Walter A. Schrading, MD, FACEP, FAWM
Associate Professor, Department of Emergency Medicine
University of Alabama at Birmingham
The role of simple handheld pulse oximetry for home testing during the COVID-19 pandemic has been debated, nevertheless they are in use. The effectiveness of such a strategy, however, is only as good as the technology. Clearly, oxygen saturation is an important 5th vital sign when evaluating a patient with symptoms of COVID-19. Our emergency department has experienced multiple patients who look relatively well yet are severely hypoxic. A patient’s oxygen saturation drives many of the decisions for further treatment, including supplemental oxygen, hi-flow nasal cannula, BiPAP, and intubation. In an opinion piece written in the NY Times, a well-known emergency medicine airway management expert, Rich Levitan, advocated for the use of widespread or home pulse-oximetry screening as an early warning sign of worsening of the clinical course of COVID-19.(1) Mentions in the lay press of the use of pulse oximetry since the beginning of the pandemic spiked sales of pulse oximeters by 527% as early as January 20th, 2020.(2) This demand has made the devices hard to find.
With the devices out of stock, many people are turning to their smart phones. The third most popular paid app on the iPhone app store was a pulse ox app in late April.(3) This app claims to be able to measure blood oxygenation using the phone’s camera and light source (with a disclaimer about not being a medical device). If this technology allowed you to measure your own oxygen saturation accurately, then having a pulse ox measurement would be only as far away as your smart phone.
Because of my interest in Wilderness Medicine and the possibility that iPhone apps could allow a provider in an austere environment easy access to a pulse ox, I studied several of these devices in patients in the emergency department. An actual pulse oximeter uses two different wavelengths of red light to measure the differential absorption of oxygenated vs. deoxygenated hemoglobin. It also has the light source on the opposite side of the finger as the measuring sensor. My concern was that a white light iPhone camera which reflects the light back to the same camera position on the iPhone would cause errant readings, particularly in persons who are hypoxic. If you were to download one of these apps onto your phone, you will get normal looking readings when you place it on your own finger (you are presumably not hypoxic) as shown in the image. But if you were trying to utilize the device to find out if you were hypoxic because you had contracted COVID-19 and it gave errant readings, this would conceivably be dangerous. With an enterprising group of medical student co-authors I studied this question. We compared 3 devices. Two of them used the onboard camera and light source to provide a reading. The other, for comparison, had a separate finger clip that used red light and attached to the iPhone to display the results.
Out of 191 patients in the entire sample the correlation coefficients for the two devices utilizing the onboard iPhone camera were less than 0.12 (if you do not remember your statistics, that is a poor correlation with the control reading). The device which had a detachable red-light source fared better but was only 0.6. When we looked at the group of patients who were hypoxic (n=68), the correlations were even worse. The sensitivity of the two with onboard cameras for detecting hypoxia (spO2 <94%) was at best 7.4%. The detachable device worked better but misclassified hypoxia vs. no hypoxia in a quarter of all the patients. We concluded that these devices had limited to no ability to detect hypoxia and should not be relied upon in any situation, regardless of how austere, to measure oxygen saturation.(4) A recent review by the Centre for Evidence-Based Medicine concluded the same thing. (5)
I could not have imagined that our findings would become so pertinent or topical because of the COVID-19 pandemic. But they have. The results seem clear. Because these apps are inaccurate and cannot correctly show when a patient is hypoxic, they should not be relied on under any circumstances. For a patient at home in seclusion with COVID-19 to rely solely on this technology could be dangerous. We should discourage our patients from using apps as pulse-oximeters. Instead, they should be directed to use simple handheld devices if they can find them for sale. Further information about the accuracy of such hand-held devices will be forthcoming in a future article.
References
- Levitan R, The Infection That’s Silently Killing Coronavirus Patients, NY Times (Accessed at https://www.nytimes.com/2020/04/20/opinion/sunday/coronavirus-testing-pneumonia.html on 4/20/20)
- Khalid A, Pulse oximeters are selling out because of the pandemic. Most people don’t need them. Quartz Daily Brief 4/4/2020 (Accessed at https://qz.com/1832464/pulse-oximeters-for-coronavirus-unnecessary-but-selling-strong/ on4/26/2020)
- The Top iPhone and iPad Apps on App Store, by AP (Accessed at https://www.nytimes.com/aponline/2020/04/21/business/ap-us-itunes-apps-top-10.html on 4/26/2020)
- Jordan T, Meyers C, Schrading W. The utility of iPhone apps: A comparison with standard pulse oximetry measurement in the emergency department. Am J Emerg Med. 2019
- Tarrassenko L, Greenhalgh T. Qestion: should smartphone apps be used as oximeters? Answer: No (Accessed at https://www.cebm.net/covid-19/question-should-smartphone-apps-be-used-as-oximeters-answer-no/ on 4/26/2020)