Aggarwal, et al: Role of CT attenuation in distinguishing exudative ORIGINAL ARTICLE Diagnostic accuracy of CT Hounsfield unit in distinguishing Exudative and Transudative Pleural Effusion Anuj Aggarwal 1 , Neha Bagri 2 , Ranjan Chandra 3 , Manisha Bais Thakur 4 , Anita Rani 5 , Apurva Surana 1 , Ayush Khandelwal 1 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. KEY WORDS: Pleural effusion, Exudative, Transudative, Computed tomography, Attenuation values. Introduction The abnormal collection of fluid in pleural cavity is described as pleural effusion, which occurs due to imbalance between the secretion of fluid by parietal pleura and its absorption by the lymphatics. It can be classified into two main types- exudative and transudative. Exudative effusion is seen in patholo- gies causing increased permeability of vessels, including inflammation, malignancy, infection and Access this article online Quick Response Code: Website: www.jmsh.ac.in Doi: 10.46347/jmsh.2022.v8i1.4 1 Senior Resident, Department of Radio-diagnosis, VMMC & Safdarjung Hospital, New Delhi, India, 2 Associate Professor, Department of Radio-diagnosis, VMMC & Safdarjung Hospital, New Delhi, India, 3 Professor, Department of Radio-diagnosis, VMMC & Safdarjung Hospital, New Delhi, India, 4 Professor, Department of Medicine, VMMC & Safdarjung Hospital, New Delhi, India, 5 Director Professor, Department of Biochemistry, VMMC & Safdarjung Hospital, New Delhi, India Address for correspondence: Neha Bagri, Associate Professor, Department of Radio-diagnosis, VMMC & Safdarjung Hospital, New Delhi, India. E-mail: drnehabagri@gmail.com thromboembolism. [1] On the contrary, transudative effusion results due to a disharmony between the hydrostatic and oncotic pressures in conditions like liver cirrhosis, congestive heart failure and nephrotic syndrome. [2] The differentiation between exudative and transuda- tive effusion is crucial both for diagnosis and clinical management of the patient. [3] Conventionally, this differentiation is achieved by biochemical analysis of pleural fluid obtained by thoracentesis. [4,5] Being an invasive procedure, thoracentesis has potential iatrogenic risks of causing pneumothorax, haemothorax and minor complications like pain, hematoma formation and local site infection. There- fore, a non-invasive technique will not only avoid these potential complications but will also allow the Journal of Medical Sciences and Health/Jan-April 2022/Volume 8/Issue 1 14