Pulse Oximeter Readings and Accuracy, Plus Tips on Use

If you have a lung condition like asthma or chronic obstructive pulmonary disease (COPD), your healthcare team may recommend checking your oxygen levels at home with a pulse oximeter. This small device estimates how much oxygen is in your blood, and it alerts you if your levels drop below what’s considered normal.

But pulse oximeters aren’t always accurate. Readings can be affected by things like poor circulation, smoking, and skin thickness — even fingernail polish. Research has also shown that readings may be less accurate in people with darker skin tones.

Below, we’ll explain how pulse oximeters work, how to use them correctly, and how to interpret your results. We’ll also discuss when it’s time to contact a healthcare professional.

What is a pulse oximeter?

A pulse oximeter is a small device that clips to your finger or earlobe and measures the amount of oxygen saturation in your red blood cells (called SpO2). A pulse oximeter works by shining a special type of light through your tissue, skin, and blood. Blood and tissue will absorb some of that light. A sensor on the other side measures how much light gets through. This information is then used to calculate your SpO2.

This helpful tool grew in popularity during the COVID-19 pandemic, but it has always been useful for people with chronic breathing problems. Low oxygen is a serious symptom of many lung diseases. And it's often a sign that you need medical attention.

Prescription vs. OTC pulse oximeters

You can order a pulse oximeter online or pick one up at the pharmacy. But not all pulse oximeters are created equal. There’s a big difference between a prescription device and one that you can purchase over the counter (OTC): 

• Prescription pulse oximeters: These are tested for accuracy and reviewed by the FDA. They’re typically used in healthcare settings like doctors’ offices and hospitals. In some cases, a healthcare professional may prescribe one for at-home use.

• OTC pulse oximeters: These are sold online or in pharmacies without a prescription. They’re not reviewed by the FDA, and they may be less reliable than prescription pulse oximeters. They aren’t intended to guide medical decisions.

How do I take a pulse oximeter reading?

If a healthcare professional prescribes you a pulse oximeter, ask them to show you how to use it, when to take a reading, and how often. For most healthy people, readings will generally fall between 95% and 100% (this can vary, which we’ll cover later). Write down your numbers in a notebook, including the date and time, to track your readings. 

Here are basic instructions for most oximeters:

• Step 1: Before you clip the device onto your finger, make sure your hand is warm and your fingernail is free of polish. When you clip it on, you may feel a slight pinch or pressure, but there should be very little to no pain.

• Step 2: For best results, use a finger without nail polish or an artificial nail. Most of the time, healthcare professionals put the device on your index finger. Your middle finger is another common option. 

• Step 3: Relax your hand below heart level and sit still. Then turn on the device.

• Step 4: In a matter of seconds, the oximeter will show your pulse and oxygen levels. 

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How to read a pulse oximeter

Once you’ve taken your reading, the device will display two numbers: 

1. Oxygen saturation: The number below or next to “SpO2” is your oxygen level. 

2. Pulse: The number below or next to “HR” (heart rate) or a heart symbol is your pulse. 

If a healthcare professional has recommended you use a pulse oximeter at home, they should give you information about what SpO2 levels are OK for you. They should also give you instructions for what to do if the readings are higher or lower than expected. SpO2 will vary from person to person. So, if you’re not sure what’s right for you, it’s always best to ask.

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Here are a few general guidelines when interpreting your pulse oximeter reading:

• Check your oxygen saturation level: Most people need an oxygen saturation level of 89% or higher to keep their cells healthy. Is your reading higher or lower than this number?

• Compare it with your baseline numbers: It’s important to compare your reading with your baseline numbers (what’s normal for you). Looking at trends over time can give you important information.

• Consider the accuracy rate: Pulse oximeter readings are estimates. Even prescription oximeters can be off by as much as 4% above or below your true SpO2. There are a number of factors that can affect the accuracy of pulse oximeter reading (more on those below). 

• Listen to your body: Don’t rely on a pulse oximeter reading to tell you when to get medical attention. Shortness of breath, chest pain, and racing heart rate can all be signs of low oxygen levels — even if your reading is within a normal range.

What are normal blood oxygen levels?

