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Cadexomer Iodine — SCE Dermatology MCQ

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HardWound HealingCadexomer IodineSCE Dermatology

A patient taking dapsone for dermatitis herpetiformis develops headache, dyspnoea and slate-grey cyanosis. Pulse oximetry remains 86% despite high-flow oxygen, but arterial PaO2 is 13.8 kPa. Which test best identifies the complication?

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Correct answer: D — Measure methaemoglobin directly using arterial or venous co-oximetry

Explanation lettering: E = shown as A · C = shown as B · B = shown as C · A = shown as D · D = shown as E

A is correct. The oxygen-saturation gap—low pulse oximetry despite a preserved arterial oxygen tension—plus cyanosis during dapsone therapy suggests methaemoglobinaemia. Co-oximetry directly measures methaemoglobin and distinguishes dyshemoglobins that standard calculated arterial saturation can miss. Dapsone can also cause haemolysis, particularly with G6PD deficiency, so blood count, reticulocytes, bilirubin and haptoglobin may be relevant, but haemolysis alone does not explain this saturation pattern. Severity, symptoms and concentration guide stopping dapsone and treatment such as methylene blue, with specialist caution in G6PD deficiency.

Reference: Dapsone 50 mg tablets Summary of Product Characteristics: https://www.medicines.org.uk/emc/product/13892/smpc