A pulse oximeter is a small electronic device that noninvasively measures your blood oxygen saturation (SpO2), assessing how effectively your lungs transfer oxygen into the bloodstream. Clipped onto a fingertip or earlobe, these devices are standard in clinical settings for monitoring respiratory health. During the COVID-19 pandemic, their use expanded into homes as people sought to monitor their own respiratory status. While useful, the accuracy of a pulse oximeter reading can be influenced by several variables.
How a Pulse Oximeter Works
A pulse oximeter functions by transmitting red and infrared light through a part of your body, like a fingertip. The technology is based on the principle that oxygenated and deoxygenated hemoglobin—the protein in red blood cells that carries oxygen—absorb different amounts of light. Oxygen-rich blood absorbs more infrared light, while blood with lower oxygen levels absorbs more red light.
A sensor on the opposite side of the device measures the amount of light that passes through the tissue. The device then calculates the ratio of red to infrared light absorption to estimate the percentage of your hemoglobin that is saturated with oxygen. This calculation provides the SpO2 reading on the screen, offering a snapshot of your respiratory function.
Factors That Affect Accuracy
Several factors can interfere with a pulse oximeter’s ability to get a correct reading. Patient-related issues like poor circulation, movement, or shivering can cause incorrect results. The temperature of your extremities is also a consideration, as the device works best when the skin is warm.
Physical barriers on the finger can block the light transmission required for a reading. Dark nail polish, particularly colors like black or blue, and artificial nails can obstruct the sensor’s ability to detect light. Additionally, certain intravenous dyes used in medical procedures can alter light absorption and lead to false readings.
Skin pigmentation is a physiological factor. Pulse oximeters may be less accurate for individuals with darker skin tones because melanin can absorb light from the device. This can result in a falsely high oxygen reading, potentially masking an underlying issue. Carbon monoxide poisoning can also lead to inaccurate readings because the device cannot distinguish between hemoglobin carrying oxygen and hemoglobin bound to carbon monoxide. Bright ambient light, like direct sunlight, can also interfere with the sensor.
Comparing Medical-Grade and Over-the-Counter Devices
A distinct difference exists between pulse oximeters for clinical settings and those sold over-the-counter. Medical-grade devices are reviewed by the U.S. Food and Drug Administration (FDA) and must meet specific accuracy standards. These devices undergo clinical testing to confirm their performance, ensuring they are reliable for medical judgments. They are accurate within a 2% to 4% margin of error compared to an arterial blood gas test.
Many devices sold directly to consumers are for wellness, sports, or aviation purposes and are not intended for medical use. These over-the-counter products may not have undergone the same rigorous FDA testing for accuracy. Consequently, their reliability can vary, and they may not provide the same level of precision as a medical-grade device.
Best Practices for an Accurate Home Reading
To get a reliable measurement from a home pulse oximeter, rest for at least five minutes to allow your heart rate and breathing to stabilize. Ensure that your hands are warm and relaxed, and keep your hand resting below the level of your heart. It is also important to remove any nail polish or artificial nails from the finger you plan to use.
When taking the reading, sit still and avoid talking or moving, as motion can interfere with the sensor. Place the oximeter so that your finger is fully inserted and touches the internal stopper. Allow the device 30 to 60 seconds for the reading to stabilize before you record the number.
Instead of focusing on a single measurement, track your readings over time to look for trends. A single low reading may not be cause for alarm, but consistently low numbers could indicate a problem. Contact a healthcare provider if your oxygen saturation levels are persistently low or are accompanied by symptoms such as shortness of breath, confusion, or a bluish tint to your lips or skin.