Hematocrit

Hematocrit is the proportion of blood volume occupied by red blood cells. Expressed as a percentage, it is a standard component of the complete blood count and reflects the concentration of circulating erythrocytes relative to plasma.

Hematocrit is most commonly measured by centrifuging whole blood in a capillary tube to separate plasma, after which the ratio of packed red cell volume to total sample volume is read. Automated hematology analyzers estimate hematocrit from red cell measurements and report it as a percentage of blood volume.

Normal values vary by age and sex. In healthy adults, typical reference ranges are about 41–53% for men and 36–46% for women. Children have different ranges that change with age, and newborns typically have higher hematocrit values that decline during infancy.

Interpretation: a low hematocrit indicates reduced red cell mass or increased plasma volume (as in anemia or overhydration), whereas a high hematocrit suggests increased red cell mass or decreased plasma volume (as in dehydration, living at high altitude, or polycythemia).

Hematocrit is influenced by hydration status and sample handling. It should be interpreted with hemoglobin concentration and red cell indices for a complete assessment, because changes in plasma volume can affect hematocrit independently of red cell production. Differences between measurement methods can also occur.