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What Is Oxygen Saturation?

Red blood cells (RBCs) travel from the lungs through the arteries to bring oxygen to the cells. Then they travel through the veins to go back to the lungs, where they load up on more oxygen and make another trip to the cells. Arterial blood is red because it is rich with oxygen. Oxygen saturation is simply the amount of oxygen in the blood.

  1. Oxygen Saturation as a Percentage

    • Hemoglobin is a protein on the RBC that binds to oxygen, and each unit can carry up to 4 molecules of oxygen. Oxygen saturation measures the concentration of oxygen in the blood: It is expressed as a percentage of how much oxygen the RBCs are carrying compared with how much they could carry. One hundred hemoglobin molecules can carry a maximum of 400 oxygen molecules (100 x 4 = 400). If they carry only 360, then oxygen saturation is 90% (400 / 360 = 90%).

    Different People, Different Numbers

    • For healthy individuals, oxygen saturation is about 95 to 100%. If there are lung or heart problems (such as pneumonia or heart failure) or anemia, saturation can drop to 80% or even lower, and an individual may become short of breath. Supplemental oxygen may be administered. People with chronic lung problems may have low baseline saturation levels.

    Pulse Oximetry

    • Pulse oximetry is one of two methods to measure oxygen saturation. It is simply a clip that goes on the finger, toe or earlobe--the result is almost instantaneous, and there is no pain or discomfort. Light from the probe measures the redness of arterial blood to determine oxygen saturation. Pulse oximeters may not work if the person's skin is cold, and if the probe is on her finger or toes, it may not register if she is wearing dark nail polish.

    Arterial Blood Gas

    • Arterial blood gas (ABG) offers more information than pulse oximetry--such as pH and carbon dioxide level--and it is definitely more painful. ABG tests are usually done by drawing blood through the artery in the wrist, whereas most blood draws are done from the veins. The person doing the ABG must first check for a pulse, and firm pressure must be applied afterward to prevent bleeding. Test results come back in about 15 minutes.

    Assess the Entire Situation

    • When assessing a person, it is important to examine the whole situation. Oxygen saturation may be 98%, but if an individual is struggling to breathe--his respirations are shallow, rapid and labored--medical intervention is needed right away. On the other hand, someone with chronic obstructive pulmonary disease (COPD), a respiratory disorder, may have a saturation level of 80% and still be quite comfortable.

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