Another device that has become more common for home use is the pulse oximeter to measure blood oxygen levels.

These monitors estimate oxygen saturation of haemoglobin in arterial blood by measuring the absorption of red and infrared light passing through skin. The reading is done typically with the monitor clipped to a finger, although devices can also be designed for use on ear lobes, the nose or forehead.¹⁷

Readings rely on the significant difference with oxyhaemoglobin (oxygenated haemoglobin) absorbing more IR light and less red light than deoxyheamoglobin; the difference in levels of red and IR light are much more affected than other colours in the spectrum.

Factors which can affect the reading include:^17,18

• skin tone – darker skin may overestimate oxygen saturation
• movement
• poor blood perfusion through the tissue (eg due to cold temperature, the patient being hypotensive, vasoconstriction, eg with catecholamine drugs, low blood volume, cardiac failure, peripheral vascular disease)
• positioning of the device
• conditions affecting the iron ion in the haemoglobin (methaemoglobinaemia or sulfhaemoglobinamia)
• chemical pigments
• sepsis and septic shock
• ambient light hitting the sensor.

Factors which may give a false normal or elevated level of oxygen saturation include carbon monoxide poisoning or sickle cell anaemia.¹⁷

False low readings may arise from factors such as venous pulsations, inherited forms of abnormal haemoglobin, severe anaemia, excessive movement, nail varnish, henna, tattoos, or intravenous pigmented dyes.

The MHRA advises that the relative change in a patient’s reading may be of greater significance to clinical management than the absolute value.¹⁸