What question did this study set out to answer?

To highlight an unusual case of transudative malignant pleural effusion caused by lung adenocarcinoma.

May 20, 2026

B80-5-35 Transudative Malignant Pleural Effusion Secondary to Micropapillary Lung Adenocarcinoma: An Unusual Presentation

Key Points

To highlight an unusual case of transudative malignant pleural effusion caused by lung adenocarcinoma.
Case presentation of a middle-aged female with lung mass and cytology positive transudative effusion.
Pleural fluid analysis conducted post-thoracentesis, revealing elevated WBC count and specific chemistry values.
Imaging studies including CT chest were performed to assess the extent of disease and guide management.
Pleural fluid contained 3800 WBC/μL with 51% lymphocytes and a consistent transudative profile according to Light's criteria.
Cytology of the pleural fluid confirmed metastatic adenocarcinoma with micropapillary architecture.
CT imaging revealed mediastinal adenopathy and atelectasis likely contributing to transudative characteristics.

Abstract

Abstract Background Most malignant pleural effusions are exudative in nature; however, transudative profile has rarely been described. We present a case of atypical presentation of lung mass with cytology positive transudative effusion. Case Patient is a middle-aged female, with a 12-pack-year smoking history and cannabis use disorder, who presented with progressive shortness of breath, cough and hypoxemia. Patient had a similar presentation three weeks prior; CTA chest revealed left lower lobe consolidation with a moderate pleural effusion. She was treated with antibiotics and thoracentesis was deferred, given small simple appearance of the fluid. Workup including MRSA nasal swab, urine legionella corresponding serum LDH and protein were 194 U/L and 5.5 g/dL, consistent with transudative effusion. Pleural fluid cytology showed metastatic adenocarcinoma with micropapillary features, consistent with lung origin. She underwent further workup for staging and continues to follow up for adenocarcinoma management. Discussion While malignant pleural effusions are typically exudative, approximately 3-5% may present with transudative characteristics, as seen in our patient. Several mechanisms have been proposed, including direct effects of the malignancy, coexisting systemic illnesses, or a combination of both. In the early phase of pleural involvement, mediastinal lymphatic obstruction can limit protein accumulation, resulting in a transudative profile. Over time, increasing vascular permeability may shift the effusion toward an exudative pattern. Additional factors such as endobronchial obstruction, hypoalbuminemia, tumor embolism, or superior vena cava syndrome have also been described. Concurrent cardiac, hepatic, or renal disease can further contribute to this atypical presentation. In our case, CT imaging demonstrated atelectasis and endobronchial obstruction, findings that likely played a role in the fluid’s transudative nature. The micropapillary variant of adenocarcinoma, known for early lymphatic and pleural dissemination, may also explain the discordance between cytology and fluid chemistry. Therefore, pleural fluid analysis should always be interpreted in clinical context, as transudative characteristics do not exclude an underlying malignancy. Conclusion Our case illustrates that a transudative pleural effusion can in rare instances be the initial manifestation of lung adenocarcinoma. Hence, one should maintain a high index of suspicion when imaging or clinical findings suggest an underlying malignancy. This abstract is funded by: None

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