Carboxyhaemoglobin (COHb) is formed when carbon monoxide (CO) binds to the ferrous iron found in haemoglobin. Haemoglobin’s affinity for CO is 218 times greater than that for oxygen (O2), which results in CO displacing O2 during competition for haem binding sites. Low concentrations of CO in inhaled air can cause rapid formation of COHb; 0.1% CO can result in 50% of haemoglobin converting to COHb and notable clinical symptoms will arise within one hour. Levels of 0.2% CO can lead to death within a few hours. Although CO binds to haemoglobin at a slower rate than O2, its greater binding affinity means that it is released 10,000 more slowly from haemoglobin.
Methaemoglobin (MetHB) arises when the iron component in haemoglobin is oxidised so that it is in the ferric state (Fe3+). MetHb is unable to bind oxygen and therefore cannot participate in respiratory function. MetHb is found in red blood cells and is formed by glycolysis, however reduction of the oxidised haem prevents accumulation and its concentration is usually less than 1% of the total haemoglobin. MetHb is restricted to the red blood cells and is only released during haemolysis, when it can be found in the plasma and urine. Even low levels of MetHb can cause the oxygen dissociation curve to shift towards the left, which can result in tissue anoxia.
The main causes of pathological increase in MetHb concentration are:
- Increased rate of formation.
- Decreased rate of COHb reduction to haemoglobin. Congenital and idiopathic methaemoglobinaemia caused by deficiency of coenzyme factor I. Congenital type is rare and is due to a defect in NADH diaphorase or NADPH diaphorase.
- Familial inherited type. Inherited in an autosomal dominant pattern, this causes a defect in the synthesis of the prosthetic globin chain, which results in haemoglobin that can be oxidised very easily. This is known as haemoglobin M. In homozygotes this condition is incompatible with life.
Non – smokers: <1.5%
Smokers: 1.5 – 5%
Methaemoglobin: 0.2 – 0.6%
- Heparinised whole blood collected into a syringe is the specimen of choice, however whole blood EDTA specimens can be used as a suitable alternative.
- Methaemoglobin analysis must be performed within 1 hour of venepuncture.
- Three patient identifiers from
- N.H.S. number
- Unit Number