Pulse oximeters are used with all anesthetics and with most patients in emergency and urgent care settings, as well as to assess blood oxygenation in patients with respiratory difficulties.

 

As a response to COVID-19 the NHS implemented the COVID Oximetry @home service providing prescription pulse oximeters to support people at home who have been diagnosed with coronavirus and were most prone to deteriorating. Meanwhile, small-size and low-cost pulse oximeters have become increasingly available to consumers, while many patients undergoing oxygen therapy at home are using them to titrate their oxygen flow or adjust the medications.

 

The accuracy of consumer grade or over-the-counter (OTC) devices has often not been evaluated by the Food and Drug Administration, raising questions as to whether they can reliably substitute medical grade or prescription devices.

 

What is the difference between medical-grade and non medical-grade pulse oximeters?

 

Consumer-grade or Over-the-counter (OTC) oximeters are available online or in consumer stores and are intended for general purpose use such as fitness, wellness or aviation. Phones including smartphone apps for estimating blood oxygen through the phone’s hardware are also considered consumer-grade. These devices are not intended for medical use as they are not cleared by the FDA.

 

Medical-grade or prescription oximeters on the other hand receive 510(k) clearance and are required to undergo clinical testing for confirming their accuracy.

 

Technically speaking, what makes a pulse oximeter medical grade is compliance with the ISO 80601–2–61:2017. Complying with this standard requires for the accuracy root mean square error (Arms) between measured values of blood oxygen saturation (SpO2) and those of the reference arterial blood gas values to be no greater than 3% for a transmittance pulse oximeter (finger wrap or clip probe), or 3.5% for ear clips and reflectance oximeters (wrist bands, smart-watches or arm wraps). Separate clauses of the ISO standard outline the requirements for testing the SpO2 or Pulse Rate accuracy of devices claiming motion performance or low perfusion performance.

 

To put these percentages into perspective, readings of 95% and above are considered normal, whereas readings of 94% and below indicate desaturation. Yet, it is only for readings below 88%* that a person can qualify for a portable oxygen concentrator and avoid costly and timely ambulance and hospital visits. Whereas, a reading of 80% or below during a 6 minute walk test is one of the termination criteria for testing. The SpO2 reading from a pulse oximeter is also commonly used to determine oxygen supply adjustments for people who are already undergoing oxygen therapy. Apparently, percentage points matter and the accuracy of a pulse oximeter can be of make or break significance.

 

Medical grade pulse oximeters are also normally considerably more expensive than over-the-counter (OTC) oximeters. Apart from the additional one-off costs of undergoing clinical testing, their higher price reflects the higher quality of components some of which also affect the final accuracy of the readings (for example passive components with tighter tolerances or higher quality materials of the sensor’s optical shields). Other features contributing to a price increase in medical grade devices are the use of application specific ICs, higher resolution screens, as well as the inclusion of additional features such rate respiration rate readings.

 

*Under Medicare, in a chronic stable state.

 

Accuracy of medical-grade pulse oximeters

 

Even for a medicial grade pulse oximeter however there are limitations which cause the lab setting accuracy to greatly worsen in a real-world application. A medical grade pulse oximeter that reads 90% may be up to 4% above or below the true oxygen saturation in the blood, with the accuracy decreasing further as the person desaturates below 90% and even further for readings below 80%.

 

Some factors affecting the real-world accuracy of pulse oximeters are the following:

 

1. First of all, pulse oximetry by design operates on the ideal assumption that only oxygenated hemoglobin molecules exist in the blood, whereas in reality both HbCO (Carboxyhemoglobin) or MetHb (Methaemoglobin) can exist. If HbCO and MetHb are present in significant quantities they prevent the binding of oxygen to hemoglobin, which in turn changes the blood’s light absorption properties and causes the pulse oximeter to give erroneous SpO2 readings. For example, smokers who have increased amounts of carbon monoxide in their blood are reported to show false high readings for up to 4 hours after smoking. In such cases confirming the presence of methemoglobinemias with a Co-oximeter should provide a more accurate assessment.

 

2. Ambient light from the environment or other probes can be present and at varying degrees, affecting the amount of light detected by the sensor in the probe of the pulse oximeter. Covering the probe and making sure to block unwanted sources of light that interfere with the sensor can provide more accurate readings.

 

3. Low blood perfusion due to cold extremities or excessive hypotension degrade the signal quality and so do motion artifacts, sometimes making it impossible to capture the pulsating blood signal and therefore providing false readings or no readings at all.

 

4. Last but not least, a 2020 report of two cohorts confirmed that people with dark skin pigmentation had 3 times the frequency of occult* hypoxemia compared to white patients, a conclusion which necessitates the use of pulse oximetry in conjuction with other clinical and patient data for the purposes of triaging patients and adjusting supplemental oxygen levels.

 

*Meaning, an arterial oxygen saturation of less than 88% despite a false elevated oxygen saturation of 92% to 96% from a pulse oximeter.

 

Does the difference really matter?

 

The inherent limitations of pulse oximetry technology for estimating blood oxygenation indeed make one wonder if the differences between a medical grade pulse oximeter and a consumer-grade pulse oximeter really matter. In light of this question, whenever a member of our online COPD community asks whether phone apps or specific pulse oximeter brands are any reliable or what should they make of a particular SpO2 reading, the answer is always, it depends:

For general purpose use, the readings from a pulse oximeter should not be used as a substitute for a diagnosis based on absolute thresholds but instead as tools to track one’s longer-term patterns. Although an individual reading from an OTC oximetry may not be as accurate as that of a medical-grade device, a pattern of readings can be insightful in detecting abnormalities. Moreover, a 2018 study on the use of non medical-grade oximeters confirmed that spot measurements of SpO2 by non medical-grade pulse oximeters were satisfactory to rule out hypoxemia for a population unlikely of being hypoxic, i.e where oxygen saturation of 90% or more was measured.

If a person wishes to qualify for assistive technology such as a CPAP machine or a portable oxygen concentrator, or when a person is already undergoing oxygen therapy and uses the oxygen saturation readings to adjust their oxygen supply, then medical-grade oximeters are required, despite the inherent inaccuracies of the technology discussed above. Besides, management and treatment of COPD is a multi-variate process in which pulse oximeter readings are used in conjuction with, and not in replacement of, other pulmonary function tests or the patient’s history and symptoms.

 

Bridging the Gap

 

The high cost of a medical-grade and multi-feature pulse oximeter device should not discourage people from having access to technology that allows for self-management and the transition to a patient-doctor paradigm where patients have full knowledge and control over their management plan. Biostealth’s hardware products are consumer-grade devices intended for general wellness and are rigorously designed to meet the accuracy and ease-of-use expectations of consumers. We believe that Biostealth’s accessible technology with its up-to-standard accuracy and design can lead the way forward to the re-invention of COPD Management.

 

At Biostealth, we are designing a wellness platform that allows users to track their biometric data, consult a medical expert and seek community support, all in one place. We have found these practices to be useful when performed individually but much more so when they are integrated together in a holistic health management plan.

 

Note: This article is for informational purposes only and is not intended as medical advice, or as the substitute for the medical advice of a physician.