Your doctor may order a blood test to look for clues such as anemia (low red blood cell count), a finding that might explain shortness of breath or suggest a chronic condition such as lung cancer. Or, a blood test might reveal an elevated white blood cell count, perhaps indicating a lung infection.
When the lungs are not functioning properly, the blood gases (amount of oxygen and carbon dioxide in the blood) may be affected. One way to measure blood gases is to take a blood sample from an artery. A noninvasive alternative is pulse oximetry, which measures the level of oxygen in the blood.
In this test, a sensor placed on the fingertip or earlobe directs a beam of light through the tissues and then measures the amount of light absorbed by hemoglobin, a pigment in the blood that carries oxygen. This gives an estimation of the blood oxygen level.