The hematocrit (HCT), or packed cell volume (PCV) represents the proportion of blood composed of red blood cells, expressed as % (vol/vol). It is the quickest and most accurate measure of the red cell component of blood. Traditionally, it is determined by measuring the height of the red cell column in a microhematocrit tube following centrifugation (see fig. at right). Automated analyzers (such as the Advia) calculate the HCT by multiplying the red cell count and the mean red cell volume, both of which are measured directly by the machine.

Examination of the "crit tube" can also provide subjective information about the color and clarity of the plasma (icterus, hemolysis, lipemia), and the size of the "buffy coat" (which contains WBC and platelets).

Additionally, one can score and break the tube as desired to remove the plasma for refractometric protein estimation, or to extrude the buffy coat for smear-making. The "buffy coat smear" has the advantage of providing a concentrated preparation of nucleated cells, which can be useful if looking for low-incidence cell-types of potential significance (e.g., mast cells).

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