Unexplained acute severe methaemoglobinaemia in a young adult.

This report describes the case of an otherwise healthy young adult female, who presented with a 12-h history of progressive bluish discolouration of lips and limbs. She denied ingesting or inhaling any drug or substance. A high PaO2 in the presence of 'cyanosis' and 'dark blood' led to suspicion of methaemoglobinaemia. Co-oximetry revealed the methaemoglobin level to be 47%. A urinary screen for drugs of abuse was negative and blood methaemoglobin reductase activity was within the normal range. The aetiology was traced to dapsone detected in the urine by gas chromatography/mass spectrometry. The therapeutic and diagnostic approach in such patients is discussed.