Question: My wife bought a pulse oximeter to monitor for potential COVID symptoms (low oxygen?) and I am skeptical.  Is there any evidence that pulse oximeters work for COVID-19 monitoring?  

Answer: We do have some evidence that pulse oximeters are useful for monitoring COVID symptoms!  Pulse oximeters cannot, however, be used as a COVID diagnostic tool.  Low blood oxygenation indicates a health problem, but does not tell you the cause.  Meanwhile, regular oxygenation levels do not indicate absence of infection.  I’ll describe the evidence in a moment, but let me first describe what a pulse oximeter is/does.  

Biology and Pulse Oximeters: Blood is oxygenated in the lungs and is then pumped by the heart to the tissues. Hemoglobin in red blood cells carries oxygen throughout the body.  When hemoglobin is carrying oxygen, it is called “saturated.”  And blood that is full of saturated hemoglobin is arterial blood (oxygenated blood).  Meanwhile, deoxygenated blood is venous blood.  Arterial blood is bright red due to its oxygenation whereas venous blood is dark red.  In typical arterial blood, 98% of haemoglobin is saturated (SpO2), however when a person doesn’t get enough oxygen (hypoxia), saturation levels are much lower.  NIH’s Clinical Treatment Guidelines consider COVID-19 cases to be moderate illness when </=94% of hemoglobin is saturated, and severe illness when <94% of hemoglobin is saturated.  To monitor oxygenation levels, the pulse oximeter painlessly goes on a person’s finger and works by shining small beams of red light through the finger tissue– through the blood– to measure the changes of light absorption based on hemoglobin saturation (the light passes differently through arterial and venous blood).  For more on this topic, you may find this WHO slidedeck helpful.  

Pulse Oximeters and COVID: As mentioned above, pulse oximeters are not a diagnostic tool.  That said, they can be very useful for monitoring disease severity and for indicating when you need to seek medical attention.  A paper published by the Academy of Emergency Medicine, Novel Use of Home Pulse Oximetry Monitoring in COVID‐19 Patients Discharged From the Emergency Department Identifies Need for Hospitalization, found that “home pulse oximetry monitoring identifies need for hospitalization in initially nonsevere COVID‐19 patients… Half of patients who ended up hospitalized had SpO2 < 92% without worsening symptoms. Home SpO2 monitoring also reduces unnecessary ED revisits.”  Another paper published by Applied Clinical Informatics, Remote Patient Monitoring Program for Hospital Discharged COVID-19 Patients, found that remote patient monitoring that includes pulse oximetry “reduces readmissions for patients with COVID-19 and provides scalable remote monitoring capabilities upon hospital discharge.”  And finally, an article published in BMJ,  What items should be included in an early warning score for remote assessment of suspected COVID-19?, found that pulse oximetry is an important component of an early warning prediction– indicating when a patient needs to seek higher level medical care.  All that to say, if you have COVID or COVID-like symptoms and are managing at home (including after hospital discharge) you should use a pulse oximeter to help monitor disease severity (note: pulse oximeters are pretty inexpensive and easy to acquire).  And if your oxygenation levels drop, you must seek immediate medical care!