Measurement of blood oxygen levels (SpO2)

Every cell in your body needs oxygen to function. Oxygen molecules are taken up by your red blood cells in your lungs and transported to the furthest cells in your body. Blood oxygen levels show how much of your red blood cells are using their total oxygen-carrying capacity. 

The higher the value, the better. If the value is 98, it means your body is working at 98% of its potential oxygen-carrying capacity. A value of 50 would mean that you are only working at half.

  • the maximum value of oxygen saturation is 100% (i.e. there is no such thing as 105% saturation)
  • between 96-100% is a healthy, i.e. normal, value, nothing to do.
  • A value between 90-95% indicates that oxygenation is reduced. You should arrange a general cardiopulmonary check-up with your GP. There’s no need to rush, but it’s worth doing soon.
  • At 88% or lower, see a doctor immediately. If you have a known medical condition, start treatment as discussed with your doctor. For example, oxygen administration.

Measurement of blood oxygen levels can be very important in many conditions

In some diseases, little oxygen reaches your cells. Therefore, vital cellular metabolic processes, such as energy production, do not take place. Your body functions worse and worse. The first task is to find out what is causing the problem and treat it.

Once you know the cause and treatment has started, you need to check your blood oxygen levels regularly. This is done with a simple device called a pulse oximeter.

Your blood oxygen level may be reduced for the following reasons

  • your heart can’t circulate blood properly,
  • your lungs are not getting enough oxygen,
  • your red blood cells are not functioning properly and therefore cannot absorb and/or transport enough oxygen.

Take regular blood oxygen measurements

in respiratory disease

  • chronic bronchitis
  • obstructive pulmonary disease (COPD)
  • asthma (during attacks)
  • allergy (during seizures)
  • lung tumour
  • for pneumonia, etc
  • Recent publication suggests that in some patients, oxygen saturation can drop below 80% in the early stages of Covid-19 infection, while the person is asymptomatic. In these cases, pulse oximetry may be an early indicator of disease.

in heart disease

  • heart failure,
  • cardiomyopathy,
  • heart muscle inflammation,
  • atrial fibrillation – absolute arrhythmia,
  • arrhythmia in attacks, etc.

for red blood cell disease:

  • low red blood cell count or dysfunction
  • a problem with haemoglobin (the molecule responsible for oxygen transport)
  • lack of certain substances (iron, vitamin B12, folic acid), insufficient or inadequate red blood cell formation and function

Pulse oximeter – a tool to measure oxygen saturation

Main types of pulse oximeters

  • finger clips
  • portable
  • desktop (clinical)

All pulse oximeters use the same measurement principle and provide nearly identical accuracy of blood oxygen level measurement.
Portable and desktop devices usually offer additional features that are important for doctors. For home use, it is generally not worth buying a clinical device. For home use, finger clip-on or portable devices are recommended. The latter if an alarm function is needed-

Always measure at rest!

Please note! The cheap fingertip devices are ONLY suitable for measuring blood oxygen levels at rest! They do not measure accurately when in motion.

For measurement on the move, you need a pulse oximeter with a so-called motion-tolerant algorithm. These are for clinical use and not for home use, and can cost hundreds of Euros.

If you have a finger-clip device, don’t move or walk around during the measurement. Keep the hand with the pulse oximeter on it completely still. This will give you a true result.

When you put the device on your finger and switch it on, it will show nothing for a while and then quickly change the numbers. This is normal and don’t worry about it! The device needs about 20 seconds to determine the average value. Read the value displayed 20-30 seconds after it is turned on.

Things that affect blood oxygen levels

Artificial nails and nail polish can interfere with the measurement because they can absorb too much light. You get a falsely low reading or no reading at all. Place the device on your finger where there are no false nails or paint.

Cheap pulse oximeters don’t give accurate readings during exercise. So-called motion-tolerant algorithms are provided by clinical (and very expensive) devices. This means that if you walk, talk or gesticulate while the measurement is being taken, the device cannot detect the pulse wave and will not give a reading or the reading will keep changing. As I wrote before, take the measurement in a sitting position, with your arm resting on the table, without moving. 50-60 seconds of measurement is enough at a time. You cannot measure with a finger-tip device during sports or exercise.

Heart disease (cardiomyopathy, heart failure) is caused by poor peripheral circulation, i.e. your heart is so weak that it cannot “push” blood to your finger. The tips of your fingers, your fingernails and even your lips are purple, indicating that fresh blood has not been delivered. Inexpensive pulse oximeters cannot accurately measure low blood flow.

If you have a heart rhythm disorder (atrial fibrillation, absolute arrhythmia), your heartbeat is very irregular, so the strength of successive pulses varies erratically. Sometimes it is very weak or even absent, sometimes it is strong. The cheap machines can’t cope with this either. Either they show no reading at all or they jump around, unintelligible.

Test position changes affect the value. A related medical study has looked at the relationship between body position and blood oxygen levels. They found that the value was higher in a sitting position than in a lying position.  So if your blood oxygen level is 3 to 5 times higher when you are sitting, that is perfectly normal. This is because the blood circulation changes with body position, the pulse wave is flattened when lying down and is stronger when sitting due to gravity. The air intake of the lungs also changes, because in a lying position the lungs are less able to expand, so they can take in less oxygen. Always measure in a sitting position and compare only with the sitting position.

In some cases there may be a difference between the two hand measurements. This could be explained, for example, by a malformation, narrowing or dilatation of the arteries on one side. In general, I recommend that you always measure in the same place. For example, on the middle or index finger of your left hand. Take the measurement in the middle of the finger in the same position.

How the pulse oximeter works simply

The device has a tiny light source and a sensor, placed opposite each other.
The light source emits two beams of light (red and infrared wavelengths). The light passes through your finger. The sensor monitors how much of the light emitted is ‘absorbed’ by the cells and the blood flowing through them. From this, it can determine what percentage of your total oxygen carrying capacity is being used up.
In simple terms, if each oxygen binding site has oxygen attached to it, then oxygen saturation is 100%. If only half of them are carrying oxygen, the device will read 50%.


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