Cyanosis is the abnormal blue discoloration of the skin and mucous membranes, caused by an increase in the deoxygenated haemoglobin level to above 5 g/dL. Patients with anaemia do not develop cyanosis until the oxygen saturation (SaO2) has fallen to lower levels than for patients with normal haemoglobin levels and patients with polycythaemia develop cyanosis at higher oxygen saturation levels.
This is a companion discussion topic for the original article at https://patient.info/doctor/history-examination/cyanosis