Blood Oxygen (SPO2) Calculator

How is your blood oxygen level (SpO2) measured?

How does a pulse oximeter work?

One of the easiest ways to do this is by using a pulse oximeter¹. This is a small device that pops on to a finger. By shining lights through the nail (a process that you can’t actually feel and doesn’t hurt at all), it measures how much of the Haemoglobin in the blood in your finger has oxygen attached to it. It gives the result as a percentage, with 100% meaning all of the Haemoglobin coming through the area has oxygen attached.

Advantages of using a pulse oximeter

There are lots of advantages to using a pulse oximeter compared to other ways of testing oxygen levels – it isn’t invasive, it doesn’t hurt, you don’t need other equipment to process the reading, and the devices are relatively inexpensive and widely available. For this reason, they are used by clinical staff in all settings both in the hospital and in the community, as well as by many people in their own homes.

Disadvantages of using a pulse oximeter

However, there are potentials for inaccuracies. For instance, we know that the readings of all pulse oximeters around the world are potentially affected by nail varnish, cold fingers and pigmented skin. This is why the NHS advises that, for people of colour using pulse oximeters to monitor a Covid-19 infection, it is important to monitor for a change in your normal reading. This can be significant, even if the number itself is still considered to be in the normal range². And of course, it is helpful to remove nail varnish and warm up any cold hands before using a device.

Pulse oximeter alternatives

While pulse oximeters are still very useful and appropriately the first line in assessing oxygen levels in the vast majority of cases, another option is by taking a blood sample from an artery (usually the wrist is used) and analysing this with a special machine. This is known as an Arterial Blood Gas, or ABG. This can be quite painful and is typically only done in more extreme circumstances, for instance if someone is unwell in the Emergency Department. This is the most accurate way of knowing the exact oxygen levels in the blood and also gives other markers of how unwell someone may be, such as a lactate level.

A third option for measuring oxygen levels is checking a Capillary Blood Gas, or CBG. This is when blood is collected from the ear, often using a pinprick. This can be useful when someone’s oxygen levels are low and increased accuracy is needed compared to using a pulse oximeter. For instance, it is often helpful for people who suffer from chronic lung disease, who may need repeated assessments of oxygen levels.

Causes of low blood oxygen

There are lots of potential causes of a low reading on a pulse oximeter. These include:

• Chest infection
• An infection elsewhere that is damaging the body (sepsis)
• Asthma attack
• Blood clot on the lungs
• Chronic lung disease (e.g., bronchitis or emphysema)
• Other lung abnormality such as accidentally inhaling something, fluid in the lungs (pulmonary oedema), fluid in the membrane around the outside of the lungs (pleural effusion)
• Heart attack or heart failure
• High altitude
• Obstructive sleep apnoea
• Artificially low readings due to cold fingers, nail varnish, incorrect oximeter size for age, etc.

Why else could my blood oxygen (SpO2) level be low?

Other instances where the actual oxygen levels in the blood may be low but a normal reading is showing in a pulse oximeter include:

• If you remember, the pulse oximeter gives a reading as a total percentage of the Haemoglobin that has oxygen attached. Anaemia, or ‘low blood count’, is the medical term for having less Haemoglobin than normal. As an analogy, picture Haemoglobin as an Uber, oxygen as the passenger inside, and the town represents a whole body. Say Town A has 100 Ubers, and 98 of them are carrying passengers. Town B has only 10 Ubers. If all 10 of them are full, their ‘SpO2’ is 100% but they are carrying way fewer people around than Town A’s. It’s the same issue with someone with anaemia – their reading may look great as proportionally lots of their Haemoglobin has oxygen attached, even though the actual amount of oxygen they are carrying may be quite low.
• Carbon monoxide poisoning and heavy smoking can both increase the amount of a type of Haemoglobin in the blood that doesn’t actually carry oxygen, called carboxyhaemoglobin, or COHb. Think of it like an Uber driver who for some reason refuses to take passengers. Due to the mechanics of how all pulse oximeters work, these non-functioning molecules sneak past undetected or even by posing as hard-working ones. This essentially leads to artificially high readings that don’t reflect the true, lower oxygen levels present.

How to increase your blood oxygen level

If your blood oxygen level is newly low, please seek medical attention as appropriate as this is not normal and can be a sign of something serious. Ring your NHS GP or 111 for mild symptoms or attend A&E or ring 999 for life-threatening symptoms.

If you normally have a low blood oxygen level and you’d like to increase it, then really this is done by treating the underlying cause. For someone with sleep apnoea, this would involve using a breathing (CPAP) machine at night. For others, it may mean ensuring good compliance with and optimisation of medication.

The main lifestyle change that can boost oxygen levels is quitting smoking. Even for people with severe lung disease, it is never too late to quit – this can still have a positive effect on your health. By quitting, you not only start to protect the lungs from the toxic chemicals, you also start to get rid of that non-functional Haemoglobin that doesn’t carry oxygen, boosting efficiencies all round.