Methaemoglobinaemia occurs when red blood cells contain methaemoglobin at levels higher than 1%. Methaemoglobinaemia results from the presence of iron in the ferric form instead of the usual ferrous form. The ferric form is unable to bind oxygen, and its presence results in a decreased availability of oxygen to the tissues. Methaemoglobinaemia can be congenital or acquired.


Methaemoglobinaemia can be congenital or acquired.

 Congenital causes of methaemoglobinaemia include:

  • Glucose-6-phosphate dehydrogenase (G6PD) deficiency
  • Pyruvate kinase deficiency
  • Cytochrome b5 oxidase deficiency
  • NADH methaemoglobin reductase deficiency

Acquired methaemoglobinaemia is usually due to exposure to a drug or oxidising agent. These include:

  • Sodium nitroprusside
  • Nitroglycerin
  • Amyl nitrate
  • Local anaesthetics, e.g. benzocaine, prilocaine
  • Antimalarials, e.g. primaquine, chloroquine
  • Cyclophosphamide
  • Paracetamol
  • Dapsone
  • Sulfonamide antibiotics

Clinical features

The clinical features of methaemoglobinaemia are proportional to the methaemoglobin level:

  • <15%: Skin (grey-blue cyanosis) and blood (chocolate-brown) blood colour changes only
  • 15-30%: Anxiety, headache, weakness, tachycardia, dizziness
  • 30-70%: Myocardial ischaemia, arrhythmias, seizures, coma
  • >70%: Usually fatal

Pulse oximetry

Pulse oximetry is usually falsely elevated in the early stages and cannot be relied upon. Once levels reach 30%, the oxygen saturations generally fall to around 80-85%, which is due to the combined light absorption of both oxyhaemoglobin and deoxyhaemoglobin. The oxygen saturations will not be responsive to supplemental oxygen.


All patients should have a serum methaemoglobin level taken to confirm the diagnosis and guide management.


Patients should be treated if they are symptomatic or if they are asymptomatic with methaemoglobin levels >25%. Management of methaemoglobinaemia typically involves:

  • Administration of IV fluids, e.g. 1-2 L 0.9% saline
  • Administration of IV methylene blue 1% solution 1-2 mg/kg
  • Referral to the intensive care team



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