For most healthy adults and children, normal oxygen levels are generally 95% or above. But some people can have a lower acceptable SpO2, especially if they have a medical condition like COPD or heart failure. Your level might also be lower if you live at a high elevation. 

Here’s how to interpret SpO2 readings. 

How accurate is a pulse oximeter reading?

It depends. Pulse oximeters estimate your oxygen level. They don’t measure it directly. Even under ideal conditions, readings can be off by several percentage points.

According to the National Institutes of Health (NIH), a pulse oximeter reading may be up to 4% higher or lower than your true oxygen level. For example, a reading of 90% could actually reflect a true SpO2 between 86% and 94%. Readings also tend to be less reliable when oxygen levels drop below about 90%.

What factors can affect the accuracy of pulse oximeter readings?

Several factors can interfere with pulse oximeter accuracy:

1. Poor circulation or cold hands: Reduced blood flow can lead to falsely low readings. Warming or gently massaging your hands may help.

2. Bright light: Direct sunlight or strong indoor lighting can interfere with the device’s sensor. Take readings indoors, away from bright light.

3. Smoking: High levels of carbon monoxide from smoking can cause readings to appear falsely high. The pulse oximeter can mistake carbon monoxide for oxygen. 

4. Darker skin tones: Pulse oximeters may miss low oxygen levels more often in people with darker skin. The FDA has proposed updated recommendations that would require more inclusive testing and clearer labeling to help address this issue. But most devices on the market today were cleared before these recommendations. 

5. Nail polish or artificial nails: Evidence is mixed, but removing polish or using an earlobe or unpolished toe may improve reliability.

So, if you have cold hands, smoke, or have a darker skin tone, it’s especially important to track your oxygen level trends over time. And pay close attention to your symptoms, rather than just relying on pulse oximeter numbers. This is especially true if the numbers don’t match the way you feel. 

How can I tell if my pulse oximeter reading is inaccurate?

It’s very difficult to know if your pulse oximeter is inaccurate without comparing it with a reading from a medical-grade device or a blood sample. But it’s important to remember that pulse oximeter readings are only an estimate of your oxygen levels.

If your pulse oximeter reading is very different from what it usually is and you feel fine, it’s possible that it’s just an inaccurate reading. Likewise, if you feel lousy but your pulse oximeter reading is normal, it’s best to trust your instincts. 

Keep in mind that some conditions — like methemoglobinemia — can cause low oxygen levels without causing you to feel short of breath. So, if your oxygen levels are reading low but you feel fine, it’s probably still a good idea to reach out to your healthcare team to discuss what’s happening.

What are the dangers of an inaccurate pulse oximeter reading?

An inaccurate reading may prevent you from getting the medical attention you need. You may think you’re doing fine because your reading is normal. But in reality, your oxygen levels could be dangerously low.

Anyone monitoring their oxygen levels, especially people with darker skin, should understand that a pulse oximeter is an imperfect device. It shouldn’t be the only deciding factor in seeking care.

When should I contact a healthcare professional?

If your pulse oximeter reading is lower than normal for you and you feel unwell, it’s always best to touch base with your healthcare team.

If you have an underlying lung condition and your healthcare team has recommended home pulse oximetry, you should have a clear plan of action for how to handle a lower-than-average reading. If not, then it’s worth asking a healthcare professional for one. 

Even if your pulse oximeter reading is normal but you feel unwell, trust your instinct. This is especially true if you have an underlying lung condition or if you think you could have a serious lung infection. 

These are signs that indicate your oxygen levels could be low — whatever your pulse oximetry readings show:

• A bluish color to your lips, nails, or face

• Shortness of breath or trouble breathing

• Chest pain or tightness

• A worsening cough

• Restlessness or discomfort

• A racing or fast pulse

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Written by:

Farzon A. Nahvi, MD

Farzon Nahvi, MD, is an emergency medicine physician and author of “Code Gray: Death, Life, and Uncertainty in the ER.” He works at Concord Hospital in Concord, New Hampshire, and teaches at the Geisel School of Medicine at Dartmouth.

Edited by:

Sophie Vergnaud, MD

Sophie Vergnaud, MD, is the Senior Medical Director for GoodRx Health. A pulmonologist and hospitalist, she practiced and taught clinical medicine at hospitals in London for a decade before entering a career in health education and technology.

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