How does a pulse oximeter work?

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

• Position the probe correctly. Depending on the device, this will usually be on the finger, toe, or earlobe. Make sure it is well-fitted, and not being forced on: for example, try the second toe rather than the big toe. If the probe is too loose or too tight, it may allow extra light in, which will impact the reading.
• Make sure you're sitting still. Shaking or shivering will affect the reading, and may make it seem much lower than it actually is. 
• Remove any nail polish. If you're wearing colored nail polish, this might affect the reading. Finger tattoos or henna dye can also affect the reading: in this case, choose a different finger, toe, or earlobe. Cold fingers or toes can also cause false readings. 

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.

from Microsoft News