PaO₂ represents the partial pressure of dissolved oxygen in the blood. In the UK PaO₂ is measured in kilopascals (kPa). In normal subjects, the PaO₂ is between 10-13kPa, although this can differ somewhat with age with it being naturally lower in older subjects.

However, in subjects where the PaO₂ is less than 8kPa they are considered to be hypoxaemic (often termed hypoxic). In some subjects, such as patients with chronic lung disease, they may be perfectly able to function with a low PaO₂ level. The cut off of 8kPa is deemed a risk factor and this person may be suitable for long term oxygen therapy.

The arterial oxygen saturation (SaO₂) from the blood sample measures how well saturated the available haemoglobin is with oxygen. The SaO₂ value should correspond closely with the SpO₂ value measured by pulse oximetry. However, you must be aware that some slight differences may occur as the SaO₂ is calculated, whereas the SpO₂ is estimated. In practice the differences between these are negligible typically within around 2%. The normal range for both the SaO₂ and SpO₂ is between 95-100%.