ICU · Toxicology

Methaemoglobinaemia: methylene blue, G6PD deficiency, and co-oximetry

Also known as Methaemoglobinaemia · MetHb · Methaemoglobin · Methylene blue

Methaemoglobin is oxidised haemoglobin unable to bind oxygen (Twine). Acquired MetHb 2% (Ash-Bernal). Cyanosis unresponsive to 100% oxygen (Coleman). Patients typically asymptomatic <10% MetHb (Twine); dyspnoea/nausea/tachycardia at ≥30%, stupor approaching 55%, 70% usually fatal (Coleman). Dapsone 42% of one 138-case series (Ash-Bernal). Diagnose by co-oximetry. Pulse oximetry plateaus at 84–86% once MetHb reaches ~35% in Barker dog work. First-line methylene blue 1–2 mg/kg of 1% solution IV (Coleman/Sikka); NYC poison-centre median 1 mg/kg, 98% improved (Rothenberg). Avoid or do not rely on methylene blue in known/suspected G6PD deficiency (Sikka, Sawaftah); ascorbic acid and exchange/transfusion are alternatives. High-dose methylene blue can haemolyse even when G6PD is normal (Mehmood). MAO-inhibitor serotonin-syndrome risk with serotonergic drugs (McMillan). Dapsone recurrence: infusion plus repeated charcoal (Dawson).

Checked against its sources on 3 Sept 2026