Methemoglobin
A form of haemoglobin in which the iron has been oxidised so it can no longer carry oxygen. A small amount is normal, but a high level (methemoglobinemia) — usually from certain drugs, chemicals or a rare inherited enzyme deficiency — starves tissues of oxygen and can be dangerous.
What it measures
Haemoglobin carries oxygen only when its iron is in the correct (ferrous) chemical state. Methemoglobin is a form in which that iron has been oxidised (to the ferric state) so it can no longer bind oxygen — and, worse, it makes the remaining normal haemoglobin release oxygen less readily too. The body normally keeps methemoglobin very low (under about 1.5% of total haemoglobin) using an enzyme system that continuously converts it back. Methemoglobin is measured directly by a device called a co-oximeter, usually on a blood-gas sample, and is reported as a percentage of total haemoglobin. A raised level (methemoglobinemia) is usually acquired: certain medicines (some local anaesthetics such as benzocaine, dapsone, nitrates and others), industrial chemicals, and some contaminated waters (nitrates, especially dangerous in infants) can oxidise haemoglobin faster than the body can reverse it. Rarely it is inherited, from a deficiency of the reducing enzyme or an abnormal haemoglobin. The tell-tale sign is a bluish-grey skin colour (cyanosis) that does not improve with oxygen, and blood that is characteristically 'chocolate-brown'. A high methemoglobin can cause breathlessness, headache, dizziness and, at high levels, be life-threatening — it has a specific antidote (methylene blue).
What a high value can mean
- Drug or chemical exposure — local anaesthetics (benzocaine), dapsone, nitrates and others that oxidise haemoglobin; the commonest cause.
- Nitrate/nitrite exposure — including contaminated water, especially dangerous in infants.
- Inherited methemoglobinemia — a deficiency of the reducing enzyme or an abnormal haemoglobin.
- Key clues — bluish-grey skin unresponsive to oxygen, and chocolate-brown blood.
What a low value can mean
- Low / normal (under ~1.5%) — the expected result; haemoglobin is in its normal oxygen-carrying form.
When to discuss with a doctor
Methemoglobin is a targeted test, usually sent when someone has bluish-grey skin that does not improve with oxygen — particularly after a new medicine (such as a topical anaesthetic), a chemical or nitrate exposure — or when a pulse oximeter reads oddly low and stubborn despite oxygen. A significantly raised level is a medical emergency: it starves tissues of oxygen and, above certain thresholds, is treated urgently, often with the specific antidote methylene blue (which must be used carefully, and is avoided in G6PD deficiency). Anyone with suspected methemoglobinemia needs immediate medical assessment. Because inherited forms exist, an unexplained persistently raised level warrants specialist review. Mediora.AI can surface a reported methemoglobin level alongside haemoglobin and blood-gas results so the pattern is visible; a high level with symptoms is an emergency handled by your doctor or emergency care.