Haematocrit (HCT) is the percentage of total blood volume occupied by red blood cells. It is a fundamental component of the full blood count and directly reflects the oxygen-carrying capacity of the blood. Haematocrit is closely related to haemoglobin concentration and red blood cell count.

Low haematocrit indicates anaemia — the red blood cells are diluted either because too few are being produced, they are being destroyed too rapidly, or blood has been lost. High haematocrit can indicate polycythaemia (excess red blood cell production), dehydration, or chronic hypoxia (as seen at high altitude or in chronic lung disease), where the body produces more red blood cells to compensate for low oxygen.