A pulse oximeter measures two things: your blood oxygen saturation level (SpO2) and your pulse rate. It’s a small device, usually clipped onto your fingertip, that uses light to estimate how much oxygen your red blood cells are carrying. Some models also display a third number indicating signal strength.
How It Works
The device shines two beams of light through your fingertip: one red and one infrared. Oxygenated hemoglobin (the protein in red blood cells that carries oxygen) absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light. By comparing how much of each light wavelength passes through your finger, the oximeter calculates the percentage of hemoglobin that’s carrying oxygen. That percentage is your SpO2 reading.
The device also detects the pulsing of blood through your finger with each heartbeat, which is how it calculates your pulse rate. This pulsing signal creates a small waveform on some oximeters called a plethysmograph, or “pleth.” If that waveform looks erratic or weak, the SpO2 number on screen may not be reliable. Hospital-grade monitors display this waveform so clinicians can judge whether the reading is trustworthy.
What the Numbers Mean
For most people, a normal SpO2 reading falls between 95% and 100%. This means that at least 95 out of every 100 hemoglobin molecules in your blood are loaded with oxygen. A reading below this range is called hypoxemia, meaning your blood oxygen is lower than expected.
If you’re using a home pulse oximeter and your reading drops to 92% or lower, that warrants a call to your healthcare provider. A reading of 88% or lower is a medical emergency. Keep in mind that some people with chronic lung conditions may have a baseline that runs slightly lower than 95%, so the thresholds that matter for you may differ.
The pulse rate number (labeled PR) works like a basic heart rate reading. It tells you how many times per minute your heart is beating, based on the pulsations detected in your fingertip.
When Pulse Oximeters Are Used
In hospitals, pulse oximetry is one of the most common monitoring tools. It’s used during surgery, in recovery rooms, and for patients with conditions that affect blood oxygen levels, including COPD, asthma, pneumonia, heart failure, lung cancer, and anemia. It’s also used to screen for sleep apnea, where breathing repeatedly stops and starts during sleep.
At home, people with chronic lung or heart conditions often use pulse oximeters to track their oxygen levels day to day. They became widely purchased during the COVID-19 pandemic as a way to catch early signs of dropping oxygen before symptoms like severe shortness of breath appeared.
Factors That Affect Accuracy
Pulse oximeters estimate oxygen levels. They don’t measure them directly the way a blood draw does, and several things can throw off the reading.
Nail polish is one of the more common culprits. Research testing ten different polish colors found that most caused a clinically meaningful drop in the SpO2 reading. Green, brown, black, and blue polishes were the worst offenders, lowering readings by 3 to 5 percentage points. Red nail polish was the only color that didn’t change the reading. If you’re monitoring your oxygen regularly, bare nails give the most reliable result.
Other factors that can skew readings include cold hands or poor circulation (which weakens the signal through the fingertip), movement during the reading, and placing the sensor too tightly. A weak or erratic signal often shows up as fluctuating numbers on the display.
Skin Tone and Measurement Bias
There is growing evidence that pulse oximeters perform differently across skin tones. The FDA has acknowledged that current scientific evidence shows accuracy differences between individuals with lighter and darker skin pigmentation. In some cases, the devices overestimate oxygen levels in people with darker skin, which could mask dangerously low readings.
The FDA has proposed updated testing requirements for manufacturers, including evaluating device performance across a wider range of skin tones and increasing the number of study participants. Under these proposed rules, devices that demonstrate comparable accuracy across skin pigmentations would carry a prominent label saying so. Until these changes take full effect, it’s worth knowing that a pulse oximeter reading may be slightly higher than your true oxygen level if you have darker skin, and a borderline reading deserves extra attention.
Home Devices vs. Medical-Grade Devices
The over-the-counter pulse oximeters sold at pharmacies and online are not held to the same testing standards as prescription, medical-grade devices used in hospitals. The FDA classifies them differently, and consumer models may have wider margins of error. A typical medical-grade oximeter is accurate to within about 2 percentage points. Consumer models can vary more, especially at lower oxygen levels where accuracy matters most.
For general wellness tracking, a consumer device gives you a useful ballpark. But if you’re relying on one to manage a chronic condition, a prescription-grade device tested against arterial blood samples provides a more dependable reading. Regardless of which type you use, take readings while sitting still, with a warm hand, on a finger without nail polish, and wait for the number to stabilize for at least 10 to 15 seconds before reading it.