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Methaemoglobinaemia D004

Last amended 
30 June 2015
Current RMA Instruments:
Reasonable Hypothesis SOP
47 of 2010
Balance of Probabilities SOP
48 of 2010
Changes from Previous Instruments:

SOP Bulletin 144

ICD Coding:
  • ICD-9-CM Codes: 289.7
  • ICD-10-AM Codes: D74.8

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. In can be congenital or acquired. The SOP covers the acquired symptomatic form only. 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.

Is specific diagnostic evidence required to apply the SOP?Yes.

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%.

Are there sub-factors that require specific information? – No.
Additional diagnoses covered by SOP
  • Nil
Conditions excluded from SOP
  • Congenital methaemoglobinaemia
Unconfirmed diagnosis

If, after applying the above information, you are unable to confirm the diagnosis, you should then seek medical officer advice about further investigation.


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.