Hematocrit

Hematocrit is the fraction of a blood sample's volume occupied by red blood cells. It can be expressed as a dimensionless fraction or as a percentage:For example, $0.45\mathrm{mL}$ of red blood cells in $1.00\mathrm{mL}$ of blood corresponds to $H=0.45$, or 45%. It is a volume fraction, rather than the number of cells per unit volume or the concentration of hemoglobin.

The denominator matters. In an idealized sample, adding $0.50\mathrm{mL}$ of cell-free plasma to that mixture without changing its red-cell volume reduces hematocrit to $0.45/1.50=0.30$. Thus a change in hematocrit does not by itself establish a change in the total number of red blood cells. Interpretation depends on both cell volume and the surrounding fluid.

Local measurements also need a specified sampling context. Capillary tube hematocrit is the instantaneous fraction of a small vessel's internal volume occupied by red blood cells; it need not equal the value in a collected systemic sample. C. Desjardins and B. R. Duling perfused capillaries with heparinase and observed at least a twofold rise in tube hematocrit without a significant change in red-cell velocity. Vehicle and heat-denatured-enzyme controls did not reproduce the response.^[ref]

That experiment links local blood-cell distribution to the endothelial glycocalyx. A local increase can therefore reflect how cells occupy a vessel, without requiring production of new cells during the observation.