Pathology Handbook




Clinical Indications

Investigation of possible carbon monoxide poisoning

The data quoted in a letter  from the Chief Medical Officer and Chief Nursing Officer dated 7th September 1998  (PL/CMO/98/5, PL/CNO/98/8) was as follows:

Carbon monoxide is produced continuously in the body as a by-product of haem breakdown.   This leads to a normal baseline COHb concentration of about 0.5%.   In pregnancy and especially in haemolytic anaemias this can rise towards 5%.   Cigarette smoking lead to COHb concentrations of up to about 13% in heavy smokers

The Department of Health has produced a leaflet called 'Are you at Risk' to inform the public about carbon monoxide.

Request Form 

Request on ICE


On request

Specific Criteria


Patient Preparation

Ideally, blood should be collected as soon as possible after exposure; in an emergency situation if a patient has received oxygen before arrival the carboxyhaemoglobin levels may be misleadingly low. 

In patients being investigated for domestic poisoning samples must be collected as soon as possible after exposure to the combustion products; if there is a delay in leaving the house and having the test done results may be misleadingly low

Turnaround Time

Same day 


Green top (heparin) tube


2 mL


Vacutainer green top


Samples should be transported to laboratory immediately.

Causes for Rejection

Unlabelled sample
Not meeting specific criteria for analysis. 

Reference Range

Normal: Up to 2%

Smokers: up to 13%

In pregnancy levels may be up to 5%

Elimination half-life

On air:  250 minutes

On 100 % oxygen: 50 minutes

On Hyperbaric oxygen: 22 minutes

Samples should be collected as soon as possible after exposure