Article: Hematocrit

The hematocrit (Ht or HCT) and packed cell volume (PCV) are measures of the proportion of blood volume that is occupied by red blood cells. It is normally between 0.42 and 0.52 for males and between 0.36 and 0.48 for women[1].

Packed cell volume diagram

The packed cell volume can be determined by centrifuging. Heparinized blood in a capillary tube (also known as a microhematocrit tube) is typically centrifuged at 10,000 RPM for five minutes.[2] This separates the blood into layers. The volume of packed red blood cells, divided by the total volume of the blood sample gives the PCV. Because a tube is used this can be calculated by measuring the lengths of the layers.

With modern lab equipment the hematocrit 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. An estimated hematocrit as a percentage may be derived by multiplying the hemoglobin level in g/dL times three and dropping the units. [3].

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.

There have been cases where the blood for testing was inadvertently drawn from the same 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 (that is, mostly red cells) rather than the diluted serum. In this case, the hematocrit measurement will be artificially very high.

In mammals, hematocrit is independent of body size.

Elevated hematocrit

In cases of dengue fever, 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.

Polycythemia vera is associated with elevated hematocrit.

Chronic obstructive pulmonary disease (and other pulmonary conditions associate with hypoxia) may elicit an increased production of red blood cells. This increase is mediated by the increased levels of erythropoyetin by the kidneys in response to hypoxia.

Lowered hematocrit

Lowered hematocrit can imply significant hemorrhage (for example, in an ectopic pregnancy.)

See also

• Hematology
• Blood tests