Abstract Introduction Pleural effusions secondary to malignancy are typically exudative by Light’s criteria. Transudative effusions associated with small cell lung carcinoma (SCLC) are exceedingly rare and may delay diagnosis. We present a case of metastatic SCLC manifesting as a transudative pleural effusion, underscoring the importance of diagnostic vigilance when pleural fluid chemistry and clinical suspicion diverge. Case Presentation A 78-year-old man with hypertension and a 50-pack-year smoking history presented with progressive dyspnea, productive cough, right-sided chest pain, and a 30-lb weight loss over one month. He was vitally stable and afebrile. Computed tomography angiography of the chest revealed a large right pleural effusion with near-total collapse of the right lung, occlusion of the right mainstem bronchus, and a right hilar/mediastinal mass with possible cardiac extension. Thoracentesis drained 1.85 L of serous fluid. Pleural analysis demonstrated protein 2.2 g/dL, LDH 80 U/L, glucose 97 mg/dL, and pH 7.28, meeting transudative criteria (serum protein 6.6 g/dL, LDH 190 U/L). A repeat sample three days later remained transudative (protein 1.6 g/dL, LDH 71 U/L).Cytologic analysis of both specimens revealed metastatic neuroendocrine tumor consistent with small cell carcinoma. Bronchoscopic biopsy confirmed SCLC with endobronchial obstruction of the right mainstem bronchus. Brain MRI showed bilateral basal ganglia-enhancing lesions, consistent with metastases. The patient was initiated on carboplatin-etoposide chemotherapy with planned radiotherapy for brain metastases. Discussion Malignant pleural effusions are typically exudative, yet rare transudative variants may occur through mechanisms such as venous or lymphatic obstruction, elevated systemic pressure, or early pleural involvement without inflammation. In SCLC, mediastinal compression of vascular and lymphatic structures can result in a transudative profile despite malignant infiltration. This case underscores that reliance solely on Light’s criteria may obscure the diagnosis of malignancy. When clinical or imaging findings raise suspicion for neoplasia, cytologic analysis remains essential even in biochemically transudative effusions. Recognizing this atypical presentation is vital to avoid delayed diagnosis and initiate timely therapy in aggressive cancers like SCLC. Conclusion Transudative pleural effusion does not rule out malignancy. In patients with risk factors and imaging findings suggestive of lung cancer, pleural cytology is warranted regardless of Light’s classification. Comprehensive evaluation of atypical effusions is crucial to prevent delayed recognition of aggressive malignancies such as SCLC. This abstract is funded by: None
Alfarrajin et al. (Fri,) studied this question.