Blood Gas
Methaemoglobin
Methaemoglobin is haemoglobin in which the iron has been oxidised from the ferrous (Fe2+) to the ferric (Fe3+) state, leaving it unable to bind and transport oxygen. At the point of care it is reported by CO-oximetry as a percentage of total haemoglobin and is used to detect and monitor methaemoglobinaemia.
Why it is measured
A raised methaemoglobin fraction lowers the blood's effective oxygen-carrying capacity and can be triggered by oxidising drugs and chemicals such as dapsone, benzocaine, prilocaine and nitrites. Rapid measurement supports timely recognition of methaemoglobinaemia and the clinical decision to investigate or treat.
| Typical range | Indicative adult reference: approximately 0 to 1.5% of total haemoglobin, with some methods reporting up to about 2%. Symptoms are uncommon below roughly 10%. Ranges vary by analyser and method, so always apply the reporting laboratory's or device's own intervals. |
|---|---|
| Sample | Arterial or venous whole blood collected in balanced or lithium heparin for benchtop CO-oximetry, analysed promptly to avoid pre-analytical change. Pulse CO-oximetry instead uses a non-invasive fingertip optical sensor (SpMet) with no blood draw. |
| Turnaround | CO-oximetry on a blood gas analyser is typically reported in under 1 to 2 minutes alongside the gas and co-oximetry panel. Pulse CO-oximetry (SpMet) gives a continuous, near real-time reading at the bedside. |
Point of care devices that report it
- Radiometer ABL90 FLEX PLUS and ABL800 FLEX (CO-oximetry module)
- Siemens Healthineers RAPIDPoint 500e (CO-oximetry)
- Werfen GEM Premier 5000 (CO-Ox)
- Roche cobas b 221 and cobas b 123 (CO-oximetry module)
- Nova Biomedical Stat Profile Prime Plus (CO-Oximetry)
- Masimo Rad-57 handheld pulse CO-oximeter (non-invasive SpMet)
Questions, answered
Will a standard pulse oximeter detect a high methaemoglobin?
No. A conventional two-wavelength SpO2 oximeter can read inaccurately when methaemoglobin is raised and tends to plateau around the mid-80s percent regardless of the true level, so it can mask the problem. Quantifying methaemoglobin needs CO-oximetry, either on a blood gas analyser or with a multi-wavelength pulse CO-oximeter that reports SpMet. This is general educational information and not advice about any individual reading.
What can cause methaemoglobin to be elevated?
Acquired causes include oxidising drugs and chemicals such as dapsone, topical local anaesthetics like benzocaine and prilocaine, nitrites, nitrates and some aniline dyes. Inherited causes include cytochrome b5 reductase deficiency and haemoglobin M variants. This list is for education only and is not a diagnosis of any specific case.
How quickly is a high methaemoglobin result acted upon?
Operationally, the value is produced within the rapid turnaround of a blood gas or pulse CO-oximetry reading so it can be correlated promptly with the patient's symptoms and oxygen saturation. The thresholds for concern and any treatment are defined in local clinical and POCT protocols rather than by the device itself.