HaematocritHematocrit · Packed cell volume

Haematocrit is the percentage of blood that consists of red blood cells — typically around 40 to 50 per cent in men and 36 to 46 per cent in women. It moves closely with haemoglobin and is measured on any standard full blood count.

It matters in both directions. Low values indicate anaemia and reduced oxygen delivery; high values thicken the blood, raising viscosity and the risk of clotting. That upper risk is why haematocrit is monitored in people on testosterone therapy, which reliably raises it, and why it is a flagged marker in anabolic steroid use.

In practice it is also sensitive to hydration: dehydration concentrates the blood and raises the measured value without any change in red cell mass, which is why a sample taken after a hard session or a sauna can mislead.