Abstract Background Pleural effusions are commonly caused by infection, malignancy, rheumatologic disease, occupational exposure, or heart failure. Light’s Criteria remain the standard for distinguishing transudative from exudative effusions. Malignant pleural effusions (MPEs) are almost universally exudative; however, rare cases of cytology-positive transudative effusions have been reported, accounting for approximately 1-3.8% of MPEs. Recognition of this phenomenon is critical, as reliance on biochemical classification alone may delay cancer diagnosis. Case A 92-year-old man with hypertension, atrial fibrillation, hypothyroidism, and untreated bladder cancer presented with progressive dyspnea, lower extremity edema, and hypoxia. Physical exam revealed volume overload, and he was admitted for acute decompensated right-sided heart failure. Echocardiography showed right ventricular and atrial dilation with elevated pulmonary artery systolic pressure (55-60 mmHg). Chest imaging demonstrated a large right pleural effusion and bilateral pulmonary nodules.Thoracentesis yielded 2.2 L of clear yellow fluid with LDH 92 U/L, protein 3.4 g/dL, serum LDH 525 U/L, and serum protein 7.3 g/dL, meeting transudative criteria by Light’s. Despite this, pleural cytology revealed malignant cells consistent with metastatic carcinoma, favoring a pancreatobiliary or gastrointestinal origin. CT thorax showed bilateral pulmonary nodules, mosaic attenuation, and interlobular septal thickening, consistent with lymphangitic carcinomatosis. The patient declined further oncologic work-up or treatment and was discharged with supportive care. Discussion This case demonstrates that malignancy can present with transudative pleural effusion, particularly in advanced disease with concurrent heart failure or lymphangitic spread, altering pleural permeability. Possible mechanisms include early pleural involvement, coexisting systemic factors (e.g., hypoalbuminemia, elevated hydrostatic pressures), or diuresis-induced dilutional effects. Cytology should be considered even for transudative effusions when clinical suspicion for malignancy exists. Conclusion While Light’s Criteria are useful for effusion classification, cytology remains essential in evaluating unexplained or recurrent transudative effusions. This case highlights the importance of maintaining diagnostic vigilance to avoid missed malignant etiologies. This abstract is funded by: None
Coritt et al. (Fri,) studied this question.