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-base status of your blood. The pH of your blood decreases (becomes more acidic) with increased amounts of CO2 and other acids,…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-base 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 can also 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, analysis of venous blood may be carried out. For babies, capillary blood from heel pricks may be 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 may 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 the blood vessels of the umbilical cord. 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 a while 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. Is any test preparation needed to ensure the quality of the sample? Typically, no test preparation is needed. However, if you are on oxygen therapy, the oxygen may either be turned off for 20 to 30 minutes before the collection for a “room air” test or, if this cannot be tolerated or if the doctor wants to check your oxygen levels with the oxygen on, the amount of oxygen being taking will be recorded. This is usually expressed as fraction of inspired oxygen in percent (e.g., 30% FIO2) or as litres of O2 flowing per minute. See MoreSee Less
