Investigating the Extent to which Discrepancies in SpO2 Readings Occur due to Differences in Patient Skin Pigmentation

Blog, Journal for High Schoolers, Journal for High Schoolers 2021

Authors

Hillary Khuu, Jun Hyun Park, Carolina Pavlik, Sylvia Chin

Abstract

Pulse oximeters measure the level of oxygen saturation, also known as SpO2, in a person’s arterial blood to diagnose patients with diseases such as anemia, chronic obstructive pulmonary disease (COPD), and sleep disorders. They were developed in 1974 and are the primary method for measuring oxygen saturation. However, some studies show that pulse oximeters may display inaccurate readings when used on individuals with darker skin tones. In light of the COVID-19 pandemic, where pulse oximeters have been frequently used as self-diagnosing tests, solving technical issues with pulse oximeters has become ever so crucial.

Our project aims to discover the extent to which pulse oximeters work more faultily on individuals with darker skin tones and to determine what factors cause inaccurate readings. To obtain an intimate look into pulse oximetry use in hospitals today, we repurpose a dataset of 400 patients and cross-analyze several demographic points to pulse oximeter accuracy. Furthermore, we explore engineering methods to minimize discrepancies in SpO2 readings that may occur due to variances in patient skin pigmentation by creating our own pulse oximeters.

Background

Pulse oximetry uses a technique called photoplethysmography, which uses red and infrared light to detect variations in blood volume [3]. A pulse oximeter shines a red light (wavelength of around 660 nm) and an infrared light (wavelength of around 940 nm) through the finger it is placed on, and a photo sensor on the opposite side of the finger absorbs the lights. Digital programs can determine the ratio between oxygenated and deoxygenated blood based on these light absorption levels and therefore provide patients with their SpO2 levels. This process is possible because healthy, oxygenated blood cells have a bright red color, whereas unhealthy, deoxygenated blood cells have a darker color, and thus absorb a different amount of red and infrared light. A healthy level of oxygen saturation hovers between 90% and 100%. An SpO2 reading of 94% indicates that 94% of a person’s red blood cells are healthy, and 6% lack oxygen [2].

There are two methods of pulse oximetry: reflective and transmissive. Reflective pulse oximetry is a process in which light is emitted through a finger, bounces off reflective material on the opposite side of the finger, and is then received by a photodiode located on the same side as the light emitters. Transmissive pulse oximetry, the more popularized and accurate method, involves the lights being emitted through a finger and a photodiode receiving them on the opposite side of the finger [4].

The accuracy of FDA-cleared pulse oximeters is between 2% and 3% of arterial blood gas values. Factors that may affect the accuracy of pulse oximeter readings include poor circulation, skin pigmentation, skin thickness, skin temperature, tobacco use, and fingernail polish [7].

In the 1970’s, when pulse oximeters were being created and tested, the population on which they were used was not racially diverse. Recently, research reports have concluded that pulse oximeters are more likely to miss low oxygen levels on darker skinned individuals compared to lighter skinned individuals. These researchers used the Fitzpatrick scale, a method of categorizing skin tone based on the ability to burn and tan. Their findings suggest that these devices may not be equally accurate on all skin tones. One explanation as to why the potential for racism in pulse oximetry has not been publicized until recently is because of the current U.S. Food and Drug Administration (FDA) requirements. As of 2021, the FDA requires at least two test subjects or 15% of subjects (whichever amount is larger) to be “darkly pigmented” [8].

Methods and Materials

  • Dataset from BMC Pulmonary Medicine journal’s “A multicentre prospective observational study comparing arterial blood gas values to those obtained by pulse oximeters used in adult patients attending Australian and New Zealand hospitals”
  • 10 of Newark’s MCL053PD red LED
  • 4 of Newark’s OP165A infrared emitter
  • 4 of Newark’s TEFD4300 photodiode
  • 1 of Newark’s A000066 Arduino Uno Board
  • 10 of Newark’s MCF 0.25W 10K ohm resistor
  • 10 of Newark’s MCCFR0W4J0331A50 330 ohm resistor
  • 1 of Newark’s 759 jumper wires
  • 4 of Newark’s SSL-LX5093UWW white LED
  • 1 USB-A to USB-B cable
  • 1 I2C 16×2 Arduino LCD Display Module
  • 1 breadboard
  • Male to female jumper wires

Our project focused on examining the relationship between pulse oximetry readings and potential racial bias due to variations in skin pigmentation. To investigate, we contacted Janine Pilcher of “A multicentre prospective observational study comparing arterial blood gas values to those obtained by pulse oximeters used in adult patients attending Australian and New Zealand hospitals,” which was published in the BMC Pulmonary Medicine journal, and obtained her dataset of 400 patients [5]. While her research tested the accuracy of pulse oximeters (which provides the SpO2 value) against arterial blood gas tests (which provides the SaO2 value) for determining oxygen saturation levels, her team recorded information on skin tone as well, making the dataset viable for our project.

