I mentioned in a that I had purchased a home pulse oximeter and had used it to monitor my oxygen saturation (SpO2) levels during the time I had COVID-esque symptoms recently. Personally, I felt the device was returning accurate information and was helpful in reassuring me that I did not require intervention.
I never completely answered whether you should utilize one. Reading between the lines, though, one might have gathered that I felt the home oximeter was a useful device to gather personal data that (preferably in conjunction with other signs and symptoms along with physician input) could help determine if one had COVID-19 that required a visit to the emergency room.
To be useful in home monitoring, the pulse oximeter, of course, must be sufficiently accurate that it allows proper decision-making. Thus, we would like to know how accurate a cheap pulse oximeter is, like the one I purchased online, that is not cleared by the FDA for medical use.
There has been a rapid evolution in the world of pulse oximetry. Pulse oximeters are being widely used in a variety of clinical settings because of their ease of use, portability, and applicability.
For Medical Use or Not For Medical Use
The FDA considers pulse oximeters to be medical devices that require a prescription. To obtain FDA labeling for "medical use," the manufacturers must submit their devices to rigorous testing on human volunteers. Accurate pulse oximeters utilize correction factors based on the in vivo comparison of arterial hemoglobin oxygen saturation obtained from direct measurement of arterial blood gases with what the pulse oximeter obtains over a wide range of oxygen saturations.
These correction factors help account for causes of known variability, including anemia, light scattering, venous and tissue pulsation by mechanical force from nearby arteries, pulsatile variations in tissue thickness in the light path other than in the arteries, nail polish, and skin pigmentation.
Because they lack validation by such rigorous testing, the (relatively) inexpensive pulse oximeters sold in drugstores or over the internet are specifically labeled not for medical use (NMU). These NMU pulse oximeters generally can be purchased now for $20 or so; but in late spring after a New York Times opinion piece suggested the great value of having one during COVID-19, there was a run on oximeters and prices rose as supplies dropped.
Although I can't find any NMU statement on the box or the manufacturer's , the Zacurate-brand NMU pulse oximeter I purchased says (in slightly bold letters) near the front of the instruction manual:
"This pulse oximeter is not a medical device and is not intended to diagnose and/or treat any medical condition or disease. It is intended for non-medical use by healthy people to monitor their pulse rate and blood oxygen levels. It is for sports and/or aviation use. People who need SpO2 and pulse rate measurements because of a medical condition should consult with their physician."
Exactly how one would use the pulse oximeter in sports is not clear to me: The devices become extremely inaccurate with any motion of the fingers. When I wore my NMU pulse oximeter with even slow walking, it told me my oxygen saturation had dropped into the 80s.
What Does Science Say?
At least three studies have looked at the accuracy of non-approved pulse oximeters.
The first, , evaluated six low-cost pulse oximeters (Contec CMS50DL, Beijing Choice C20, Beijing Choice MD300C23, Starhealth SH-A3, Jumper FPD-500A, and Atlantean SB100 II) "available for purchase from popular consumer retailers."
This study has been widely reported as demonstrating that NMU pulse oximeters are inaccurate and unreliable. However, although four of the six oximeters did not meet FDA standards for accuracy, the authors wrote that two "unexpectedly" did meet accuracy standards defined by the FDA and International Organization for Standardization: the Beijing Choice C20 and Contec CMS550DL.
Furthermore, all the NMU pulse oximeters worked pretty well when SpO2 was above 90%, where most individuals without severe lung disease would run. "The magnitude of the oximeter error in all 6 oximeters tested here was relatively small at saturations >90% and probably of no clinical significance," the researchers wrote.
However, at SpO2 below 90%, there were significant errors, and two of the devices locked into a normal SpO2 even as the true levels became very low or hypoxemic.
A sister product to one of those accurately-performing NMU pulse oximeters, Contec's CMS50D, was selected in a and compared to a much more expensive gold-standard, bedside pulse oximeter.
The reference medical-grade monitor cost 400 times that of the CMS50D. I found the CMS50DL selling for $29.95 at .
