Pulse Oximetry Insights: Vital Signs in Medicine

Pulse oximetry is a non-invasive method used to measure the oxygen saturation level, or SpO2, of arterial blood. The device estimates the percentage of hemoglobin binding sites in the bloodstream that are occupied by oxygen. In general internal medicine, pulse oximetry is considered a crucial tool for rapid and continuous monitoring of a patient’s oxygenation status, especially during surgery, in intensive care, and in emergency medicine. The typical pulse oximeter consists of a sensor that is placed on a thin part of the patient’s body, such as a fingertip or earlobe. The sensor uses light-emitting diodes to shine both red and infrared light through the tissue, while a photodetector on the opposite side measures the amount of light that passes through.

Healthy adults usually have oxygen saturation levels between 95% and 100%. Levels below 90% are considered low and may suggest hypoxemia, a condition where blood oxygen is insufficient to meet the body’s needs. Pulse oximetry readings in this range prompt immediate clinical attention, as sustained low oxygen saturation can compromise organ function, particularly in the brain and heart. The non-invasive nature of pulse oximetry means it can be used for frequent or continuous monitoring without causing discomfort, and it quickly became a standard part of patient assessment in hospital settings during the 1980s.

Despite its widespread adoption, there are well-established limitations to pulse oximetry that can impact its clinical reliability. One critical limitation is that pulse oximetry does not directly measure the partial pressure of oxygen in the blood. Instead, it estimates saturation based on the light absorption characteristics of oxygenated and deoxygenated hemoglobin. This indirect method is generally accurate within about 2% of the true value for healthy individuals, but the margin of error can increase in critically ill patients or in those with certain physiological or environmental factors.

A major factor affecting the accuracy of pulse oximetry is poor circulation at the measurement site. When blood flow to the fingertip or earlobe is reduced, as can occur in patients with vasoconstriction, hypothermia, or when using vasopressor medications, the device may not detect an adequate pulsatile signal. This can lead to erroneously low readings or a complete inability to produce a measurement. In such cases, clinicians may be forced to seek alternative sites or methods to assess oxygenation.

Skin pigmentation is another recognized source of error. Multiple studies have found that pulse oximeters can overestimate oxygen saturation in individuals with darker skin tones, particularly at lower saturation levels. One study found that black patients were three times as likely as white patients to have undetected low arterial oxygen saturation, with pulse oximeter readings indicating normal values when blood gas measurements showed hypoxemia. This discrepancy is believed to result from increased melanin scattering the photons of light used in the measurement process. Manufacturers historically calibrated pulse oximeters using predominantly lighter-skinned subjects, which contributes to a systemic bias in measurements for people of color.

Nail polish, particularly darker colors, can also interfere with the light absorption process, causing inaccurate readings. Even ambient light in the examination room can affect sensor performance, especially for consumer-grade devices. Motion artifact presents another common challenge. In patients who are shivering, restless, or moving, the physical motion can make it difficult for the pulse oximeter to distinguish between true arterial blood pulses and noise, resulting in false alarms or spurious measurements.

Consumer-grade pulse oximeters and mobile apps are now widely available for home and personal use. However, these devices generally have lower accuracy than clinical-grade equipment. The sensors and algorithms used in consumer devices are often less sophisticated, and studies have shown that they may be off by several percentage points compared to hospital-grade monitors. Some smartwatches now include pulse oximeter functions, but these are intended for general wellness and fitness tracking rather than for clinical diagnosis or management of respiratory conditions.

Mobile app-based pulse oximeters rely on the smartphone’s camera and flashlight to estimate oxygen saturation, rather than using dedicated red and infrared light sources. These apps are especially prone to inaccuracies, as the phone’s camera cannot reliably distinguish between the wavelengths of light necessary to differentiate between oxygenated and deoxygenated hemoglobin. At least one study has suggested that these app-based solutions are not sufficiently reliable for clinical decision-making.

It’s important to note that pulse oximetry does not measure the actual concentration of oxygen in the blood, nor does it reflect the total amount of oxygen available to tissues. For example, in cases of severe anemia, a patient’s hemoglobin may be nearly 100% saturated with oxygen, but the total amount of hemoglobin—and thus the oxygen-carrying capacity of the blood—may be dangerously low. Additionally, substances such as carbon monoxide can bind to hemoglobin and be misinterpreted by the device, causing falsely reassuring readings while the patient is, in fact, hypoxic.

For a more detailed and invasive measurement, clinicians use arterial blood gas analysis. This procedure involves drawing a sample of arterial blood, typically from the radial artery in the wrist, and directly measuring not only oxygen saturation, but also the partial pressures of oxygen and carbon dioxide, blood pH, and bicarbonate levels. Arterial blood gas analysis provides a comprehensive view of the patient’s respiratory and metabolic status, but requires technical skill, causes discomfort, and cannot be performed continuously.

Pulse oximetry and arterial blood gas analysis thus serve complementary roles in clinical practice. Pulse oximetry offers rapid, ongoing, non-invasive monitoring, which is essential in dynamic or acute care settings. Arterial blood gas analysis provides definitive, multi-parameter assessment, which is critical when precise values are needed for medical decision-making, such as in severe respiratory distress or when managing complex acid-base disorders.

The limitations of pulse oximetry have real-world consequences for patient care. Inaccurate readings due to skin pigmentation, movement, or poor perfusion can delay the recognition and treatment of hypoxemia. Studies have shown that patients from racial and ethnic minorities may be less likely to receive timely supplemental oxygen because their true oxygen saturation is underestimated by pulse oximetry. In the context of the COVID-19 pandemic, this limitation became especially significant, as pulse oximeters were used to monitor patients both in hospital and at home for dangerous drops in oxygen levels.

In summary, while pulse oximetry is a powerful and essential tool for measuring oxygen saturation, its limitations—including the effects of poor circulation, skin pigmentation, nail polish, movement, and the use of consumer-grade devices—must be considered during clinical assessment. The most specific finding is that in one hospital-based study, black patients with COVID-19 were found to have occult hypoxemia in 28.5% of cases, compared to 17.2% of white patients, when using pulse oximetry alone.