A pulse oximeter is a small, noninvasive device used to monitor respiratory well-being in clinical settings and at home. This electronic instrument clips onto a finger or toe and provides a rapid estimate of how efficiently the lungs transfer oxygen to the bloodstream. Understanding the device’s readings is important for monitoring health, especially for individuals managing respiratory conditions. This guidance explains how the device works, how to interpret its readings, and what to do if results raise concern.
Understanding Pulse Oximetry
The core measurement provided by the device is the peripheral oxygen saturation, abbreviated as SpO2. This percentage represents the proportion of hemoglobin molecules in the arterial blood that are bound to oxygen. The device operates by emitting two different wavelengths of light—red and infrared—through the tissue.
Oxygenated hemoglobin absorbs more infrared light, allowing more red light to pass through, while deoxygenated hemoglobin does the opposite. The sensor detects the ratio of light absorption to calculate the SpO2 percentage, providing an indirect estimate of the oxygen content in the blood. The pulse oximeter also measures the pulse rate, displayed in beats per minute (BPM), derived from the pulsating flow of blood in the arteries.
Interpreting SpO2 Levels
For most healthy adults and children, the expected reading falls within a specific range. This range indicates that the respiratory system is functioning optimally and delivering sufficient oxygen to the body’s tissues. The interpretation of these numbers is categorized into three levels for quick reference.
Normal Range
A reading of 95% to 100% is considered the normal oxygen saturation level for a healthy individual. This percentage confirms that the vast majority of hemoglobin is saturated with oxygen. Readings consistently within this range indicate proper respiratory function and require no immediate action. Older adults, particularly those over 70, may have a slightly lower saturation level, though 95% is still acceptable for that age group.
Acceptable/Monitoring Range
Saturation levels between 90% and 94% suggest a mild reduction in blood oxygen, often called mild hypoxemia. Readings in this range warrant increased monitoring and contact with a healthcare provider, especially if the reading remains low after retesting. For individuals with chronic lung diseases, such as COPD, a target SpO2 of 92% to 94% may be an acceptable baseline, but this must be confirmed with a physician.
Hypoxemic/Critical Range
Any reading that drops below 90% signals a significantly low oxygen level and indicates severe hypoxemia. This level suggests that the body’s tissues may not be receiving the necessary oxygen supply. Immediate medical attention is necessary if saturation levels fall to this point or lower. A reading of 88% or below is concerning and signals the need to seek emergency medical care.
Common Causes of Inaccurate Readings
The accuracy of a pulse oximeter reading can be compromised by several technical and physiological factors that interfere with the light-based measurement. Poor peripheral perfusion, which is reduced blood flow to the extremities, is a common culprit. If the hands are cold or blood pressure is low, the device may struggle to detect a strong arterial pulse, resulting in a falsely low or erratic reading.
Movement, even slight shivering or tremor, can disrupt the sensor’s ability to measure the light accurately, leading to motion artifact. The presence of dark-colored nail polish, such as black or blue, or artificial nails can block the light transmission, causing an inaccurately low saturation percentage. Strong ambient light, such as direct sunlight, can also interfere with the photodetector, corrupting the signal.
Certain physiological conditions can also result in a misleadingly high reading. In cases of carbon monoxide poisoning, the device cannot differentiate between oxygenated hemoglobin and hemoglobin bound to carbon monoxide, leading to an artificially high saturation number despite true oxygen deprivation. Additionally, dark skin pigmentation can sometimes lead to an overestimation of the true oxygen saturation level.
Actionable Steps for Low Oxygen Levels
When a pulse oximeter displays a low reading, the initial step is to troubleshoot the device and its placement. Ensure the finger is warm, still, and free of nail polish or artificial enhancements. After correcting any potential interference, allow the probe to remain in place for at least a minute to ensure the reading has stabilized.
If the low reading persists, reposition the body to a more upright posture, such as sitting up straight, to maximize lung expansion and oxygen intake. Practicing slow, deep breathing exercises, like pursed-lip breathing, can also help improve oxygen exchange. If the retested reading is 93% or 94%, and you are experiencing new shortness of breath or confusion, contact a primary care physician immediately.
A consistent reading of 92% or below requires immediate emergency medical services, especially if accompanied by severe symptoms. These symptoms include difficulty breathing, chest pain, or bluish discoloration of the lips or face. Do not delay seeking help for critical readings, as a rapidly dropping saturation level can indicate a serious respiratory event. The goal is to obtain rapid medical assessment to address the underlying cause.