We used the Fitzpatrick scale as a reference for determining different levels of human skin pigmentation. The Fitzpatrick scale classifies human skin color into six categories that are dependent on the skin’s melanin concentration and reaction to UV rays. A low number on the scale generally indicates a lighter skin tone that burns more often than tans, while a high number generally indicates a darker skin tone that tans more often than burns [1].

We repurposed Pilcher’s dataset and organized the data by demographic information (i.e. Fitzpatrick type, gender, hospital location). We applied formulas to calculate the average percent error between the SpO2 and SaO2 measurements. We converted these computations into charts and plotted the patient Fitzpatrick scale skin types against the average percent error between SpO2 and SaO2 measurements to interpret whether there were discrepancies in the accuracy of measurements of different skin tones.

Additionally, we furthered our understanding of how pulse oximeters function by assembling our own pulse oximeters with instructions from Giulio Pons’ “Really Homemade Oximeter Sensor” [6]. Although the guide was excellent, some modifications were required due to accessibility issues. For example, instead of using a pre-assembled KY-039 sensor as the basis of the light emitting and sensing portion of the circuit, our group individually connected a red LED, infrared LED, and photodiode to create the sensor. Also, some parts of the code were edited to fit the parameters of our own pulse oximeter. Eventually, we were able to construct a working pulse oximeter that successfully detects heart rate as well as SpO2 readings.

Results

We reorganized a dataset of 400 patients from Pilcher’s study in Australia and New Zealand, which compared the accuracy of pulse oximeters to arterial blood gas tests. From this dataset, we were able to organize patients on their Fitzpatrick scale classification, gender, hospital location, and percent error between SpO2 and SaO2 readings.

When creating our pulse oximeters, a significant challenge we faced in ensuring accurate results was correctly positioning the LEDs and photodiode. The red and infrared LEDs had to be placed on the top of the finger and the photodiode on the bottom so that the lights could be absorbed properly. Because of accessibility issues due to COVID-19, a casing could not be 3-D printed for the pulse oximeter, which required us to manually position the sensors, as well as tape certain components together for stability. Correctly placing the light emitters and sensors was crucial since incorrect placement could result in inaccurate readings or no readings at all. Also, the lack of at-home soldering material required us to intertwine the metal wires rather than solder them together, which proved to be difficult and tedious at times. As shown below in Figure 1, one member of our research group obtained a heart rate of 91 beats per minute and an SpO2 reading of 95%.

Figure 1: Our Homemade Pulse Oximeter

Some studies show that pulse oximeters display results with larger errors when used on individuals with darker skin complexions. However, a slight downtrend in percent errors is visible as the Fitzpatrick scale increases (see Figure 4), disproving our hypothesis. These contradictory results can be greatly attributed to the fact that the Australian and New Zealand study mostly conducted tests on patients that were types I through IV on the Fitzpatrick scale, with only one patient that was a V category and zero patients that were a VI category, as shown in Figure 3. Therefore, since most data points were obtained from patients with lighter skin tones, the study does not demonstrate a holistic view of the global community. Furthermore, Figure 5 shows that the average percent error varied for each gender, and Figure 6 shows that the average percent error varied for each hospital location. Such discrepancies in factors other than skin tone could also have contributed to the contradictory downtrend in average percent errors as skin tone became darker.

Conclusion

While the results of our research suggest that patients between types IV and V on the Fitzpatrick scale obtain more accurate pulse oximeter results than those between types I and III, different results may have been obtained if an equal number of patients from each Fitzpatrick scale were tested and the population size of each category was substantially greater. Our results highlight the necessity of diverse test subjects and new FDA requirements. The FDA requires only two or 15% of patients to be “darkly pigmented” but fails to specify what qualifies as “darkly pigmented” [8]. To clarify, the FDA should list specific Fitzpatrick scale numbers. In addition, the requirements should be adjusted so that the test subjects in each Fitzpatrick scale category make up around 15% of the total in order to reach equal representation. These adjustments should be applied to all IoMT devices, not solely pulse oximeters, to remove biases based on racial traits.

Additionally, the study was conducted in only the Australian and New Zealand regions, which further strengthens our belief that further research should be conducted on a diverse pool of subjects in order to represent all groups of the world. While our data suggests that percent errors between SpO2 and SaO2 readings decline as Fitzpatrick scale increases, we would need to globally expand our project to definitively trust the results.

