In the hospital, a more invasive but more accurate test is used. It is called blood gas.
Blood gas tests measure the amount of atmospheric gases—usually oxygen and carbon dioxide—in the bloodstream by drawing blood from the arteries. Most other blood tests draw blood from the veins, which is easier and safer for the patient.
Arterial gases—rather than venous—measure the potential for hemoglobin to supply oxygen and to remove carbon dioxide. Since carbon monoxide is neither used by nor easily removed from the bloodstream, it can be tested through either arterial or venous blood.
Blood gas tests are considered more accurate than pulse CO-oximetry. Even though oximetry is useful for identifying patients at the scene who potentially have carbon monoxide poisoning, blood gases should be obtained to confirm carboxyhemoglobin levels.