Arterial blood gas

Arterial blood gas measurement is a test that can be done to determine the amount of oxygen, carbon dioxide and bicarbonate in the blood, as well as the pH of the blood. Its main use is in pulmonology, as many lung diseases feature poor gas exchange, but it is also used in nephrology (kidney diseases) and electrolyte disturbances.

Brief theory

Arterial blood is taken from any easily accessible artery (typically either radial, brachial or femoral) or out of an arterial line. The syringe is prepacked and contains a small amount of heparin, to prevent coagulation.

The sealed syringe is taken to a blood gas machine. The machine aspirates this blood from the syringe and measures the pH and the partial pressures of oxygen and carbon dioxide and the bicarbonate concentration, as well as the oxygen saturation of hemoglobin. Some machines can also measure glucose, lactate, hemoglobin and limited electrolytes (sodium and potassium).

The results are usually available within five minutes, and are now ready for interpretation. The pH will indicate if a patient is acidotic (pH <7.35) or alkalotic (pH > 7.45), while the oxygen and carbon dioxide partial pressure readings will indicate whether the cause is respiratory problems (increased/decreased CO2), or metabolic (such as ketoacidosis), and whether or not there's been any compensation by the body.

Other results can determine oxygen saturation, blood sugar status, and other metabolites in the blood.

Reference ranges

Analyte Range
pH 7.35 - 7.45
pO2 10.0-13.0 kPa or 75-100 mmHg
pCO2 4.7-6.0 kPa or 35-45 mmHg
HCO3- 22 - 30 mmol/l
Base excess -2 to +2 mmol/l


These ranges are only guidelines. Consult your local laboratory for definitive ranges.

Lactate levels are often included on blood gas machines in neonatal wards; infants often have elevated lactic acid.


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