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Methaemoglobinaemia D004

Last amended 
11 March 2019
Current RMA Instruments
Reasonable Hypothesis SOP
17 of 2019
Balance of Probabilities SOP
18 of 2019
Changes from previous Instruments

SOP Bulletin 207

ICD Coding
  • ICD-9-CM Codes: 289.7
  • ICD-10-AM Codes: D74.0
Brief description

Methaemoglobinaemia is a problem with the haemoglobin in red blood cells that interferes with oxygen carrying capacity. It is either asymptomatic or manifests with hypoxaemia. It can be congenital or acquired. The SOP covers the acquired symptomatic form and the heterozygous form of NADH-methaemoglobin reductase deficiency restricted to red blood cells, but not other forms of congenital methaemoglobinaemia. This condition is normally temporary, being amenable to drug treatment within a period of days. However, if the hypoxaemia has caused organ damage, there may be persistent effects. Death can ensue in severe cases. See further comments, below.

Confirming the diagnosis

This diagnosis is based on a specific methaemoglobin blood test together with evidence of the presence of clinical manifestations. The methaemoglobin level is normally less than 1%, with skin colour changing at 10%, cerebral ischaemia occurring at > 15% and death possible at > 60%.

The relevant medical specialist is a general physician or haematologist.

Additional diagnoses covered by SOP
  • Heterozygous form of NADH-methaemoglobin reductase deficiency restricted to red blood cells

Conditions excluded from SOP
  • Congenital methaemoglobinaemia (other forms)
Clinical onset

This is an acute onset condition related to the ingestion of specific drugs or agents.  Clinical onset will be when relevant symptoms such as lightheadedness, headache, tachycardia, fatigue or dyspnoea first developed or clinical signs, particularly cyanosis with pale, gray or blue colored skin, lips, and nail beds, were first noticed.

Clinical worsening

Worsening is likely to be temporary, unless death occurs.  Worsening would be evidenced by an increase in the methaemoglobin level or a worsening of symptoms or signs.


There is naturally some methaemoglobin in the blood. The SOP concerns an abnormally high level of methaemoglobin which is at a level to cause clinical symptoms.

The technical detail is that haemoglobin is normally required to collect oxygen at the lungs and deliver it at the tissues. The haemoglobin does this by utilising a co-ordinate covalent bond involving the metal iron in its ferrous (Fe++) state. If the iron is oxidised to the ferric (Fe+++) state it holds onto the oxygen tightly and will not give up the oxygen at the tissues. As such the ferric state in methaemoglobin makes the haemoglobin useless for oxygen transport.

The ferrous state in haemoglobin is constantly threatened by normal environmental oxidants (2% per day) but is kept reduced by endogenous reducing agents.

If the level of the environmental oxidants is abnormally high the balance between oxidation and reduction in the red cell shifts producing a lower level of useful haemoglobin and abnormally high level of useless methaemoglobin.