Vol. 3, Issue 1, Part A (2021)

Background: Pleural effusion is defined as an abnormal fluid collection in the pleural cavity. In healthy people, pleural cavity contains a small amount of fluid secreted by the parietal pleura (0.25 ml/kg). As the fluid amount increases, lung compliance reduces due to the increased lung capillary pressure. It may lead to patient dyspnea. Pleural effusion can be transudate or exudate. Differentiation between the transudate and exudate is important for clinical management. Abnormal fluid accumulation in the pleural space results from increased permeability of the capillary bed. Thoracentesis is an invasive diagnostic method for obtaining pleural fluid for analysis. This procedure is associated with iatrogenic complications. With the help of computed tomography (CT) density of pleural fluid can be assessed by measuring Hounsfield unit (HU). Type of pleural fluid can be predicted based on HU value of the pleural fluid. Also assessing features like pleural thickening and septation within pleural effusion. This study was conducted with an aim to differentiate between exudative & transudative pleural effusion based on the CT Hounsfield unit. Methodology: An observational cross-sectional study was conducted in the department of radiodiagnosis consisting of 48 patients with the pleural effusion after clearance from the institutional ethical committee. CT Thorax was performed by a 16 slice MDCT machine. HU value of pleural effusions were measured at three different locations on three different axial slices on the mediastinal window, mean of three HU was calculated and correlated to pathological report. Results: The median HU value of exudate was 17.94 and transudate was 8.03. The median CT HU value was higher in exudate than that of transudate. The p-value found was 0.001. There was a statistically significant difference in the HU value between the exudate and transudate. According to the Light’s criteria, out of 48(100%) patients with pleural effusions, 36(75%) pleural effusions were exudate and the remaining were transudate. The overall relationship between the pleural fluid protein/total protein and CT HU was found using Spearman’s rho correlation coefficient. As the pleural fluid protein/total protein ratio increased, the CT HU value also increased. There was a moderate positive correlation between the pleural fluid protein/total protein ratio and CT HU value (rho=0.55, p=0.001). Conclusion: The exudative and transudative effusion showed a significant difference in the CT HU value. Therefore, the CT HU value can help the radiologist to predict the type of pleural effusion which will further help in the final diagnosis of disease condition along with associated findings in lung and mediastinum. Also, thoracocentesis can be considerably avoided in patients who are at risk of developing pneumothorax and hemothorax.