Hemothorax had significantly higher attenuation values and P/A ratios than did pleural effusion or empyema (P < 0.001, respectively). In differentiating hemothorax from pleural effusion, excellent accuracies were obtained with an area under the ROC curve (AUC) of 0.964 (95% CI: 0.931 similar to 0.998) for HU values and 0.951 (95% CI: 0.914 similar to 0.988) for P/A ratios. The optimal cutoff values were 15.6 HU (sensitivity: 86.8%; specificity: 97.4%) and 30.0% (sensitivity: 94.7%; specificity: 83.3%). To distinguish hemothorax from empyema, good accuracies were obtained with an AUC of 0.866 (95% CI: 0.797 similar to 0.935) for HU values and 0.870 (95% CI: 0.801 similar to 0.938) for P/A ratios. The optimal cutoff values were 15.9 HU (sensitivity: 86.8%; specificity: 71.2%) and 56.0% (sensitivity: 76.3%; specificity: 90.4%). Conclusions: CT attenuation values and P/A ratios are distinguishable between hemothorax, pleural effusion, and empyema. Copyright (C) 2016, Taiwan Society of Emergency Medicine. Published by Elsevier Taiwan LLC.