Methaemoglobin

Methaemoglobin is an abnormal type of haemoglobin that does not bind oxygen well. Normally 1-2% of people's haemoglobin is methaemoglobin, a higher percentage than this can be genetic or caused by exposure to various chemicals and depending on the level can cause health problems. A higher level of methaemoglobin will tend to cause a pulse oximeter to read closer to 85% regardless of the true level of oxygen saturation. For more information see:

Moyle JTB (2002) Pulse Oximetry 2nd ed. BMJ books, London