Oximetry: How To Measure & Oxygen Saturation Levels

Oximetry is a test that allows you to measure the oxygen saturation in the blood, i.e., the percentage of oxygen transported in the blood. This test can be done at the hospital or at home with a pulse oximeter, and it’s important if you suspect you may have 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, make sure you get a doctor to assess you. Low blood oxygenation can indicate the need for hospital treatment and may pose a life risk if not corrected properly.

There are two forms of measuring oxygen saturation:

1. Using a pulse oximeter

Oximetry: How To Measure & Oxygen Saturation Levels

This is the most common form of measuring oxygen saturation, as it is a non-invasive method. This method measures oxygen levels through a small device, called a pulse oximeter, which is placed in contact with the skin, generally on the tip of the finger.

The main advantage of this method is that you will not need to collect a blood sample, and therefore it is not invasive. As well as saturation levels, this device can also measure other vital data, 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 developed 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. The oximeter can be bought at medical supplies stores.

2. Doing an arterial blood gas test (ABG)

Oximetry: How To Measure & Oxygen Saturation Levels

Arterial blood gas test is an invasive form of measuring blood oxygen, as it is done through a blood sample, and, therefore, a needle is necessary. Therefore, this type of test is less frequent than pulse oximetry.

The advantage of the arterial blood gas test is that it is a more accurate measurement of the oxygen saturation levels in the blood, as well as providing other measurements, such as the quantity of carbon gas, pH and the quantity of acid and bicarbonate in the blood. 

  • How it works: first a blood sample is collected and then this sample is taken to be measured in a specific device in a lab. The blood vessels that are most used for this type of measurement are the radial artery, in the wrist, or the femoral, in the inner thigh, but other arteries can be used as well.

This type of measurement is usually only used in cases in which the patient needs to be monitored continually or more precisely, which is more common with big surgical procedures, serious heart disease, arrhythmia, generalized infection, sudden blood pressure increases, or in cases of respiratory insufficiency.

Normal 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 the saturation to be between 90 and 95%, and this is no reason to worry.

When saturation values dip below 90% this may indicate a reduction in oxygen levels in the body due to the presence of a serious disease that is able to reduce the efficiency of gas exchanges between the lungs and the blood. Examples of this are asthma, pneumonia, emphysema, heart insufficiency or neurologic diseases, and even COVID-19 complications.

In arterial blood gas tests, partial oxygen pressure (Po2) is also assessed, as well as oxygen saturation levels. Po2 should be between 80 and 100 mmHg.

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

Precautions for a precise reading 

It’s very important that the devices that measure oxygen saturation are regularly calibrated, to avoid corrupted readings. In addition, when using the pulse oximeter, there are some precautions that will help avoid an inaccurate reading:

  • Avoid using nail varnish or fake nails, as they interfere with the passage of the light sensor;
  • Keep your hand relaxed 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.

Before doing a test, the doctor should still investigate other diseases such as anaemia or poor circulation, which can interfere with the measurement of blood oxygen.

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References

  • SOCIEDADE BRASILEIRA DE PNEUMOLOGIA E TISIOLOGIA. Informações ao Paciente – Oximetria de pulso. Available on: <https://sbpt.org.br/portal/publico-geral/doencas/oximetria-de-pulso/>. Access in 27 May 2019
  • WHO. Using the Pulse Oximeter. 2011. Available on: <https://www.who.int/patientsafety/safesurgery/pulse_oximetry/who_ps_pulse_oxymetry_tutorial2_advanced_en.pdf>. Access in 27 May 2019
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