Is my pulse oximeter accurate?

How to Use a Pulse Oximeter, and When the NHS and FDA Say to Get Help

Warm hands, no nail polish, sit still and wait for a steady number. How to take a pulse oximeter reading the way the FDA describes it, what the numbers on the screen are, and the thresholds NHS England published for when to call 111 or 999.

7 min read 5 sources

A fingertip pulse oximeter looks foolproof: clip it on, read the number. But the number can be thrown off by cold fingers, nail polish, a bit of movement or a reading taken too soon. This guide covers how to take a reading the way the FDA describes it, what the numbers on the screen are, and what the NHS and FDA say about when to get help.

We are not doctors, and nothing here tells you what your own readings mean. If a doctor or nurse has asked you to monitor your oxygen, their instructions come first.

Before you start

Know why you are checking. The FDA’s first piece of advice is to “follow your health care provider’s advice about when and how often to check your oxygen levels.” In the UK, the MHRA goes further: it “does not recommend that members of the public use oximeters at home unless they have been advised to do so by a qualified clinician, have been shown how to take an accurate measurement, and they are providing results for clinical review.”

Use an oximeter made for the job. The FDA says oximeters sold as general wellness or sporting and aviation products “are not reviewed or evaluated by the agency.” In the UK, NHS England’s home oximetry leaflet says to “make sure it has a CE mark, UKCA mark or CE UKNI mark and that the batteries have enough power.” Our pulse oximeter picks are all FDA cleared as medical devices.

Read the instructions that came with it. The FDA lists following the manufacturer’s instructions for use as its second step.

How to take a reading

These steps are the FDA’s, from its consumer guide to pulse oximeters, with the MHRA’s list of things that can throw a reading off.

  1. Warm your hand. The FDA says to make sure your hand is “warm, relaxed, and held below the level of the heart.” The FDA lists skin temperature and poor circulation among the factors that can affect accuracy, and the MHRA lists low perfusion (poor blood flow).
  2. Take off nail polish on the finger you will use. The FDA lists fingernail polish as a factor that can affect accuracy, and the MHRA adds henna dye and tattoos.
  3. Put it on properly. The MHRA advises using an appropriate device and making sure it fits securely: too tight causes constriction, and too loose lets light in. It also lists bright light falling on the sensor as a problem, so keep bright light off the sensor.
  4. Sit still. The FDA’s words: “Sit still and do not move the part of your body where the pulse oximeter is located.” Movement is on the MHRA’s list too.
  5. Wait for a steady number. The FDA says to “wait a few seconds until the reading stops changing and displays one steady number.” Do not take the first number that appears.
  6. Write it down. The FDA suggests writing down your oxygen level with the date and time of the reading, “so you can track any changes and report these to your health care provider.” NHS England’s leaflet says the same: “Keeping a record of regular readings makes it easier to see any changes.”

Reading the screen

The FDA explains that most pulse oximeters show two or three numbers:

  • SpO2, your oxygen saturation, shown as a percentage. The FDA calls it “the most important number.”
  • PR, your pulse rate, similar to your heart rate.
  • A third number, on some models, for the strength of the signal. On many oximeters this is labeled PI, for perfusion index.

Make sure you know which number is which before you write anything down. On a small screen, the oxygen and pulse numbers are easy to mix up, especially if the display rotates.

On what the number usually is, the FDA says: “Oxygen saturation values are between 95% and 100% for most healthy individuals but sometimes can be lower in people with lung and heart problems, for example.” It adds that levels are “generally slightly lower for those living at higher altitudes.”

Watch the trend and how you feel

Every body we cite says the same thing: an oximeter reading is one piece of information, not the whole picture.

  • The FDA says: “Do not rely only on a pulse oximeter.” Keep track of your symptoms or how you feel.
  • The UK’s independent review of medical devices recommends that patients using an oximeter at home “look at changes in readings, rather than just a single reading, to identify when oxygen levels are going down and they need to call for assistance.”
  • The MHRA notes that “the relative change in a patient’s reading may be of greater significance to clinical management than the absolute value.”

