Pleural effusion: characterization with CT attenuation values and CT appearance

The purpose of this study was to assess the utility of CT in characterizing pleural effusions on the basis of attenuation values and CT appearance.

We retrospectively analyzed 100 pleural effusions in patients who underwent chest CT and diagnostic thoracentesis within 48 hours of each other. On the basis of Light's criteria, effusions were classified as exudates or transudates using laboratory biochemistry markers. The mean value in Hounsfield units of an effusion was determined using a region of interest on the three slices with the greatest quantity of fluid. All CT scans also were reviewed for the presence of additional pleural features such as fluid loculation, pleural thickening, and pleural nodules.

Twenty-two of the 100 pleural effusions were transudates and 78 were exudates. The mean attenuation of the exudates (7.2 HU; [SD] 9.4 HU; range, 21-28 HU) was not significantly lower than the mean attenuation of the transudates (10.1 HU; 6.9 HU; range, 0.3-32 HU), (p = 0.24). None of the additional CT features accurately differentiated exudates from transudates (p > 0.1). Fluid loculation was found in 58% of exudates and in 36% of transudates. Pleural thickening was found in 59% of exudates and in 36% of transudates.

The clinical use of CT attenuation values to characterize pleural fluid is not accurate. Although fluid loculation, pleural thickness, and pleural nodules were more commonly found in patients with exudative effusions, the presence of these features does not accurately differentiate between exudates and transudates.