Smartphone Biosensor With App Meets FDA/ISO Standards for Clinical Pulse Oximetry and Can Be Reliably Used by a Wide Range of Patients

Abstract

Background: Millions of smartphones contain a photoplethysmography (PPG) biosensor (Maxim Integrated) that accurately measures pulse oximetry. No clinical use of these embedded sensors is currently being made, despite the relevance of remote clinical pulse oximetry to the management of chronic cardiopulmonary disease, and the triage, initial management, and remote monitoring of people affected by respiratory viral pandemics, such as severe acute respiratory syndrome coronavirus 2 or influenza. To be used for clinical pulse oximetry the embedded PPG system must be paired with an application (app) and meet US Food and Drug Administration (FDA) and International Organization for Standardization (ISO) requirements.

Research question: Does this smartphone sensor with app meet FDA/ISO requirements? Are measurements obtained using this system comparable to those of hospital reference devices, across a wide range of people?

Study design and methods: We performed laboratory testing addressing ISO and FDA requirements in 10 participants using the smartphone sensor with app. Subsequently, we performed an open-label clinical study on 320 participants with widely varying characteristics, to compare the accuracy and precision of readings obtained by patients with those of hospital reference devices, using rigorous statistical methodology.

Results: "Breathe down" testing in the laboratory showed that the total root-mean-square deviation of oxygen saturation (Spo₂) measurement was 2.2%, meeting FDA/ISO standards. Clinical comparison of the smartphone sensor with app vs hospital reference devices determined that Spo₂ and heart rate accuracy were 0.48% points (95% CI, 0.38-0.58; P < .001) and 0.73 bpm (95% CI, 0.33-1.14; P < .001), respectively; Spo₂ and heart rate precision were 1.25 vs reference 0.95% points (P < .001) and 5.99 vs reference 3.80 bpm (P < .001), respectively. These small differences were similar to the variation found between two FDA-approved reference instruments for Spo₂: accuracy, 0.52% points (95% CI, 0.41-0.64; P < .001) and precision, 1.01 vs 0.86% points (P < .001).

Interpretation: Our findings support the application for full FDA/ISO approval of the smartphone sensor with app tested for use in clinical pulse oximetry. Given the immense and immediate practical medical importance of remote intermittent clinical pulse oximetry to both chronic disease management and the global ability to respond to respiratory viral pandemics, the smartphone sensor with app should be prioritized and fast-tracked for FDA/ISO approval to allow clinical use.

Trial registry: ClinicalTrials.gov; No.: NCT04233827; URL: www.clinicaltrials.gov.

Keywords: chronic disease management; remote clinical pulse oximetry; respiratory viral pandemics; smartphone sensor with app.

Figure 1

Smartphone with embedded photoplethysmography sensor suite and plastic case with braille guide for digit placement used as the test instrument.

Figure 2

Plot of data collected from 10 volunteers during a “breathe down test” (see Methods). Each point on the plot represents one pair of a phone reading and Masimo Radical 7 reading. The root-mean-square deviation of bias between the two instruments in this test was 2.2 oxygen saturation counts. The upper and lower limits of agreement per Bland and Altman are shown as dashed lines. A second-order polynomial fit of the data is shown as a solid line. Spo₂ = oxygen saturation.

Figure 3

A and B, Comparison of root-mean-square deviation (RMSD) and SD of the test (in-phone) and reference (Spot Vital Signs unit; Welch Allyn) measurement systems, for heart rate (A) and Spo₂ (B), in the outpatient study (n = 250) and inpatient study (n = 70). The error bars correspond to 95% CIs. RMSD decomposes into SD and bias (see Methods, Equation 1). The reference system has no bias, by definition, and thus RMSD equals SD. See Figure 2 legend for expansion of abbreviation.