Furthermore, while the six categories of the Fitzpatrick scale are beneficial for simplicity, they are oversimplified and do not account for important nuances between darker skin pigmentations as type V and VI were added many years after the lighter tones [9]. The Fitzpatrick scale is the current scientific standard, and while it has room for improvement, it is superior to its predecessor, the Von Luschan’s chromatic scale (VLS). VLS consists of 36 categories, as opposed to the Fitzpatrick scale’s 6, so each category is far more specific, making it more challenging to have a classification for every skin pigmentation. Also, the large number of categories has led to inconsistent results when classifying skin tones. However, one benefit to VLS is that it categorizes skin based on pigmentation. In contrast, the Fitzpatrick scale classifies based on the ability to burn and tan, which may not always correlate with the expected skin pigmentation and does not correspond to any race [10]. Therefore, the FDA should require that the race of the subjects be recorded in conjunction with their Fitzpatrick scale classification to ensure equal representation in the research.

Future Directions

Through our project, we hope to contribute to the mitigation of racial bias within the medical field in order to improve the lives of all patients. Because COVID-19 has limited human interaction and the ability to work hands-on, we were not able to reach as far into our goals as originally planned. However, future work would include obtaining our own dataset with an equal number of individuals from each Fitzpatrick category to ensure a diversely represented pool of subjects. Testing pulse oximeters on a diverse population would allow us to detect potential discrepancies in SpO2 readings between Fitzpatrick categories.

Furthermore, we would continue to develop stringent recommendations for FDA requirements to ensure safety and accuracy for all patients regardless of skin tone. In addition, we would consult with pulse oximetry experts on their opinions and experiences with inaccurate SpO2 readings that result from variances in skin pigmentation as a method of confirming our hypothesis. We would then continue to modify our pulse oximeters so that skin pigmentation would no longer have any effect on pulse oximeter accuracy. We would test our novel pulse oximeters on the aforementioned group of diverse subjects to confirm the efficiency and accuracy of our successful modification. To further eliminate racial biases and inequities in healthcare, we intend to expand access to our pulse oximeters by making them inclusive and affordable for everyone.

References

  1. Beveridge, Chloe. “Determining your skin type on the Fitzpatrick scale.” Current Body Editorial, 4 May 2018, http://www.currentbody.com/blogs/editorial/determining-your-skin-type-on-the-fitzpatrick-scale. Accessed 21 July 2021.
  2. Chan, Edward D., et al. “Pulse oximetry: Understanding its basic principles facilitates appreciation of its limitations.” ScienceDirect, vol. 107, no. 6, 2013, pp. 789-99, https://doi.org/10.1016/j.rmed.2013.02.004. Accessed 21 July 2021.
  3. Cheriyedath, Susha, M.Sc. “Photoplethysmography (PPG).” Edited by Yolanda Smith, B.Pharm. News Medical, 27 Feb. 2019, http://www.news-medical.net/health/Photoplethysmography-(PPG).aspx. Accessed 21 July 2021.
  4. Nitzan, Meir et al. “Pulse oximetry: fundamentals and technology update.” Medical devices (Auckland, N.Z.) vol. 7 231-9. 8 Jul. 2014, doi:10.2147/MDER.S47319
  5. Pilcher, Janine et al. “A multicentre prospective observational study comparing arterial blood gas values to those obtained by pulse oximeters used in adult patients attending Australian and New Zealand hospitals.” BMC pulmonary medicine vol. 20,1 7. 9 Jan. 2020, doi:10.1186/s12890-019-1007-3
  6. Pons, Giulio. “Really Homemade Oximeter Sensor.” Project Hub, 13 May 2020, create.arduino.cc/projecthub/giulio-pons/really-homemade-oximeter-sensor-7cf6a1. Accessed 21 July 2021.
  7. “Pulse Oximeter Accuracy and Limitations: FDA Safety Communication.” U.S. Food and Drug Administration, 19 Feb. 2021, http://www.fda.gov/medical-devices/safety-communications/pulse-oximeter-accuracy-and-limitations-fda-safety-communication. Accessed 21 July 2021.
  8. “Pulse Oximeters – Premarket Notification Submissions [510(k)s]: Guidance for Industry and Food and Drug Administration Staff.” U.S. Food and Drug Administration, 4 Mar. 2013, http://www.fda.gov/regulatory-information/search-fda-guidance-documents/pulse-oximeters-premarket-notification-submissions-510ks-guidance-industry-and-food-and-drug. Accessed 15 July 2021.
  9. Sharma, Ajay N., and Bhupendra C. Patel. “Laser Fitzpatrick Skin Type Recommendations.” National Center for Biotechnology Information, 11 Mar. 2021, http://www.ncbi.nlm.nih.gov/books/NBK557626/. Accessed 8 Aug. 2021.
  10. “Von Luschan’s chromatic scale.” Wikipedia, 20 June 2021, en.wikipedia.org/wiki/Von_Luschan%27s_chromatic_scale. Accessed 1 Aug. 2021.

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