Co-Oximetry
Definition
Spectrophotometric analysis of blood that simultaneously quantifies oxyhaemoglobin, deoxyhaemoglobin, carboxyhaemoglobin, methaemoglobin, sulphhaemoglobin, and total haemoglobin by their distinct absorption wavelengths. The primary diagnostic instrument for CO, cyanide-associated colour change, and H2S poisoning.
- Method
- Multi-wavelength spectrophotometry of blood
- Measures
- Oxy-, deoxy-, carboxy-, met-, and sulfhaemoglobin plus total Hb
- Primary use
- Carbon monoxide, cyanide, and H2S poisoning diagnosis
Common questions
How does co-oximetry differ from a standard pulse oximeter?+
A pulse oximeter reads only two wavelengths and cannot distinguish carboxyhaemoglobin or methaemoglobin from oxyhaemoglobin, so it can show a falsely normal or near-normal reading in carbon monoxide poisoning. Co-oximetry uses multiple wavelengths and resolves each haemoglobin species separately.
Why does co-oximetry matter in a cyanide poisoning case?+
Cyanide itself does not bind haemoglobin in a way co-oximetry detects directly, but the test is run alongside cyanide analysis because CO and cyanide exposure often coexist, especially in fire deaths, and CO-Hb elevation helps establish the exposure timeline.
Related terms
- 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...
- Carboxyhaemoglobin (COHb)
- 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 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...
- 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...
- GC-FID
- Gas chromatography with flame ionisation detection. The standard analytical method for quantifying cyanide and methanol in blood and tissue specimens. Provides both...
- Knockdown Pattern (H2S)
- The scene distribution of multiple H2S victims clustered at the entrance or access point of a confined space, as first responders and...
- Sulphhaemoglobin (SHb)
- A modified haemoglobin species in which one or more pyrrole rings is sulphonated by reaction with hydrogen sulphide. Produces green-brown blood and...