Patient Basics: Oxygen Saturation Test

Originally published by Harvard Health.

What is the test?

Your red blood cells carry oxygen through your arteries to all of your internal organs. They must carry enough oxygen to keep you alive. Normally, when red blood cells pass through the lungs, 95%-100% of them are loaded, or “saturated,” with oxygen. If you have lung disease or other types of medical conditions, fewer of your red blood cells may be carrying their usual load of oxygen, and your oxygen saturation might be lower than 95%.  If your blood oxygen saturation is too low, you may need to be given oxygen to breathe.

How do I prepare for the test?

There is no preparation necessary.

What happens when the test is performed?

An estimate of your oxygen saturation can be made easily and painlessly with a clip that fits on your finger. This clip shines a light through one side of your finger; a detector measures the light that comes through the other side. This machine can make a good estimate of your oxygen saturation because blood cells that are saturated with oxygen absorb and reflect light differently than those that are not. Blood cells are a bright red when they are loaded with oxygen, and they change to a bluish color when they are no longer carrying a full load of oxygen. The finger clip machine cannot give a perfect measurement of your oxygen saturation; it can give only a rough estimate, and its measurement can be affected by things as simple as red nail polish on your finger.

A better test for measuring your oxygen saturation is an arterial blood gas test. For this test, a small sample of blood must be drawn directly out of an artery. Most other blood tests use blood that is drawn out of a vein, so this test is a little different. The artery that is sampled most often is the radial artery in your wrist, the one you can feel when you take your pulse. To draw blood from this artery, your doctor or a technician feels your pulse before inserting the needle. Some patients find that it hurts a little more to have blood taken from an artery instead of a vein, but the procedure takes only a few seconds. Your arterial blood can be directly tested for its oxygen level, and other tests (such as the level of carbon dioxide and the pH of the blood) can be done as well.

What risks are there from the test?

Measurement made with a finger clip has no risks. The risks of an arterial blood gas test are very small. Even temporary injury to your artery is unlikely to cause a problem, because most patients pump blood to their hand through more than one artery.

Before drawing your blood, your doctor may do a brief physical examination to make sure that you still get good blood flow to your hand even when one wrist artery is blocked. To do this test, the doctor presses down first on both sides of your wrist to block blood flow, until your hand becomes pale. Then he or she lifts off the pressure from one side to see if that is enough to let your hand turn pink again.

Must I do anything special after the test is over?

You will need to have pressure held over the artery for a few minutes after the blood is drawn, because arteries are more likely than veins to bleed afterward.

How long is it before the result of the test is known?

The results of the arterial blood gas test are processed very quickly and are available within 15 minutes in most laboratories. The finger clip estimate of oxygen is available immediately.