Dapsone-induced methemoglobinemia: “Saturation gap”—The key to diagnosis
An 8-year-old girl, a known case of immune thrombocytopenic purpura, with treatment history of oral prednisolone 20 mg/day and dapsone 50 mg/day since 1 year, was planned for laparoscopic splenectomy.\nPre-operative assessment revealed cushingoid features, skin discoloration all over the body and splenomegaly [].\nShe had a platelet count of 20,000/cmm but the other hematological variables were normal. Oxygen saturation on room air was 88%. During maintenance of general anesthesia with a 60:40 (nitrous oxide:oxygen) gas oxygen ratio, the SpO2 rose to 92%. Vital signs, physical examination including bilateral air entry was normal, end tidal carbon dioxide and airway pressures were unremarkable. Intraoperative ABG analysis revealed a PaO2 of 260 mmHg, pH 7.40, PaCO2 38 mmHg, HCO3 22.4 meq.l-1 and SaO2 of 99%. Filter paper test done on the patient's blood comparing it with blood from a normal patient [] which showed that the patient's blood remained dark even after exposure to atmospheric oxygen.\nDeoxyhemoglobin can be distinguished from methHb by this simple bedside test. By placing 1 or 2 drops of the patient's blood on a white filter paper: Deoxyhemoglobin brightens after exposure to atmospheric oxygen, but methHb does not change color.[]\nShe was transferred to the pediatric intensive care unit (ICU) with oxygen therapy by polymask at 5 l/min, as SpO2 remained 86% on room air. Blood analysis showed methHb levels of 28.6%. Dapsone was stopped and the patient remained asymptomatic with supplemental oxygen by facemask. As the methHb levels were less than 30% and the patient was stable, therapy with methylene blue was not considered.