An 82-year-old Chinese female known to have erythrodermic pemphigus with background of psoriasis was started on dapsone for 4 months prior to admission. She was then admitted due to delirium and desaturation. On admission, oxygen saturation was 89% on room air, which subsequently improved with high flow oxygen therapy. The full blood count showed hemoglobin 11.2 g/dL (reference range: 13.6–16.6 g/dL), white blood cells 23.9 × 10⁹/L (reference range: 4.0–9.6 × 10⁹/L), platelets 362 × 10⁹/L (reference range: 150–360 × 10⁹/L) and reticulocyte counts 220.8 × 10⁹/L (reference range: 25–85 × 10⁹/L). Markers of hemolytic anemia were present, including an elevated LDH at 730 U/L (reference range: 270–550 U/L), elevated total bilirubin 54 mmol/L (reference range: 5–30 mmol/L) and low haptoglobulin at <30 mg/dL (reference range: 36–200 mg/dL) but direct Coombs test was negative. Peripheral blood film showed significant numbers of irregularly contracted red cells, bite cells and fragmented cells, suggestive of oxidative hemolysis. There were also Howell-Jolly bodies and Papenheimer bodies seen suggestive of possible dyserythropoiesis. Blood methemoglobin level was elevated at 8.2%. Her G6PD assay was normal. In view of clinical findings of oxidative hemolysis and methemoglobinemia, dapsone was stopped. She was then started on folic acid and vitamin C at 1 gram twice a day. Two days after, her oxygen saturation improved, and a repeat blood methemoglobin level improved to 1.8%.

Dapsone is known as a cause of drug induced hemolytic anemia and methemoglobinemia due to N-hydroxylation to a hydroxylamine derivative that is directly toxic to RBCs.^{1, 2} The amine metabolites of dapsone are capable of oxidation of heme iron, resulting in acquired methemoglobinemia and hemoglobin denaturation forming Heinz bodies which are removed by splenic macrophages, forming bite cells and irregularly contracted cells.³ In addition, given the chronic consumption of dapsone for 4 months, compounded with the long standing oxidative hemolysis, the features observed in the patient's blood film such as the presence of Howell-Jolly and Pappenheimer bodies probably represent dyserythropoiesis secondary to erythroid stress.

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