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 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.