Diagnostic Use
Methaemoglobin is haemoglobin with the iron oxidised to its ferric state. It is unable to bind oxygen and causes hypoxia and cyanosis at high concentrations.
It is increased in inherited disorder from NADH cytochrome b5 reductase deficiency.
Methaemoglobin can also be increased due to drugs and environmental exposures including chloroquine, isoniazid, nitrates, nitrites, sulphonamides, benzocaine, and aniline dyes.
Infants have increased susceptibility to forming methaemoglobin even without inherited NADH cytochrome B5 reductase deficiency because their RBCs have half the amount of cytochrome B5 reductase compared with adult; HbF more susceptible to oxidation than HbA; more alkaline GI tract promotes growth of nitrite producing gram negative bacteria. Infants (especially <3 months, premature, low weight) can develop acquired methaemoglobinaemia of >10% from gastroenteritis.
Considerations when treating with Methylene Blue:
– Methylene blue can negatively interfere with methaemoglobin results, particularly if the methaemoglobin is <30% of total haemoglobin. To minimise interference, wait 60 minutes after administration of IV methylene blue before repeating methaemoglobin measurement.
– Caution is advised if the patient has known or suspected G6PD deficiency. Refer to TOXINZ Supportive Care Methemoglobinemia guidelines.
Pulse oximetry interference:
– Methaemoglobin interferes with pulse oximetry, causing falsely high or falsely low oxygen saturation readings (especially apparent at methaemoglobin of ≥30% total haemoglobin).
– Methylene blue negatively interferes with pulse oximetry readings.
Reference Intervals
Adult <1.5%
| Methaemoglobin Level |
Symptoms |
| <3% |
Most likely asymptomatic |
| 3-10% |
Possible presence of detectable cyanosis, asymptomatic |
| 10-20% |
Patients with pre-existing conditions* may have symptoms (see next entry), most will be asymptomatic |
| 20-50% |
May include light-headedness, headache, confusion, dyspnoea, tachycardia, and fatigue |
| 50-70% |
Life threatening. Coma, seizures, arrythmias, acidosis, respiratory depression, delirium |
| >70% |
High Risk of mortality |
* Conditions that affect oxygen transport including anaemia, lung and heart disease can exacerbate toxicity |
In infants the expected level of methaemoglobin is slightly higher than that of adults, particularly in premature neonates.
Test Method
Principle: Blood gas analysis
Analyser: Radiometer ABL800 FLEX or ALB90