We can get a good estimation of how much oxygen a person has in the blood by a little device called a pulse oximeter, while clips painlessly on a person’s finger tip (there’s an adapter for a child’s ear lobe). This can estimate whether our red blood cells (RBCs) are carrying enough oxygen, the way they should. The result is given as the percent of our RBCs that are being covered with oxygen the way they should, called an Oxygen Saturation percent (the “O2 Sat”).
Simply stated, we interpret the results as follows:
- 95% to 100% — Normal
- 89% or below — Low oxygen (danger)
- 90% to 94% — Can’t really tell
There are various reasons the result may be misleading. In carbon monoxide (CO) poisoning, the value comes out normal because the device measures both gases, while the CO kills us. In another condition methemoglobinemia (caused by certain diseases and medications), the result looks bad but we have plenty of O2.
The same thing can happen with nail polish. One of my regular patients with asthma, who didn’t look particularly sick, had an O2 Sat of 90%. Her nails were painted thickly red. So I turned the device upside-down, with the sensor on her fingerpad instead of her nailbed, and the result jumped to 95%. In Emergency Rooms they simply remove the polish with acetone (no fussing around with ER staff — they also gleefully cut expensive clothes off trauma victims).
Also beware in rare patients with severe asthma or COPD, the O2 Sat can be falsely reassuring, because it won’t tell us if carbon dioxide is building up. With too much carbon dioxide, people get sleepy, and slowly stop breathing. Then the O2 Sat will fall, but that may be too late.
A recent report found that dark skin can make values between 89%-96% range look higher than they really in about 1 in 10 persons. That would be bad if ICU staff developed a false sense of security. It would also be bad if you bought a device on-line to monitor yourself, and ignored your symptoms because the numbers looked good. Feeling short of breath is always more important than a number.
See NPR article: https://www.npr.org/sections/health-shots/2020/12/16/947226068/dark-skin-can-lead-to-errors-with-pulse-oximeters-used-in-covid-19-treatment#:~:text=Live%20Sessions-,Pulse%20Oximeters%20Can%20Give%20False%20Readings%20In%20COVID%2D19%20Patients,normal%20when%20they’re%20not.