What is being tested?
When you breathe, you bring oxygen (O₂) into your lungs and release carbon dioxide (CO₂). The bicarbonate test measures the total carbon dioxide in your blood [present in three forms: carbonic acid (H₂CO₃, CO₂ dissolved in blood, and HCO₃^- (bicarbonate, the dominant form)]. HCO₃^- is an electrolyte that is excreted and reabsorbed by your kidneys. Its main job is to help maintain the acid-base balance (pH) in your body and secondarily to work with sodium, potassium, and chloride to maintain electrical neutrality at the cellular level.

The bicarbonate test, since it measures all three forms at once, gives your doctor a rough estimation of acid-base balance. This is usually sufficient, but measurements of gasses dissolved in the blood may be done if more information is needed. Bicarbonate may be measured along with sodium, potassium, and possibly chloride in an electrolyte profile as it is the balance of the three of four that gives your doctor the most information.

How is the sample collected for testing?
A blood sample is drawn by needle from a vein in the arm.