How Do You Measure Blood Oxygen Levels?

If you’ve been to the ER, your primary care doctor or an emergency clinic, you’ve probably seen a medical professional place a small, clip-on electronic device on your finger to get a baseline of your vitals.

This small device, called an oximeter or pulse ox, measures your blood oxygen level, which is the percentage of oxygen red blood cells carry in the bloodstream. This measurement is known as oxygen saturation.

When you inhale or take a breath, your lungs expand. Oxygen in molecular form, O₂, travels through your lungs and is picked up by hemoglobin, a protein molecule found in red blood cells that transports oxygen throughout the body.

A single hemoglobin molecule can carry up to four oxygen molecules and transport them through the bloodstream, arteries and capillaries before dropping them off at your cells, which use O₂ for energy production.

Once the cells use O₂, they need to get rid of carbon dioxide (CO₂), which is a waste product. So the hemoglobin molecule picks up the CO₂ and carries it back to your lungs, where it’s exhaled.

How does the oximeter work? A medical professional first places the small clip-on device on the tip of your index finger.  A light source within the device shines a red light through your finger. The hemoglobin inside your finger absorbs some of this light, and that is what is detected on the other end of the device.

The oximeter essentially measures how much light hemoglobin absorbs at two different wavelengths. This is called the absorption spectrum of hemoglobin. One wavelength represents hemoglobin that carries oxygen, called oxyhemoglobin, and the other represents hemoglobin without oxygen, called deoxyhemoglobin.

These measurements of the two wavelengths are shown as a percentage reading on a machine the oximeter device connects to. That percentage represents the many hemoglobin molecules carrying oxygen molecules in the volume of blood measured from the tip of your finger. A reading of 0% means the hemoglobin carries no oxygen, whereas 100% means it carries as much oxygen as possible.

Normal oxygen saturation for the average person is between 95% and 100%. Levels below 90% are considered lethal and may indicate oxygen deficiency, called hypoxia, which requires immediate medical attention.

Skin pigmentation from melanin in the body can affect the oxygen saturation reading on oximeter devices. The FDA is currently addressing this matter with draft guidance to improve the performance of pulse oximeters across skin tones.  

So next time you watch your favorite medical drama or visit your primary care doctor, you’ll know how your blood oxygen levels are measured.