Findings were similar to the earlier study and the NMU pulse oximeter worked well during normoxia: "This pragmatic study demonstrated that a fingertip pulse oximeter was accurate (within 3% SpO2) in perioperative patients with normal oxygenation (SpO2 ≥93%) compared with a bedside pulse oximeter."
Again, once the oxygen levels dropped, however, the NMU pulse oximeter values differed from the reference, with the researchers writing that "accuracy deteriorated with progressive hypoxaemia. A measurement of <93% on the portable device is cause for concern, and should prompt further investigation and management of hypoxia if necessary."
Despite similar findings to the earlier study of NMU pulse oximeters, these authors concluded that the devices could be useful for doctors and patients to use "when ruling out hypoxemia:"
"Since small quantitative differences in SpO2 may not be clinically meaningful when oxygen saturation is nearly complete, NMU pulse oximeters may be helpful for family physicians and their patients to use when ruling out hypoxemia despite being labeled as not for medical use and sold without prescription. Moreover, the widespread availability of NMU pulse oximeters and their relatively low cost compared with MU pulse oximeter devices serves to improve access to rapid assessment of systemic oxygenation in many patients when it would otherwise be impractical."
, published in 2018 in the Annals of Family Medicine, evaluated eight NMU pulse oximeters (see below for brands) and compared them to a pulse oximeter approved for medical use. They also found the devices worked well during normoxia with "no meaningful differences in the displayed oxygen saturations between the MU pulse oximeter and the NMU pulse oximeters in the range from 90% to 99%, and this is consistent with laboratory findings from a prior study."
Here are the eight NMU pulse oximeters studied. Note the Contec CMS50DL appears again.
The authors commented:
"Thus, when confirming normoxia or ruling out hypoxemia, spot measurement of SpO2 by NMU pulse oximeters appeared satisfactory among patients in a population where hypoxemia was unlikely. Because pulse oximeter measurements of oxygen saturation are less accurate for measurements below 90%, patient management decisions regarding oxygenation should be verified using a device intended for medical use whenever possible."
Correct Usage During COVID-19
Was it reasonable for me to use SpO2 from an NMU pulse oximeter I purchased from Amazon to aid in my assessment of a potential COVID-19 episode?
Having dealt with pulse oximeters in a hospital setting, I have a good feel for issues that interfere with their accuracy. It's not uncommon to see measured SpO2 dropping to alarmingly low levels during a procedure despite everything else about the patient (respiratory rate, skin color, heart rate, blood pressure) looking fine. Often, a repositioning of the pulse oximeter on the finger, a move to another finger, or repositioning of the arm swiftly corrects the erroneous reading.
With any pulse oximeter, it's important to verify that the device is properly positioned in order to get accurate, actionable information. Patients need to be aware of this and other factors that might give falsely low values. I have a good feel for how to adjust the device to optimize the recording, maximizing the pulse volume and the SpO2 recorded based on my hospital experiences.
I think it's important to get to know your NMU pulse oximeter before using it to assess a potential COVID-19 illness. Wear it at different times during the day (at rest) and determine what your baseline normal SpO2 is. For me (and most individuals without significant lung disease), levels consistently run between 97% and 100%.
During my illness, I would measure my SpO2 twice daily and at times when I felt short of breath. When I felt the worst, I noted the SpO2 had dropped to 95%. Within 24 hours it rebounded and I recorded >96% thereafter.
If the SpO2 had progressively dropped and consistently showed values <90%, I would have contacted my primary care physician and described the constellation of signs (pulse rate, respiratory rate, blood pressure, and SpO2) and symptoms (shortness of breath, cough, headache, fatigue, etc.) that I had and sought his advice on what to do.
If I had fully researched this topic prior to my impulse purchase on Amazon, I would have purchased the CMS50DL, since it has good bona fides from multiple studies.
Check back soon for the Skeptical Cardiologist's views on the new Apple Watch Series 6 and its blood oxygen sensor capability.
Anthony C. Pearson, MD, is a noninvasive cardiologist and professor of medicine at St. Louis University School of Medicine. He blogs on nutrition, cardiac testing, quackery, and other things worthy of skepticism at , where a version of this post first appeared.