FDA Safety Communication: Pulse oximeter accuracy and limitations
The U.S. Food and Drug Administration (FDA) released a safety communication informing patients and healthcare providers that although pulse oximetry is useful for estimating blood oxygen levels, pulse oximeters have limitations and a risk of inaccuracy under certain circumstances that should be considered.
The COVID-19 pandemic has caused an increase in the use of pulse oximeters, and a recent report (Sjoding et al.) suggests that the devices may be less accurate in people with dark skin pigmentation. Patients with conditions such as COVID-19 who monitor their condition at home should pay attention to all signs and symptoms of their condition and communicate any concerns to their healthcare provider.
Recommendations for Healthcare Providers include:
· Be aware that multiple factors can affect the accuracy of a pulse oximeter reading, such as poor circulation, skin pigmentation, skin thickness, skin temperature, current tobacco use, and use of fingernail polish. Review the information in the sections below to better understand how accuracy is calculated and interpreted.
· Refer to the device labeling or the manufacturer’s website to understand the accuracy of a particular brand of pulse oximeter and sensor. Different brands of pulse oximeters and even different sensors (finger clip versus adhesive) may have a different accuracy level. Pulse oximeters are least accurate when oxygen saturations are less than 80 percent.
· Consider accuracy limitations when using the pulse oximeter to assist in diagnosis and treatment decisions.
· Use pulse oximeter readings as an estimate of blood oxygen saturation. For example, a pulse oximeter saturation of 90 percent may represent an arterial blood saturation of 86 to 94 percent.
· When possible, make diagnosis and treatment decisions based on trends in pulse oximeter readings over time, rather than absolute thresholds.
Most pulse oximeters show two or three numbers. The most important number, oxygen saturation level, is usually abbreviated SpO2, and is presented as a percentage. The pulse rate (similar to heart rate) is abbreviated PR, and sometimes there is a third number for strength of the signal. Oxygen saturation values are between 95 percent and 100 percent for most healthy individuals, but sometimes can be lower in people with lung problems. Oxygen saturation levels are also generally slightly lower for those living at higher altitudes.
There are two categories of pulse oximeters: prescription use and over the counter (OTC). Prescription oximeters are reviewed by the FDA, receive 510(k) clearance, and are available only with a prescription. The FDA requires that these pulse oximeters undergo clinical testing to confirm their accuracy. They are most often used in hospitals and doctors’ offices, although they may sometimes be prescribed for home use. Over-the-counter (OTC) oximeters are sold directly to consumers in stores or online and include smart phone apps developed for the purpose of estimating oxygen saturation. Use of OTC oximeters has increased as a result of the COVID-19 pandemic. These products are sold as either general wellness or sporting/aviation products that are not intended for medical purposes, so they do not undergo FDA review. OTC oximeters are not cleared by the FDA and should not be used for medical purposes.
Pulse oximeters have limitations and a risk of inaccuracy under certain circumstances. In many cases, the level of inaccuracy may be small and not clinically meaningful; however, there is a risk that an inaccurate measurement may result in unrecognized low oxygen saturation levels. Therefore, it is important to understand the limitations of pulse oximetry and how accuracy is calculated and interpreted.
FDA-cleared prescription pulse oximeters are required to have a minimum average (mean) accuracy that is demonstrated by desaturation studies done on healthy patients. This testing compares the pulse oximeter saturation readings to arterial blood gas saturation readings for values between 70 to 100 percent. The typical accuracy (reported as Accuracy Root Mean Square or Arms) of recently FDA-cleared pulse oximeters is within two to threeo 3percent of arterial blood gas values. This generally means that during testing, about 66percent of SpO2 values were within two or three percent of blood gas values and about 95 percent of SpO2 values were within four to six percent of blood gas values, respectively.
However, real-world accuracy may differ from accuracy in the lab setting. While reported accuracy is an average of all patients in the test sample, there are individual variations among patients. The SpO2 reading should always be considered an estimate of oxygen saturation. For example, if an FDA-cleared pulse oximeter reads 90 percent, then the true oxygen saturation in the blood is generally between 86 to 94 percent. Pulse oximeter accuracy is highest at saturations of 90 to 100 percent, intermediate at 80 to 90 percent, and lowest below 80 percent. Due to accuracy limitations at the individual level, SpO2 provides more utility for trends over time instead of absolute thresholds. Additionally, the FDA only reviews the accuracy of prescription use oximeters, not OTC oximeters meant for general wellness or sporting/aviation purposes.
Many patient factors may also affect the accuracy of the measurement. The most current scientific evidence shows that there are some accuracy differences in pulse oximeters between dark and light skin pigmentation; this difference is typically small at saturations above 80 percent and greater when saturations are less than 80 percent. In the recently published correspondence by Sjoding, et. al., the authors reported that Black patients had nearly three times the frequency of occult hypoxemia (low oxygen levels in the blood) as detected by blood gas measurements but not detected by pulse oximetry, when compared to White patients.
It is important to note that this retrospective study had some limitations. It relied on previously collected health record data from hospital inpatient stays and could not statistically correct for all potentially important confounding factors. However, the FDA agrees that these findings highlight a need to further evaluate and understand the association between skin pigmentation and oximeter accuracy.
