The hematocrit (Ht) or packed cell volume (PCV) is the proportion of blood that is occupied by red blood cells. It is normally between 0.35 and 0.52, and slightly higher on average in males.

Packed cell volume diagram

The packed cell volume can be determined by centrifuging the blood in a capillary tube (a.k.a. a hematocrit tube), which forces the cells to one end. The length of the tube containing blood cells, divided by the length containing cells or plasma gives the PCV.

While usually used synonymously, the hematocrit is a value that is calculated by an automated analyser, and not directly measured. It is determined by multiplying the red cell count by the mean cell volume. The hematocrit is slightly more accurate as the PCV includes small amounts of plasma trapped between the red cells.

The hematocrit is not usually considered an important consideration when looking at a person's full blood count results as most people are more concerned with the hemoglobin concentration and mean cell volume. However, the hematocrit gives an indication of how concentrated the red cells are in the blood - this gives an indirect impression of whether the patient is dehydrated and also how relevant the measured hemoglobin level is.

For example, there have been cases where the blood for testing taken was inadvertently taken from the same arm as arm with the intravenous line running in a transfusion of packed red cells. In this sample, the hemoglobin measurement will be high because it is measuring the fluid being transfused (i.e. mostly red cells) rather than the diluted serum. In this case, the hematocrit measurement will be abnormally very high, out of proportion to the hemoglobin measurement.

In cases of dengue, where the full blood count is done daily, the hematocrit is monitored as a high hematocrit is a danger sign of an increased risk of dengue shock syndrome .

In mammals, hematocrit is independent of body size.