Diagnostic accuracy of CT Hounsfield unit in distinguishing Exudative and Transudative Pleural Effusion

Abstract

Background: Differentiation of exudative and transudative types of pleural effusion is crucial for management of patients. Currently, this differentiation is done with the help of biochemical analysis of the aspirated fluid after thoracentesis, which is an invasive procedure. Objective: To evaluate the utility of CT attenuation values in pleural effusion and define a threshold value to differentiate between exudative and transudative pleural effusion. Materials & Methods: A cross sectional study was done on 130 patients showing pleural effusion on CT thorax, mean attenuation was calculated using nine regions of interest on three slices. Within 48 hours of imaging, biochemical assessment was done to decide exudative or transudative nature of fluid as per Light’s criteria. A Receiver operating characteristic curve was drawn to assess the cut off CT attenuation value for distinguishing between exudative and transudative effusion and evaluate its accuracy. Result: Mean CT attenuation value of 57 exudative effusion cases was 5.9 + 6.48 HU, which was higher than 73 cases with transudative pleural effusion (2.97 + 2.69 HU). The difference was statistically significant (p value < 0.0001). The ROC curve revealed a cut off value of 7.5 HU. When the mean CT attenuation value was > 7.5 HU, the specificity and sensitivity of CT scan for diagnosing exudative effusion was 98.6 % and 59.6% respectively. Area under the curve was 0.79, which revealed moderate accuracy of this threshold value. Conclusion: CT attenuation values have moderate accuracy in differentiating between exudative and transudative pleural effusion.