A pulse oximeter is a small device that typically clips on to your finger, toe, or earlobe to measure oxygen levels in the blood. 

Pulse oximetry is a quick and easy way to determine how well oxygen is being sent from the heart and lungs out to the furthest parts of the body, which can help determine if your heart and lungs are functioning properly. 

Pulse oximeters can be used to gauge warning signs for chronic lung or heart conditions, and may help determine whether you should seek medical attention for the coronavirus. 

Here's what you need to know about what exactly a pulse oximeter measures, how it works, and how you can use it properly. 

A pulse oximeter measures the oxygen saturation of your blood. Essentially, this percentage is a calculation of how much oxygen is in the blood, with 100% being 'fully saturated' and the optimal level. 

And while this simple non-invasive medical tool is available over the counter in many pharmacies, it's not necessary for most people. 

"Unless you have a chronic heart or lung condition that affects your oxygen saturation on a regular basis, most individuals don't need one in their home," says Albert Rizzo, MD, Chief Medical Officer for the American Lung Association. 

Those with lung or heart conditions — such as chronic obstructive pulmonary disease (COPD) or heart failure — may be prescribed home oxygen therapy by their doctor, says Rizzo, and a pulse oximeter can help them to monitor whether the use of oxygen at home is improving their condition or not.

Pulse oximeters have also been used to indicate severe cases of COVID-19. That's because the coronavirus can cause a decrease in blood oxygen levels — though if this occurs, it's likely that you'll recognize other symptoms before needing to use a pulse oximeter. 

In fact, Rizzo says that oxygen saturation by itself is often not the best indicator of medical conditions, and other symptoms are just as important to tell your doctor. 

Other warning signs of low blood oxygen levels may include:  

For everyday health, you shouldn't rely on pulse oximeter readings alone, says William Berry, Research Scientist at Harvard T.H. Chan School of Public Health. The oximeter numbers are only estimates of your blood oxygen levels — and they should be taken in context. 

To get the most accurate reading on a pulse oximeter, there are a few simple steps, according to the World Health Organization:  

There are many types of pulse oximeters, which come with varying features and are sold at a range of prices, so you'll want to find the best pulse oximeter for your purposes.  

A 2016 comparison between the accuracy of the pocket or standard pulse oximeter found that the standard oximeter had a precision error of 1.8%, while the pocket version showed 2.21% error. One 2018 study of pulse oximeters used in heart patients suggested that earlobe probes had greater accuracy. 

If you think your pulse oximeter might not be accurate, Rizzo suggests checking the pulse reading or heart rate on the pulse oximeter against your own reading of your pulse.

According to WHO, a reading of 95% to 100% on the pulse oximeter is considered healthy and isn't a cause for concern. 

If you have no underlying medical conditions, your levels should generally be at 95 or higher. But for people with chronic conditions, it's important to talk with your doctor about what the appropriate reading should be for you, and when you should seek medical attention. 

For reference, lower levels of blood oxygen saturation may indicate: 

In addition, if your pulse oximeter reading drops during exercise, Rizzo says it can be a sign of an underlying lung or heart condition and you should speak with your doctor. 

There are lots of pulse oximeters on the market, but not all are equal. We test and recommend the best pulse oximeters. Check out our favorites:

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