Doctors test the levels of carbon dioxide in the blood by measuring the patient's blood bicarbonate levels, according to MedlinePlus. They draw blood from a vein on the inside of the elbow or the back of the hand. Doctors test the carbon dioxide levels in the serum, or the liquid part of blood.
To take the test, the doctor cleans an area on the back of the arm or inside the elbow with antiseptic, explains Healthline. He then wraps an elastic band around the vein so that the vein swells with blood. Using a needle, he injects into the vein and draws blood into a syringe tube.
To prepare, the patient may have to fast before the blood test, according to Healthline. The doctor may also ask the patient to stop taking any antacids or corticosteroids, since they raise the levels of carbon dioxide in the blood.
Normal levels are between 23 and 29 milliequivalents per liter, reports MedlinePlus. Lower levels may be a sign of kidney disease, methanol poisoning, Addison's disease or lactic acidosis. Higher levels may indicate breathing disorders or Cushing's syndrome. Other conditions that may alter the levels of bicarbonate in blood include alkalosis, delirium and dementia.