Pulse oximetry is a term that frequently appears online and in news reports in connection with COVID-19. But what, exactly, is it?

Basically, pulse oximetry is a painless, noninvasive method of measuring the saturation of oxygen in a person’s blood.

Oxygen saturation is a crucial measure of how well the lungs are working. When we breathe in air, our lungs transmit oxygen into tiny blood vessels called capillaries. In turn, these capillaries send oxygen-rich blood to the heart, which then pumps it through arteries to the rest of the body. 

Our organs need a constant supply of oxygen to work properly. When the capacity of the lungs to transport oxygen into the blood is impaired, blood oxygen saturation declines, potentially putting our organs in danger. A pulse oximeter can quickly detect this drop in oxygen saturation, alerting people of the need for medical intervention.

If you have ever had a physical or visited a doctor for a medical procedure, you’ve had your blood oxygen saturation measured by a pulse oximeter. More recently, the spread of COVID-19, which can cause significant drops in blood oxygen saturation, has spurred a surge in the popularity of at-home pulse oximeters. (Some people who are worried they may have—or fear contracting—COVID-19, have purchased pulse oximeters with the aim of monitoring their blood oxygen levels.)

“It’s important to remember that not all changes in pulse oximetry are related to COVID-19,” says Denyse Lutchmansingh, MD, a Yale Medicine pulmonologist. “Other lung-related issues, such as pneumonia and blood clots, can also result in low readings on pulse oximetry. Thus, persistently low readings should be discussed with a doctor.”

A small, electronic device called a pulse oximeter is clipped onto a part of the body, usually a fingertip. The device emits light that passes through the fingernail, skin, tissue, and blood. On the other side of the finger, a sensor detects and measures the amount of light that passes through the finger without getting absorbed by the tissue and blood. Using that measurement, the device calculates the oxygen saturation of the blood.

Pulse oximetry offers many advantages over traditional methods of measuring blood oxygen levels. Whereas traditional methods usually involve drawing a sample of arterial blood—a potentially painful experience for patients that requires around 15 minutes, at minimum, to analyze blood samples—pulse oximetry is noninvasive and provides near-immediate readings. What’s more, pulse oximeters can be used continuously and, therefore, can provide long-term monitoring of a person’s blood oxygen levels.

At the same time, pulse oximetry is less precise than conventional methods, such as arterial blood gas testing. Also, it does not provide as much information on other blood gases (e.g., carbon dioxide) as do tests that directly measure the blood.

Today, pulse oximeters are used across a broad range of health care settings. In general practice, they are frequently used to quickly assess someone's general health, for instance, during a routine physical examination. In fact, pulse oximeters have become so widespread that blood oxygen saturation is often referred to as the “fifth vital sign,” a piece of data collected alongside four other measurements—temperature, blood pressure, pulse, and respiration rate—to gain insight into a person’s health status.

Outside of general practice, pulse oximetry is most frequently used to monitor patients with lung and heart disorders, who are at risk of low levels of blood oxygen. In clinical settings, they are routinely used in the following situations:

• To monitor patients before, during, and after surgery, including during anesthesia
• To monitor patients on certain medications that may reduce respiration and lung function
• To assess the lung function of people with conditions that can cause reduction of blood oxygen levels, including COPD, asthma, acute respiratory distress syndrome (ARDS), anemia, pneumonia, lung cancer, cardiac arrest, and heart failure, among others
• To assess individuals with sleep disorders such as sleep apnea

When pulse oximeters are used at home, it has usually been by people with known lung conditions, who may regularly monitor their blood oxygen saturation levels with guidance from their doctors.

A resting oxygen saturation level between 95% and 100% is regarded as normal for a healthy person at sea level. At higher elevations, oxygen saturation levels may be slightly lower. People should contact a health care provider if their oxygen saturation readings drop below 92%, as it may be a sign of hypoxia, a condition in which not enough oxygen reaches the body’s tissues. If blood oxygen saturation levels fall to 88% or lower, seek immediate medical attention, says Dr. Lutchmansingh.

Note that for people with known lung disorders such as COPD, resting oxygen saturation levels below the normal range are usually considered acceptable. A physician can provide details on appropriate oxygen saturation levels for specific medical conditions.

Most pulse oximeters are accurate to within 2% to 4% of the actual blood oxygen saturation level. This means that a pulse oximeter reading may be anywhere from 2% to 4% higher or lower than the actual oxygen level in arterial blood.  

A number of factors can impair the functioning or accuracy of a pulse oximeter. Nail polish and artificial nails may block the red and infrared light emitted by the device. Certain dyes used for diagnostic tests or medical procedures can also hinder light transmission. Excessive motion—shivering, shaking, or other movement—can also cause erroneous readings.

Skin temperature and thickness can also reduce the accuracy of pulse pulse oximeters, and whether a person smokes tobacco can affect the device’s accuracy. Pulse oximetry can be less accurate for people who have dark skin pigmentation. Recent evidence suggests that pulse oximetry more frequently fails to detect hypoxemia—low blood oxygen levels—in Black patients as compared to white patients.