Vital Signs

Pulse Oximetry

Also written as: SpO2, Oxygen saturation, Pulse ox

The percentage of hemoglobin in the blood that is carrying oxygen, read by a light sensor on a finger.

What it measures

The percentage of hemoglobin, the oxygen-carrying protein in red blood cells, that is currently loaded with oxygen. A clip on the finger or earlobe shines light through the tissue and estimates saturation from how the light is absorbed, without any needle.

Why it is checked

Checked when breathing problems, lung disease, or heart disease are being assessed, and to monitor illnesses such as pneumonia. It is also used during procedures, during sleep studies, and for home monitoring of lung conditions.

What is generally associated with a higher result

Readings near the top of the scale broadly reflect well-oxygenated blood and are the expected finding in healthy lungs. High values are generally not caused by disease and are not pursued.

What is generally associated with a lower result

Broadly associated with lung conditions such as asthma flares, pneumonia, and chronic lung disease, with certain heart conditions, and with high altitude. Low readings are also broadly associated with measurement problems at the sensor rather than anything in the body.

Everyday things that move this number

Not every shift in a value reflects disease. These are ordinary factors known to affect this particular measurement:

• Nail polish or artificial nails over the sensor
• Cold hands or poor circulation in the fingers, which weaken the signal
• Skin tone, since studies document that some devices can read falsely high in people with darker skin, potentially missing low oxygen
• Movement, tremor, or a loosely fitting sensor
• High altitude, which lowers readings for everyone

Questions to take to your appointment

• Could nail polish, cold fingers, or my skin tone be affecting the accuracy of this device?
• Should the reading be checked during walking or during sleep, rather than only at rest?
• If there is doubt, would a blood test of oxygen levels give a more definitive answer?
• Is a home pulse oximeter worth using for my situation, and how should I use it correctly?

Build a printable questionnaire with your own results →

Where to read more

MedlinePlus, from the US National Library of Medicine, publishes patient-facing pages on individual lab tests and is a reliable next stop: medlineplus.gov/lab-tests .

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