This is similar to a blood test but instead of a needle going into a vein it is inserted briefly into a small artery in your wrist.  A small amount of blood is taken and sent to the laboratory to test for the amount of oxygen your blood is carrying. This gives a very good indirect measure of how your lungs are working.

 

Many conditions can cause a blood gas imbalance and while the blood gas tests do not identify the exact cause of the imbalance they will point to either a chest or metabolic problem.