Oximetry: How To Measure & Oxygen Saturation Levels

Updated on April 2022

Oximetry is a test that reveals the percentage of blood cells that are carrying oxygen, otherwise known as your oxygen saturation. This test can be done at the hospital or at home with a pulse oximeter. This test is important to complete when assessing for heart disease, neurological disease, or an illness that hinders or interferes with lung function. 

Generally, an oximetry reading above 90% indicates good oxygenation of the blood, however, you should still be assessed by a doctor if you are experiencing symptoms like shortness of breath or dizziness. Low blood oxygenation can indicate the need for hospital treatment and may be life-threatening if left untreated.

There are two ways to measure oxygen saturation:

1. Pulse oximeter

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This is the most common way to measure oxygen saturation, as it is a non-invasive method. This method measures oxygen levels through a small device, called a pulse oximeter, which is on the tip of a finger.

The main advantage of this method is that you will not need to collect a blood sample, and it does not generate any discomfort. In addition to saturation levels, this device can also measure other vital signs, such as heart rate and respiratory rate. 

  • How it works: the pulse oximeter has a light sensor which records the quantity of oxygen that goes through the blood in the area where the test is being done and readings appear in a few seconds. These sensors make immediate and regular measurements, and they are designed to be used on fingers, toes or on the ears. 

Pulse oximetry is commonly used by doctors and other health professionals during clinical assessment, mainly in cases of heart disease, neurological diseases, illnesses that cause difficulty breathing, such as pulmonary diseases, or during anaesthesia. But pulse oximetry can also be used to monitor health in case of the COVID-19 disease. A pulse oximeter can be purchased at medical supplies stores.

2. Arterial blood gas test (ABG)

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Arterial blood gas test is completed by collecting a blood sample, which makes this a more invasive way to measure oxygenation, as it requires the puncture of an artery. Although this test can reveal more information about carbon dioxide, pH and acid imbalances, pulse oximetry is more commonly used to obtain rapid readings. 

One study shows that ABGs are not more accurate than pulse oximeters for oxygen saturations above 90%, as the pulse oximeter is sensitive and can read high saturation levels without much divergence. However, ABG readings are preferable for use in patients with saturation levels below 90% and for patients in critical status. 

  • How it works: A blood sample is collected specifically from an artery and then this sample is analyzed in the lab. The most common arteries accessed for this sample are the radial artery, in the wrist, or the femoral, in the inner thigh, but other arteries can be used as well.

ABGs are usually used in cases in where the patient requires continued monitoring, such as during surgery, following a cardiovascular event, when treating sepsis or when experiencing respiratory insufficiency.

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Normal oxygen saturation levels

A healthy person, with adequate oxygen in the body usually has an oxygen saturation above 95%. However, if you have the flu or a cold, it’s common for saturation levels to dip to around 90 to 95%. This is an expected finding any is not a concern. 

When saturation values dip below 90% this means that oxygen levels have dropped significantly and that the body is not efficiently exchanging gas between the lungs and blood. This can happen with asthma, pneumonia, emphysema, heart insufficiency or neurologic diseases, and even COVID-19 complications.

In arterial blood gas tests, partial pressure of oxygen (PaO2) is also assessed, as well as oxygen saturation levels. PaO2 should be between 80 and 100 mmHg.

Learn more about normal saturation levels and what to do to increase it.

Obtaining an accurate reading 

Devices that measure oxygen saturation should be are calibrated on a regular basis to ensure accuracy. Other important considerations include:

  • Avoid using pulse oximeters on fingers with nail polish or gel nails, as they will interfere with the light sensor
  • Keep your hand relaxed, still and under the heart level;
  • Protect the device if you are in a very light or sunny area;
  • Make sure the device is well-placed (ie. when placing on a finger, ensure the entire fingernail is in contact with the light sensor)

There are other health conditions that can interfere with accurate readings, like anemia, which the doctor should confirm or rule-out when evaluating a saturation level.