Hct differs from PCV or packed cell volume in that the latter is a measured value based on a centrifuged aliquot of whole blood, whereas the Hct is a calculated value obtained by multiplying a red blood cell count (RBC) with the mean corpuscular volume (MCV). Hct is generally regarded as more accurate than the slightly higher PCV, since plasma is trapped between cells during centrifugation.
A true baseline Hct value may be hard to measure accurately for a variety of reasons. For example, when drawing a blood sample from a subject, a stress response may cause a contraction of the spleen, resulting in a momentary increase in Hct. Hct may also vary dramatically with physical activity. Average Hct values for humans range from 40 to 48%. Diving animals, and pinnipeds in particular have elevated hematocrits, with some phocid species ranging above 60%.
See also complete blood count (CBC).