Blood Oxygen (SpO2) and Pulse Oximeters, Explained Carefully

What SpO2 is

A pulse oximeter is a device that is usually placed on a fingertip. It uses light beams to estimate the oxygen saturation of the blood and the pulse rate, without having to draw a blood sample.

Oxygen saturation gives information about the amount of oxygen carried in the blood. MedlinePlus describes it this way: when the device is placed on your finger, it analyzes the light that passes through your finger to determine the percentage of oxygen in your red blood cells.

Most pulse oximeters show two or three numbers. The most important number, oxygen saturation level, is usually abbreviated SpO2 and is presented as a percentage. The pulse rate (similar to heart rate) is abbreviated PR, and sometimes there is a third number for strength of the signal.

How a fingertip pulse oximeter estimates it

The FDA says a pulse oximeter uses light beams to estimate the oxygen saturation of the blood and the pulse rate. MedlinePlus Magazine adds detail from the same idea: pulse oximeters clip onto a fingertip and send beams of red and infrared light through tissues such as the nail, skin, and blood. The amount of oxygen in the tissue — called oxygen saturation — affects how well it absorbs light. The clip’s sensor measures how much light passes through without first getting absorbed by the tissue.

That is why the FDA and MedlinePlus both describe the result as an estimate. The device does not draw blood. It works from light passing through the finger.

What the FDA says about typical values — with the FDA’s caveats

The FDA states it this way, and this page keeps the FDA’s caveats in the same place the FDA puts them: oxygen saturation values are between 95% and 100% for most healthy individuals, but sometimes can be lower in people with lung problems. Oxygen saturation levels are also generally slightly lower for those living at higher altitudes.

Read that range only together with what else the FDA says on the same pages:

• The SpO2 reading should always be considered an estimate of oxygen saturation.
• Accuracy is not the same at every level. Pulse oximeter accuracy is highest at saturations of 90% to 100%, intermediate at 80% to 90%, and lowest below 80%. Pulse oximeters are least accurate when oxygen saturations are less than 80%.
• The typical accuracy in testing is limited. The FDA says the typical accuracy (reported as Accuracy Root Mean Square, or Arms) of recently FDA-cleared pulse oximeters is within 2 to 3% of arterial blood gas values. The FDA explains that this generally means that during testing, about 66% of SpO2 values were within 2 or 3% of blood gas values and about 95% of SpO2 values were within 4 to 6% of blood gas values, respectively. That testing compared pulse oximeter readings to arterial blood gas saturation readings for values between 70% and 100%.
• The FDA’s own example: if an FDA-cleared pulse oximeter reads 90%, then the true oxygen saturation in the blood is generally between 86% and 94%.
• Real-world accuracy may differ from accuracy in the lab setting, and there are individual variations among patients, because reported accuracy is an average of all patients in the test sample.

This page does not turn any of those numbers into advice about a person’s own reading. The FDA itself says that, due to accuracy limitations at the individual level, SpO2 provides more utility for trends over time instead of absolute thresholds.

Factors that can affect a reading

The FDA lists these factors as ones that can affect the accuracy of a pulse oximeter reading:

• Poor circulation
• Skin pigmentation
• Skin thickness
• Skin temperature
• Current tobacco use
• Use of fingernail polish

On skin pigmentation, the FDA says the most current scientific evidence shows that there are some accuracy differences in pulse oximeters between dark and light skin pigmentation; this difference is typically small at saturations above 80% and greater when saturations are less than 80%. The FDA also says it has taken steps to learn more about whether accuracy is affected by a person’s skin pigmentation, including issuing draft guidance in January 2025 on performance testing and labeling for pulse oximeters used for medical purposes.

Different brands of pulse oximeters, and even different sensors (finger clip versus adhesive), may have a different accuracy level. The FDA says to refer to the device labeling or the manufacturer’s website to understand the accuracy of a particular brand.

Prescription and over-the-counter oximeters are not the same

The FDA describes two categories:

• Prescription oximeters are reviewed by the FDA, receive 510(k) clearance, and are available only with a prescription. The FDA requires that these pulse oximeters undergo clinical testing to confirm their accuracy. They are most often used in hospitals and doctors’ offices, although they may sometimes be prescribed for home use.
• Over-the-counter (OTC) oximeters are sold directly to consumers in stores or online and include smart phone apps developed for the purpose of estimating oxygen saturation. The FDA’s 2021 safety communication says these products are sold as either general wellness or sporting/aviation products that are not intended for medical purposes, so they do not undergo FDA review. OTC oximeters are not cleared by the FDA and should not be used for medical purposes.

The FDA only reviews the accuracy of prescription use oximeters, not OTC oximeters meant for general wellness or sporting/aviation purposes.

How the FDA says to take a reading

The FDA’s instructions for patients and caregivers are:

• Follow your health care provider’s recommendations about when and how often to check your oxygen levels, and follow the manufacturer’s instructions for use.
• When placing the oximeter on your finger, make sure your hand is warm, relaxed, and held below the level of the heart. Remove any fingernail polish on that finger.
• Sit still and do not move the part of your body where the pulse oximeter is located.
• Wait a few seconds until the reading stops changing and displays one steady number.
• Write down your oxygen levels with the date and time of the reading so you can easily track changes and report these to your health care provider.

Do not rely only on a pulse oximeter

This is the FDA’s central caution, stated in its own substance across its pages:

• Do not rely only on a pulse oximeter to assess your health condition or oxygen level.
• If you are using a pulse oximeter to monitor your oxygen levels at home and are concerned about the reading, contact a health care provider. It is also important to keep track of your symptoms or how you feel. Contact a health care provider if your symptoms are serious or get worse.
• When taking pulse oximeter measurements, pay attention to whether the oxygen level is lower than earlier measurements, or is decreasing over time. Changes or trends in measurements may be more meaningful than one single measurement.
• Patients with conditions such as COVID-19 who monitor their condition at home should pay attention to all signs and symptoms of their condition and communicate any concerns to their health care provider.

The FDA lists these signs or symptoms of low oxygen levels to pay attention to alongside any reading:

• Bluish coloring in the face, lips, or nails
• Shortness of breath, difficulty breathing, or a cough that gets worse
• Restlessness and discomfort
• Chest pain or tightness
• Fast or racing pulse rate

The FDA adds: be aware that some patients with low oxygen levels may not show any or all of these symptoms. Only a health care provider can diagnose a medical condition such as hypoxia (low oxygen levels).

The FDA also says pulse oximeters cannot be used to diagnose or rule out COVID-19. If you think you may have COVID-19, the FDA says to contact your health care provider or local health department about getting a diagnostic test for COVID-19.

If you think you have a problem with a pulse oximeter, the FDA encourages reporting the problem through the MedWatch Voluntary Reporting Form.