Images of the month 1: 'Soy sauce' pleural effusion: what causes black pleural fluid?

We present a case of black pleural fluid following thoracic trauma. The unusual dark colour most strikingly resembled soy sauce as independently commented upon by multiple treating physicians. The black colouration could not be fully accounted for by haemothorax or cholethorax, so other differential diagnoses were investigated, including Aspergillus niger infection and malignant melanoma. The cause, however, was thought to be due to staining of the fluid with carbon deposited in the pleural space from the non-volatilised impurities from smoking crack cocaine. A novel use of a point-of-care urine toxicology assay confirmed the presence of cocaine in the pleural fluid. Considering a broad range of differential diagnoses is needed to avoid missing important causes of unusual pleural effusions. © Royal College of Physicians 2021. All rights reserved.