Pathology Handbook

Blood gases



Clinical Indications

In the investigation of acid-base disorders and respiratory disorders with abnormalities of oxygen and carbon dioxide levels

Test Includes

pH, pCO2, pO2 are all measured

In addition, from these the base excess is calculated, together with the patient's bicarbonate level

Request Form 

Request on ICE

Give information about:

1) whether patient is on oxygen or inspired air

2) patient's temperature


On request.

Specific Criteria

An arterial blood sample is required for investigation of respiratory markers (pO2 and pCO2). 

A venous sample may be adequate for investigation of simple acid-base disorders

Patient Preparation


Turnaround Time

Brought to lab: immediate assay

The blood gas machine in A&E is available for trained staff to do their own measurements.

Rochford Ward blood gas machine is also available for trained staff to do their own measurements


Whole Blood collected into special blood gas syringes, containing anticoagulant.

Mix the blood by rotating the syringe.

After collection, the needle MUST be removed from the syringe and replaced by a cap, to prevent air entering the syringe.

Ordinary syringes must NOT be used




Blood gas syringe


Samples should be analysed immediately after collection

Air bubbles in the syringe invalidate analysis and will be rejected

Causes for Rejection

Unlabelled sample 
Delay in sample reaching laboratory
Air bubbles in sample

Needle left on syringe

Reference Range


7.35 to 7.45


4.7 to 6.0 kPa


10.5 to 14.0 kPa

Base excess

-2.5 to +2.5 mmol/L

Calculated bicarbonate

22 to 30 mmol/L