Pulse oximeters, which use infrared light to painlessly measure blood-oxygen levels, have become popular during the pandemic, since some COVID-19 sufferers experience dramatic—and dangerous—drops in blood oxygen without realizing it. Here’s what you need to know about using this device…

Pulse oximeters can’t diagnose COVID-19. A normal reading is not evidence that you don’t have COVID-19. The majority of people who are infected never see any change in their blood-oxygen levels. But if you have been diagnosed with COVID-19, low blood-oxygen levels could be a sign that the infection has led to a potentially dangerous pneumonia and that you should contact your doctor to see if you should head to the hospital.

Some people have low blood-oxygen levels all the time. That’s especially common among people suffering from chronic lung diseases such as COPD, pulmonary fibrosis and congestive heart failure. If you consistently get readings below 94, mention it to your doctor and ask him/her what pulse oximeter reading you should consider problematic, especially following a COVID diagnosis. 

To ensure an accurate reading, take your blood-oxygen levels at least a few times when you’re healthy to establish your normal level. Before you put the device on your finger, spend a few minutes moving around to get your blood pumping…and warm up your hand, such as by putting it under your armpit—cold hands are a common cause of inaccurate, low readings. Which finger you use doesn’t matter.

Warning: Fake fingernails or dark nail polish—black, purple or dark blue, in particular—interfere with accuracy. Remove these from at least one finger before using a pulse oximeter…or use the device on a toe. 

A single low pulse oximeter reading doesn’t necessarily mean a problem. Take a few readings from both hands to rule out a bad reading before calling your doctor or visiting the emergency room—one hand might have better blood circulation than the other. If you can get one reading within normal range, there’s no reason for concern—when pulse oximeters provide varied results, the highest reading is most likely to be a true reflection of your blood-oxygen levels. 

How Often to Check?

I do not recommend that the general public routinely monitor their oxygen levels. Individuals with chronic health conditions that may involve low oxygen levels should discuss monitoring programs with their health-care providers.

If an individual has been diagnosed with COVID-19 and is recovering at home, there is no perfect recipe for how often to check. Taking a reading every few hours while awake is reasonable. Make sure to check both at rest as well as during activity such as walking around. Furthermore, if new symptoms develop at any point such as chest pain or worsening cough, a quick oxygen check won’t hurt.