What is Haematocrit?

The bone marrow can put more or fewer red cells into circulation, or the cells can stay exactly as they are while the plasma around them, the liquid they float in, changes in volume. Both move the haematocrit, and the figure alone does not say whether the cells or the plasma changed.

RBC Count gives the number of red cells in the same sample and Haemoglobin the protein they carry. All three are measured against one volume of blood, so a change in plasma alone moves them together, while a change confined to the cells does not.

Iron supply and marrow activity set the cell side: without iron the marrow cannot build new cells, and blood loss or illness changes how many it turns out. Plasma volume sets the other side, and dehydration, pregnancy or a long spell of heat shifts it while the marrow carries on unchanged.

Red cells turn over across roughly 120 days, so their side of the ratio moves slowly. Plasma volume can shift within a day. One reading cannot say which side moved. A second a year later can: a change on the cell side is still present, while a shift in plasma volume has long since passed. This is Do Health's monitoring schedule, not a general recommendation for everyone.