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PTD - 209

Test Directory - Carboxyhaemoglobin



Clinical Indications

Investigation of possible carbon monoxide poisoning.

Request Form

Combined Pathology Blood form (Yellow/Black)


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


Whole blood


1 ml


Purple top (EDTA) tube

Causes for Rejection

Unlabelled sample.

Reference Range and Interpretation

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 leads to COHb concentrations of up to about 13% in heavy smokers 
(Reference: Letter  from the Chief Medical Officer and Chief Nursing Officer dated 7th September 1998  (PL/CMO/98/5, PL/CNO/98/8)

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

Lab. Handling


Processing: Test code: COHB. Store whole blood at 4C in separating fridge (CB39) prior to analysis on a blood gas analyser

Version 1.0 / April 2014                                                                                                          Approved by: Consultant Biochemist