Can computerized tomography Hounsfield unit values be useful in the differential diagnosis of pleural effusion?

Introduction: We aimed to investigate the efficacy of Hounsfield unit (HU) attenuation measured on computed tomography (CT) as a non-invasive method for pleural effusion characterization.

Methods: Patients with pleural effusion who underwent thoracic CT imaging and thoracentesis within a maximum of three days were included in this retrospective study (15 transudate and 36 exudate). By drawing a circular region of interest (ROI) on the section with the thickest pleural effusion in terms of anteroposterior diameter in the upper-medial-lower zone on axial images, a total of three HU values, one from each level, were averaged. An independent t-test was applied to the CT attenuation (HU) values for the transudate-exudate differentiation. A receiver operating characteristic (ROC) analysis was then made.

Results: The mean attenuation±standard deviation (minimum-maximum) value for the patients with transudate was 2.17±3.76 ((-7.5)-7.5) HU, whereas the mean HU value for the patients with exudate was 8.38±6.2 ((-6)-22). The independent t test made for the transudate-exudate differentiation revealed a statistically significant difference (P=0.001). In the ROC analysis carried out to determine the cut-off value of the attenuation value of pleural effusion in the transudate-exudate differentiation, the area under the curve was found to be 82.8%. When the cut-off value was taken as 2.75HU for the area under the curve, sensitivity was found to be 84%, and specificity was 60%.

Conclusion: Although CT-HU values are statistically significant in the differential diagnosis of transudate-exudate, there is still a need to establish a correlation with other tomographic findings and clinical laboratory findings.