Under acute blood loss in trauma patients, the skeletal muscle StO2 measured by NIRS correlates with blood haemoglobin (Hb) and delivery of oxygen, and can easily detect latent stage of haemorrhagic shock . Unexpectedly, chronically anaemic haematology patients with preserved oxygen extraction capability did not show low skeletal StO2 and THb index despite their severe anaemia, which is what would be predicted from normal physiological responses .