Acquired Methaemoglobinaemia Secondary to Sepsis-Induced Oxidative Haemolysis: A Case Report

Methaemoglobinaemia is a rare condition that disrupts oxygen transport by converting haemoglobin into a non-functional form. Clinical recognition can be delayed due to subtle symptoms and limitations of standard monitoring tools. A 36-year-old man developed acute shortness of breath shortly after taking antibiotics for fever and diarrhoea. Despite high-flow oxygen therapy, his oxygen saturation remained unexpectedly low. Arterial blood appeared dark brown, prompting suspicion of methaemoglobinaemia, which was confirmed by co-oximetry showing methaemoglobin levels of 19.6–24.0%. He also showed evidence of oxidative haemolysis and severe anaemia, although his glucose-6-phosphate dehydrogenase screening test was normal. Methylene blue was administered but had no therapeutic effect and was discontinued. Supportive care and intravenous antibiotics were given for presumed sepsis-related oxidative stress. The patient improved steadily and was discharged after 16 days with residual kidney impairment from the sepsis. This case illustrates the diagnostic challenge of methaemoglobinaemia, particularly in the absence of classic signs like cyanosis. A discrepancy between pulse oximetry and arterial saturation can offer an important clue. Awareness of contributing factors and thoughtful interpretation of laboratory results are essential to ensure safe and effective treatment.