All body tissues need oxygen to live. Oxygen comes into the body through the lungs. In the lungs, the oxygen attaches to hemoglobin (haemoglobin) on our red blood cells. The blood is then pumped (by the heart) around the body. Hemoglobin carries oxygen around the body.   Typically, a digital pulse oximeter is clipped on the tip of your finger. You may experience a small amount of pressure without any pain during the reading. On the inside, the machine has lights on one side and light detectors on the other side. The machine sends light through the skin to measure how much light reflects off of our red blood cells. This provides a reading of "oxygen saturation level" and heart rate. More reflection of light means more oxygen saturation. Less reflection of light means less oxygen saturation. An oxygen saturation between 95% and 100% is normal for most healthy people at sea level.     Digital pulse oximeters are easy-to-use and painless. Some versions are quite accurate. Hospitals and other medical settings have high-quality devices that measure blood oxygen within 2% to 4% of the true value. This means that the true reading could be 2-4% higher or lower than the true amount of oxygen in the blood.   According to the U.S. Food and Drug Administration (FDA), the accuracy of tested and approved pulse oximeters is highest when the result from a pulse oximeter shows 90-100% oxygen saturation. Accuracy is poor when the reading is less than 80%.

The following may also make pulse oximeter readings less accurate: - Movement during testing. - Nail polish (or artificial nails). - Dirt on the hands. - Poor circulation. - Certain medicines. - Skin thickness and temperature. - Tobacco use. - Dark-colored skin.   The quality of other non-approved pulse oximeters available at pharmacies, stores, or online can vary. Digital apps to measure blood oxygen levels can be very inaccurate and are not recommended. Data is not available for most non-approved, over-the-counter devices, or digital apps.   Doctors may recommend that patients with certain health conditions, including some patients with COVID-19, use pulse oximeters to monitor their symptoms. The U.S. FDA recommends that pulse oximeters not be relied upon to diagnose a medical condition. You should not buy a pulse oximeter unless directed by a health provider.   The U.S. FDA recommends that people with COVID-19 who use pulse oximeters at home pay attention to "all signs and symptoms of their condition."  They should communicate any concerns to their health provider.   If pulse oximetry is part of your medical care, the U.S. FDA suggests the following: - Follow the manufacturer instructions. - Make sure your finger is warm and clean (without nail polish) before attaching the machine. - Your hand should be lower than your heart (the middle of your chest). - Sit still and do not move your hand or finger where the pulse oximeter is attached. - The result is the number you see on the screen that does not change after a few seconds. - Always follow instructions provided by your health provider for when to seek medical help.   The Government of India recommends that patients with COVID-19 check their oxygen levels three or four times per day. They also advise patients to seek hospital care if oxygen levels drop below 92%.

There have been multiple viral videos and messages about pulse oximetry. Earlier in the pandemic a claim was made about a digital app that could measure blood oxygen. Digital apps to measure blood oxygen levels can be very inaccurate and are not recommended.   People have also shared videos of people using pulse oximeters to measure oxygen levels of objects like pens or biscuits (cookies). In these videos, the machine is being “tricked” into believing that it is measuring a finger or earlobe. In these cases, the light sent out from one side of the device may go around the object to the other side. In some cases, the light may be able to go through the object. The light may even seem to move (like a pulse) when the reading is being measured. In either case, trying to “trick” the machine can sometimes result in a reading even though these objects do not have red blood cells or hemoglobin. This does not mean that pulse oximeters do not work.   Low oxygen levels (hypoxia) are reported among patients experiencing severe cases of COVID-19. Pulse oximeters may be used to track oxygen levels in the blood and monitor heart rate. They are popular because they can present warning signs of an upcoming problem before it's too late. Doctors only recommend pulse oximeters for some patients who have either been diagnosed with COVID-19 or who have other health conditions.   The quality of digital pulse oximeters can vary. These devices should not be used to diagnose COVID-19. Many public health agencies, like the U.S. Centers for Disease Control, do not recommend purchasing a digital pulse oximeter to track your blood oxygen levels unless directed by a doctor. If you feel like you're getting sick or experiencing any symptoms of COVID-19, you should contact a doctor right away.