The FDA lists signs and symptoms of low oxygen levels to know about: bluish coloring in the face, lips or nails; shortness of breath, difficulty breathing, or a cough that gets worse; restlessness and discomfort; chest pain or tightness; and a fast or racing pulse. It also warns that “some people with low oxygen levels may not show any or all these symptoms,” and that “only a health care provider can diagnose a medical condition such as hypoxia (low oxygen levels).”

Skin tone matters here too. NHS England’s leaflet says there have been reports that oximeters “may show readings higher than the level of oxygen in your blood” in people with brown or black skin, which is another reason to watch the trend and your symptoms rather than one number. Our guide to pulse oximeter accuracy and skin tone covers what the FDA and the UK have found.

When to get help

The FDA’s advice does not give a number. It says to “contact a health care provider if you are concerned about the pulse oximeter reading, or your symptoms are serious or get worse.”

NHS England has published specific thresholds, in a leaflet for adults with suspected or confirmed COVID-19 who are at home rather than in hospital (version 8, April 2022). It was written for that situation; if a clinician has asked you to monitor for another reason, they may give you different numbers. Quoted exactly, it says:

Contact NHS 111 as soon as possible if:

If you use a pulse oximeter, your blood oxygen level is 94% or 93% or continues to be lower than your usual reading where your normal oxygen saturation is below 95% (re-take a reading within an hour first)

Attend your nearest A&E within an hour, or call 999, if:

Your blood oxygen levels are 92% or less (retake your reading immediately first)

The same leaflet lists other reasons to go to A&E or call 999 whatever your oximeter says, including being “unable to complete short sentences when at rest due to breathlessness” and your “breathing gets worse suddenly.” When you call, it says to give your oxygen reading if you use an oximeter.

If you are pregnant or have recently given birth, the leaflet sets a different threshold: “If your blood oxygen levels are 94% or less, you should contact your hospital immediately or call 999.”

In the US, call 911 in an emergency. The FDA’s guidance does not set an oxygen number for this; it points you to your health care provider.

Common mistakes

  • Taking the first number. Wait until it stops changing.
  • Cold hands. Warm them first.
  • Nail polish, or a false nail. Use a bare finger.
  • Moving your hand. Keep it still and below heart level, as the FDA describes.
  • A sport-only oximeter. It has not been reviewed by the FDA for medical use.

Take readings the same way each time, write them down, and keep an eye on how you feel. If you are worried about a reading or your symptoms, the FDA says to contact a health care provider, and NHS England’s leaflet puts it plainly: “If your condition worsens, do not wait but act immediately.”

Sources

  1. U.S. Food and Drug Administration, Pulse Oximeter Basics
  2. U.S. Food and Drug Administration, Pulse Oximeters
  3. NHS England, Suspected coronavirus (COVID-19): Important information to keep you safe while at home (version 8, April 2022)
  4. Medicines and Healthcare products Regulatory Agency (UK), The use and regulation of pulse oximeters (information for healthcare professionals)
  5. UK Department of Health and Social Care, Equity in medical devices: independent review, summary report

Keep reading

The Best Fingertip Pulse Oximeters, Only the FDA Cleared Ones Most pulse oximeters on Amazon are sold "for sport and aviation use only" and were never reviewed by the FDA. These fingertip oximeters are named in an FDA 510(k) clearance, with the link so you can check it yourself. Are Pulse Oximeters Less Accurate on Darker Skin? What the FDA and the UK Review Found The FDA says pulse oximeters show some accuracy differences between lighter and darker skin, and a 2024 UK review found they overestimate oxygen levels more on darker skin. What the regulators found, what the FDA's 2025 draft guidance proposes, and what they advise you to do. How Does a Pulse Oximeter Work? Pulse Oximetry Explained by the FDA and MHRA A pulse oximeter shines red and infrared light through your finger and works out how much of your blood's hemoglobin is carrying oxygen. How pulse oximetry works, what SpO2 really measures, how accurate the FDA and MHRA say these devices are, and what can throw them off.