Not all pulse oximeters are created equal; the industry must work to enhance their accuracy and reliability to ensure the best possible clinical information.

By Paul F. Nuccio, MS, RRT, FAARC


Pulse oximetry is prevalent in nearly every aspect of clinical medicine today. Often referred to as “the fifth vital sign,” it has the ability to distinguish between oxyhemoglobin (O2Hb) and deoxyhemoglobin (HHb).1 The oximeter has the ability to monitor the saturation of only arterial blood “based on the principle that the amount of red and IR light absorbed fluctuates with the cardiac cycle,” as opposed to that of the veins and capillaries, which remain relatively constant.2

History and Importance of Pulse Oximetry

The pulse oximeter has been in use for a half century, although the work leading to its development began nearly a century ago. First presented by Takuo Aoyagi in 1974, the ability to measure arterial oxygenation non-invasively is considered to be “one of the most significant contributions to the field of medical technology.”3

There are countless applications associated with pulse oximetry, in anesthesiology and critical care, in newborn medicine and adult care. In addition to hospital use, pulse oximetry is also utilized pre-hospital by first responders, as well as in the home environment. It is also used in other settings such as health clubs and athletic facilities.

Medical and health professionals from many different disciplines, along with the general public, have come to rely on the pulse oximeter to provide us with instant feedback on the oxygenation status of our patients, family members, and ourselves.

Challenges and Limitations in Accuracy

It has long been believed that discrepancies may exist when monitoring patients with low blood pressure and/or perfusion, those with dark skin pigmentation, and those who wear nail polish. These concerns, although they have existed for quite some time, were significantly highlighted during the recent COVID-19 pandemic.4 This has resulted in researchers examining these devices much more closely than perhaps had been done previously, with a particular emphasis on the over-the-counter (OTC) portable devices that can be purchased online or locally, without the need for a prescription.

In an executive summary, published by the US Federal Drug Administration (FDA) in 2024, it was acknowledged that many factors, in addition to skin pigmentation, may lead to inaccuracies reported from pulse oximetry use such as perfusion index, carboxyhemoglobin, motion artifact, nail polish, and prevalence of hypoxemia across different patient groups. These are not only reasons for concerns for FDA-cleared pulse oximeters, but even more-so for the many OTC devices that have flooded the market over the past few years.5

As mentioned, it has long been suspected that nail polish might impact the quality of pulse oximetry readings. In a 1988 publication by Cote, et al, the authors reported that although many factors may impact pulse oximetry results, nail polish can significantly alter the results, particularly depending on the type and color of nail polish used. In fact, in some instances, there may be no detection of a pulse at all (1988).6 Like many research studies, others have found different conclusions. In a paper by Aggarwal (2023), researchers found that although nail polish can impact the determination of pulse oximetry, that impact appears to be clinically insignificant.7

In a paper by Yek, et al, the authors found that gel-based manicures could result in an overestimation of oxygen saturation readings. Based on their study they recommended that gel-type polish be routinely removed as a precaution. (2019).8

Accuracy and Performance Across Different Skin Tones

A correspondence letter by Sjoding, et al (NEJM, 2020) highlighted a concern related to racial bias when monitoring patients with pulse oximetry.9 They found while studying this in a large cohort of patients, that nearly 12% of Black patients had an overestimation of oxygen saturation by pulse oximetry based on results with arterial blood samples, as compared to white patients.

According to the authors, “Our results suggest that reliance on pulse oximetry to triage patients and adjust supplemental oxygen levels may place Black patients at increased risk for hypoxemia.” In many of these patients who were ultimately classified as having occult hypoxemia, it was found that although pulse oximetry reported an SpO2 of 92% to 96%, the actual arterial oxygen saturation was less than 88%.9 Relying solely on the pulse oximeter in these certain patients could result in the withholding of supplemental oxygen when it is actually indicated. According to Cubanas (2023), “The presence of occult hypoxemia is a risk factor associated with increased mortality. It can delay diagnosis and subsequent initiation of treatment.”10

The Open Oximetry Project, founded by the UCSF’s Center for Health Equity in Surgery and Anesthesia, in conjunction with their Hypoxia Lab to “improve the safety and precision of pulse oximeters in all populations as these devices have been found to be less accurate in people with darker skin tone.” The founders invited experts from all over the world to share research and to collectively work to improve upon the shortcomings related to these issues.11

The inaccuracies related to dark skin have been particularly problematic with the surge of over-the-counter finger-tip models that in many cases, are unregulated by the FDA. In response to this, Leeb and colleagues performed a study as part of The Open Oximetry Project to evaluate the performance of the most popular finger-tip oximeters that have been made readily available for purchase by the general public.

Out of the 11 oximeters that were evaluated, five were found to function outside of the parameters considered to be acceptable by the FDA.12 In addition, nine out of the 11 studied were found to perform poorly when used on subjects with darker skin pigmentation, as opposed to those with lighter skin. A notable fact was that the researchers used both standard subjective measures of identifying skin color using scales, in addition to objective measures performed at different anatomical sites. Their study found that there were significant variations among the different oximeters that were tested, as well as widespread differences between the subjective skin color assessments and those that were objective.12 The authors strongly recommend that the subjective scales no longer be used and that only objective measures should be used to evaluate skin color.12

In a paper by Henry (2022), it was stated that “Most studies of pulse oximeters have been calculated using individuals with low skin melanin concentrations, whilst the population sample with dark pigmentation remains underrepresented.”13

Pulse oximeter manufacturers are working to mitigate these factors using different strategies with hardware sensors and software algorithm improvements. Some have already done this which was demonstrated in the Leeb study discussed previously. Therefore, publications reporting limitations of certain pulse oximeters may be specific to that manufacturer or model. 

