C45-18 Anchovy-Paste Pleural Effusion: Pseudochylothorax in a Patient With Rheumatoid Arthritis and Prior Tuberculosis

Abstract Introduction Pseudochylothorax is an uncommon chronic pleural effusion characterized by the presence of cholesterol crystals and a milky or “anchovy-paste” appearance of pleural fluid. It is most often associated with long-standing inflammatory pleural disease such as tuberculosis or rheumatoid arthritis. We present a diagnostically challenging case of pseudochylothorax in a patient with overlapping risk factors, highlighting the complexity of pleural fluid interpretation in immunosuppressed hosts. Case Presentation A 59-year-old man with a history of prior pulmonary tuberculosis and rheumatoid arthritis treated with rituximab and methotrexate was referred to the pulmonology clinic for evaluation of a recurrent left pleural effusion. The effusion had been incidentally detected on CT chest performed for routine surveillance and appeared loculated. The patient reported mild dyspnea on exertion but no fever, cough, or weight loss.Diagnostic thoracentesis yielded approximately 500 mL of dark brown, “anchovy-paste” appearing pleural fluid. Laboratory analysis revealed an exudative effusion with high protein content and numerous cholesterol crystals on polarized microscopy, consistent with pseudochylothorax. Cytology was negative for malignant cells. Given the patient’s history, the differential diagnosis included rheumatoid pleuritis, tuberculous pleuritis, and drug-induced effusion. Rheumatoid factor was within normal limits. Adenosine deaminase (ADA) was markedly elevated, and a QuantiFERON-Gold assay returned positive. Despite these findings, Infectious Disease specialists considered active tuberculosis unlikely and recommended tissue confirmation. The patient underwent video-assisted thoracoscopic surgery (VATS) with decortication and pleural biopsy. Final pathology was pending at the time of report. Discussion Pseudochylothorax arises from chronic pleural inflammation leading to cholesterol accumulation within the pleural space. The two most common etiologies are tuberculosis and rheumatoid arthritis, both of which were present in this patient, making determination of the underlying cause particularly challenging. The presence of “anchovy-paste” fluid and cholesterol crystals is characteristic, yet differentiation from empyema or true chylothorax requires biochemical and cytological analysis.This case underscores the diagnostic complexity of pleural effusions in patients with multiple chronic inflammatory disorders and on immunosuppressive therapy. Elevated ADA and positive interferon-gamma release assays must be interpreted cautiously in such contexts, as they may reflect latent rather than active infection. Ultimately, pleural biopsy remains essential to establish the diagnosis and guide management. Recognition of pseudochylothorax is important because it may mimic malignancy or infection but requires distinct management strategies. This abstract is funded by: N/A