Arterial blood gas measurement is a blood test is performed to determine the concentration of oxygen, carbon dioxide and bicarbonate, as well as the pH, in 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. As its name implies, the sample is taken from an artery, which is more uncomfortable and difficult than venipuncture.

Obtaining and processing the sample

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 or needs to be heparinised, by drawing up a small amount of heparin and squirting it out again.

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.

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. Various analyzers also test hemoglobin, electrolyte, lactate and others metabolites (urea, creat, gluc) levels.

Reference ranges

These are typical reference ranges, although various analysers and laboratories may employ different 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

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