An often-overlooked limitation of pulse oximetry is with incidences of hyperoxemia, where patients are receiving more oxygen than is necessary. Once the hemoglobin is near or at full saturation, the oximeter will be unable to assess when oxygen levels are too high. The oxyhemoglobin dissociation curve at high saturation levels becomes almost flat, resulting in the inability to detect the large changes in PaO2 that may result from small SaO2 changes. This can be problematic for some patient groups, such as preterm newborns.14

Another concern is that patients who are being over-oxygenated may develop unrelated complications that impact their breathing ability. In these instances, there may be a significant delay in the pulse oximeter to detect hypoventilation, or even apnea, due to the time it takes for oxygenation levels to drop enough to trigger oximeter alarms due again to the flattening of the oxyhemoglobin dissociation curve. This can lead to a delay in care and the potential for poor outcomes.

Future Improvements and Regulatory Considerations

The pulse oximeter is an amazing tool that has provided excellent monitoring of the oxygenation status of patients, inside as well as outside of health care institutions. The results that are displayed are relied upon to make treatment decisions. Because of this, it is of the utmost importance that the information provided by such devices are accurate and reliable for all types of patients. It is imperative that researchers and regulators become more stringent in evaluating the effectiveness of these devices.

A recent publication by Lellouche and Branson (2024), highlighted some concerns regarding the protocol recommended by the FDA in evaluating pulse oximeters. They pointed out the fact that oximeters have been known to show bias when used in patients with various types of skin pigmentation. They highlighted the fact that almost all studies evaluating pulse oximeters involved healthy subjects, and this does not necessarily relate to patients who are sick in a health care setting. They suggest that studies must be done using actual patients under variable conditions.

The authors conclude their correspondence by stating that evaluating these devices using normal healthy subjects is not appropriate and will likely miss important clinical information that is often seen in patients in the hospital. They went on to say the following: “The objective of the evaluation is not to demonstrate that, in the words of the Leibnitzian philosopher Pangloss in Candide, ‘everything is for the best in the best of all possible worlds,’ but to describe the reality, even if it is difficult to say: pulse oximeters, although essential, are not very accurate, and several confounding factors, including skin pigmentation and pulse oximeters brand, must be considered when making clinical decisions.”15

Although this previous statement may seem somewhat bold, it highlights the fact that not all pulse oximeters are created equal; the industry must work to enhance their accuracy and reliability to ensure the best possible clinical information.

RT

Paul F. Nuccio, MS, RRT, FAARC, is a contributing writer to RT and a former director of pulmonary services at Boston’s Brigham and Women’s Hospital. For more information, contact [email protected].


References

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  2. Chan E, et al. Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations. Respir Med 2013; 107:789-799.
  3. Quaresima V. et al. Ninety years of pulse oximetry: history, current status, and outlook. J Biomed Opt. 2024; 29(Suppl 3): S33307.
  4. Luks A, Swensen E. Pulse Oximetry for Monitoring Patients with COVID-19 at Home: Potential Pitfalls and Practical Guidance. Ann Am Thorac Soc 2020; (17)9:1040-1046.
  5. FDA Executive Summary: Performance Evaluation of Pulse Oximeters Taking into Consideration Skin Pigmentation, Race and Ethnicity. 2024. Fda.gov/media/175828/download.
  6. Cote C. et al. The effect of nail polish on pulse oximetry. Anesth Analag 1988. 67(7):683-686.
  7. Aggarwal A. et al. Impact of Finger Nail Polish on Pulse Oximetry Measurements: A Systematic Review. Respir Care 2023. 68(9):1271-1280.
  8. Yek J, et al. The effects of gel-based manicure on pulse oximetry. Singapore Med J. 2019. 60(8);432-435.
  9. Sjoding M. et al. Racial bias in pulse oximetry measurement. N Engl J Med 2020. 383(25):2477-2478.
  10. Cabanas A. et al. Improving pulse oximetry accuracy in dark-skinned patients: technical aspects and current regulations. Br J Anaesth 2023. 131(4):640-644.
  11. The Open Oximetry Project. Openoximetry.org/about/.
  12. Leeb G. et al. The performance of 11 fingertip pulse oximeters during hypoxemia in healthy human participants with varied, quantified skin pigment. Lancet 2024 (102).
  13. Henry N. et al. Disparities in Hypoxemia Detection by Pulse Oximetry Across Self-Identified Racial Groups and Associations With Clinical Outcomes. Crit Care Med 2022. 1;50(2):204-211.
  14. Nitzan M. et al. Pulse oximetry fundamentals and technology update. Medical Devices:Evidence and Research 2014. 7:231-239.
  15. Lellouche F. and Branson R. US Food and Drug Administration Strategy to Evaluate Pulse Oximeters: The Same Cause Will Produce the Same Effects. Am J Respir Crit Care Med 2024. 299(11):1301-1303.