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

Background/PurposeHemothorax 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.PurposeTo assess the applicability of the CT attenuation value in differentiating hemothorax from pleural effusions.MethodsIn 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.ResultsHemothorax 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∼0.998) for HU values and 0.951 (95% CI: 0.914∼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∼0.935) for HU values and 0.870 (95% CI: 0.801∼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%).ConclusionsCT attenuation values and P/A ratios are distinguishable between hemothorax, pleural effusion, and empyema.