Carboxyhaemoglobin (COHb)
Definition
The compound formed when carbon monoxide binds to haemoglobin at the oxygen-binding site, with an affinity 200-250 times greater than oxygen. Produces cherry-red blood and livor mortis. Measured by co-oximetry as a percentage of total haemoglobin. Fatal at sustained concentrations above 50-70%.
- Binding affinity vs oxygen
- 200-250 times stronger than oxygen
- Fatal threshold
- 50-70% of total haemoglobin
- Measurement method
- Co-oximetry (percentage of total haemoglobin)
- Forensic sign
- Cherry-red blood and livor mortis
Common questions
What is carboxyhaemoglobin and how is it formed?+
Carboxyhaemoglobin (COHb) forms when carbon monoxide binds to haemoglobin at the oxygen-binding site. CO binds with extremely high affinity - roughly 200-250 times stronger than oxygen - so even low CO concentrations in the air can quickly saturate a large fraction of circulating haemoglobin.
Why is carboxyhaemoglobin poisonous?+
When haemoglobin is bound to carbon monoxide, it cannot carry oxygen to the body's tissues. This causes chemical asphyxiation - the cells starve for oxygen. Sustained COHb levels above 50-70% are typically fatal.
How can carboxyhaemoglobin be detected and measured?+
COHb levels are measured by co-oximetry, which calculates the percentage of total haemoglobin that is bound to carbon monoxide. In forensic cases involving fire exposure, the presence of elevated COHb confirms the victim was alive during the fire. A telltale sign is cherry-red discoloration of blood and livor mortis.
Related terms
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- Bitter-Almond Odour
- The characteristic smell of hydrogen cyanide, detectable at autopsy in approximately 40% of people (a genetically determined anosmia to HCN). Present in...
- Cherry-Pink Lividity
- The bright pink discolouration of dependent tissues seen in CO and cyanide deaths. In CO the colour comes from COHb itself, in...
- Cherry-Red Livor Mortis
- Bright pink-red post-mortem lividity produced by the spectral absorption shift of carboxyhaemoglobin; also seen in cyanide poisoning and hypothermia. Distinguished from natural...
- Co-Oximetry
- Spectrophotometric analysis of blood that simultaneously quantifies oxyhaemoglobin, deoxyhaemoglobin, carboxyhaemoglobin, methaemoglobin, sulphhaemoglobin, and total haemoglobin by their distinct absorption wavelengths. The primary...
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- Conway Microdiffusion
- A two-chamber glass cell in which an acid in the outer ring liberates HCN from the sample and the gas diffuses into...
- Cytochrome C Oxidase (Complex IV)
- The terminal electron acceptor in the mitochondrial respiratory chain, inhibited by both cyanide and hydrogen sulphide. Inhibition prevents oxidative phosphorylation and ATP...
- Fire Tetrahedron
- The model of sustained combustion requiring four concurrent elements: fuel, oxidiser, heat, and an uninhibited radical chain reaction. Replacing the older fire...
Explained in these topics
- Chemical Asphyxia: CO, Cyanide and Hydrogen SulphideThe compound formed when carbon monoxide binds to haemoglobin at the oxygen-binding site, with an affinity 200-250 times greater than oxygen. Produces cherry-r...
- Fire Chemistry: Combustion, Oxidation and the Fire TetrahedronThe compound formed when carbon monoxide binds to haemoglobin. Blood COHb above 50-60% is typically lethal; its presence confirms that the victim was alive dur...
- Gases and Volatile Poisons: CO, Cyanide, Methanol, EthanolThe complex formed when carbon monoxide displaces oxygen on haemoglobin. CO binds haemoglobin roughly 240 times more avidly than oxygen, so even a low ambient...