What is being tested?
Blood gas tests tell your doctor if you have enough oxygen in your blood and whether or not your blood pH is balanced - not too acidic (acidosis) or too alkaline/basic (alkalosis). Blood gas tests directly measure: 

• pH – a measure of the level of hydrogen ion (H⁺), which indicates the acid/alkali status of your blood. The pH of your blood decreases (becomes more acidic) with increased amounts of CO₂ and other acids, and the pH increases (blood becomes more alkaline) with decreased CO₂ or increased amounts of bases like bicarbonate (HCO₃^-).
• PO₂ – the partial pressure of O₂ (the amount of oxygen gas dissolved in blood). Decreased PO₂ may result in an elevated PCO₂ and decreased pH
• PCO₂ – the partial pressure of CO₂ (the amount of carbon dioxide gas dissolved in the blood). As PCO₂ levels rise, blood pH levels will decrease, becoming more acidic. As PCO₂ decreases, pH levels will rise, making the blood more alkaline.

Calculations or measurements also can be done to give other values, such as: 

• O₂ saturation – a calculation of how much oxygen is bound to haemoglobin in the red blood cells and available to be carried through the arteries to nourish the body’s cells
• HCO₃^- (bicarbonate) is excreted and reabsorbed by the kidneys in response to pH imbalances and is directly related to the pH level; as the amount of HCO₃^- rises, so does the pH

How is the sample collected for testing?
Since arterial blood carries oxygen to the body and venous blood carries waste products to the lungs, the gas and pH levels will not be the same in both. Arterial blood is almost always used for blood gas analysis, but in some cases, such as for babies, whole blood from heelsticks is used.

An arterial blood sample is usually collected from the radial artery in the wrist (located on the inside of the wrist, below the thumb, where you can feel your pulse). A circulation test called an Allen test will be done before the collection to make sure that you have adequate circulation in your wrist. The test involves compressing both the radial and the ulnar wrist arteries, then releasing each in turn to watch for "flushing" (the return of blood to your hand). If your hand does not flush, your other wrist will be tested. Blood can also be collected from the brachial artery in your elbow or the femoral artery in your groin.

In newborns who experience difficulty in breathing right after birth, blood may be collected from both the umbilical artery and vein and tested separately.

If you are on oxygen therapy, the O2 will either be turned off for 20 to 30 minutes before the collection for a "Room Air" test or, if you cannot tolerate this or if the doctor wants to check your oxygen levels with the O2 on, the amount of oxygen you are taking will be recorded.

After the arterial blood has been collected you will be instructed to hold the site firmly for at least 5 minutes. Since blood pumps through the artery, the puncture will take awhile to stop bleeding. If you are taking blood thinners or aspirin, it may take up to ten or fifteen minutes to stop. The doctor will verify that the bleeding has stopped and put a wrap around your wrist, which should be left in place for an hour or so.