The commonest way to measure the level of oxygen in a person’s blood is by using a pulse oximeter; a clip that is usually placed on a fingertip and provides a value where 96-100% is considered normal.

An increasing number of studies have shown that for people with darker (pigmented) skin, pulse oximeters may over-estimate their true oxygen level. This is because the pulse oximeter shines light through the fingertip and skin tone may affect the device’s accuracy. Over-estimation of oxygen levels could under-estimate the seriousness of a person’s illness, leading to them not receiving the correct treatment and come to unnecessary harm.

This study aims to find out whether skin tone affects the accuracy of pulse oximeters and confirm whether they over-estimate the level of oxygen in people with darker skin.


EXAKT will be running as part of another larger study (called UK-ROX) that was designed to look at how oxygen levels affect patients on breathing machines in intensive care units (ICUs). Patients eligible to be enrolled in this study will be offered the opportunity to participate in the EXAKT study.

We will study several different pulse oximeters used by the NHS and for each patient, two of these will be placed on their fingers and the oxygen level will be compared to the oxygen level in a routine blood sample taken at the same time. These blood samples (‘arterial blood gas samples’) are a normal part of caring for patients in ICU and are taken several times each day. The patient’s skin tone will be measured using a special device (called a handheld spectrophotometer) that accurately measures colour. We plan to enrol approximately 900 patients to this study from 24 ICUs participating in the UK-ROX trial over 12 months.

This evaluation will have a large and immediate impact on clinical practice and on patient outcomes in the NHS and worldwide.

Key information

Chief investigator(s)

Professor Daniel Martin OBE, University of Plymouth

Mr Paul Mouncey, ICNARC


Intensive Care National Audit & Research Centre (ICNARC)


National Institute for Health and Care Research (NIHR) – Health Technology Assessment (HTA) Programme

(Project Number: NIHR135577)

Trial registration(s)

SWAT Registration: 183

Contact information

Trial manager: Tasnin Shahid


Phone: 020 4513 6238

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