Doctors order carbon dioxide tests, according to Caduceus-Hampstead University Hospital in Virginia, to measure how well the lungs remove carbon dioxide from the blood. Carbon dioxide (CO2) is a waste product of metabolism. When respiration increases, carbon dioxide levels drop. Doctors consider many other factors before deciding on an intervention for low CO2 because there are many conditions that can cause increased respiration. (Liquidlibrary/liquidlibrary/Getty Images)