How Accurate Are Pulse Oximeters Labeled Not For Medical Use?

I mentioned in my post entitled “Should You Utilize A Home Pulse Oximeter During COVID-19?” that I had purchased a home pulse oximeter and had used it to monitor my oxygen saturation (SpO2) levels during the time I had COVIDesque 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 the titular question (Should you utilize one?) but reading between the lines 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 and 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 the pulse oximeter, of course, must be sufficiently accurate that it allows proper decision-making. Thus, we would like to know how accurate is a cheap pulse oximeter (PO) like the one I purchased online which is not validated by the FDA for medical use (NMU.)

There has been a rapid evolution in the world of pulse oximetry. Pulse oximeters are being widely used in a variety of clinical setting 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” (MU) the manufacturers must submit their devices to rigorous testing on human volunteers. Accurate pulse oximeters utilize corrections 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 corrections factors help account for causes of known variability including anemia, light scattering, venous and tissue pulation by mechanical force from nearby ateries, 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 POs generally can be purchased now for 20$ or so but in late spring after a NY 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 its box or its website, the NMU PO I purchased (Zacurate) 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 Sp)2 and pulse rate measurements because of a medical condition should consult with their physician”

Exactly how one would use the PO in sports is not clear to me: the devices become extremely inaccurate with any motion of the fingers. When I wore my NMU PO with even slow walking it told me my oxygen saturation had dropped into the 80s.

What Does Science Tell Us About NMU Pulse Oximeters?

At least three studies have looked at the accuracy of nonapproved pulse oximeters.

The first published in 2016 in Anesthesia and Analgesia evaluated 6 low-cost POs (Contec CMS50DL, Beijing Choice C20, Beijing Choice MD300C23, Starhealth SH-A3, Jumper FPD-500A, and Atlantean SB100 II) “available for puchase from popular consumer retailers.”

This study has been widely reported as demonstrating that NMU POs are inaccurate and should not be relied on. However, although 4 of the 6 oximeters did not meed US FDA standards for accuracy the authors wrote:

Unexpectedly, 2 of the 6 oximeters did meet accuracy standards defined by the FDA and ISO, an A_rms<3%.

NMU POs 4 and 5 which met the FDA standard were the Beijing Choice C20 and Contec CMS550DL Furthermore, all the NMU POs 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.

However, At SpO2 below 90% there were significant errors and 2 of the devices locked into a normal SpO2 even as the true levels became very low or hypoxemic.

One of the accurately performing NMU POs, the CMS50D (Contec, USA) from that 2016 study was selected in a 2019 South African study and compared to a much more expensive gold-standard PO

The CMS50D Fingertip Pulse Oximeter (Contec, USA) was selected as the test device for this study because of its relatively low cost (~ZAR500, compared with the control bedside pulse oximeter, which costs ~ZAR200 000), its ease of availability in SA, and the fact that that it was one of two devices identified that met International Organization for Standardization (ISO) and FDA standards in healthy test subjects in a prior study.[15] The device was purchased privately by the authors.

Note that the reference medical grade monitor cost 400 times the CMS50D. I found the CMS50DL selling for 29.95 at Pulse Oximeter.org

Findings were similar to the earlier study and the NMU PO 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. As in previous studies,

Again, once the oxygen levels dropped, however, the NMU PO values differed from the reference

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 POs, 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 help- ful 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.

Another study published in 2018 in the Annals of Family Medicine evaluated 8 NMU Pos (see below for brands) and compared them to a pulse oximeter approved for medical use. They also found the devices worked well during normoxia:

” 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 8 NMU POs 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 of NMU PO During COVID-19

Was it reasonable for me to use SpO2 from an NMU PO I purchased from Amazon to aid in my assessment of a potential COVID-19 episode?

Having dealt with POs in a hospital setting I have a good feel for issues which 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 PO on the finger, a move to another finger or repositioning of the arm swiftly corrects the erroneous reading.

With any PO 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 which 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 its important to get to know your NMU PO before using the NMU PO 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, BP, and SpO2) and symptoms (shortness of breath, cough, headache, fatigue, etc.) that I had and seek 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.

Normoxemically Yours,

-ACP

N.B. Arnold Roland, MD reminds us that altitude influences SpO2.

Good review of home-based pulse oximetry- thanks. One additional factor that affects the normal range is the altitude where patients live. I am in Santa Fe, NM at 7,200 ft. and my normal range is 92-94% saturation. This has been confirmed by the FDA approved oximeters at our local hospital. Cheers.

This further emphasizes my recommendation to get to know your PO and establish your normal baseline before relying on it to monitor any illness, COVID or otherwise.