Britannica Sources used in this answer

Why can an unusual hematocrit level be an early sign of disease?

Created from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles.

Blood plasma — blood Blood sediment in test tubes, showing plasma (clear fluid), red blood cells (red), and the release of hemoglobin into the surrounding plasma (pink).

It is important to clarify that while the term "hematocrit" is sometimes colloquially confused with the erythrocyte sedimentation rate (ESR), they are distinct diagnostic procedures. The hematocrit specifically measures the percentage of total blood volume occupied by packed red blood cells, whereas the sedimentation rate measures how quickly cells settle in a tube.1

An unusual hematocrit level serves as an early indicator of disease because it reflects direct abnormalities in the composition of your blood. Because the hematocrit represents the volume of red blood cells, deviations from the normal range provide immediate evidence of underlying physiological stress:12

Low levels: A thin layer of red blood cells often indicates iron-deficiency anemia or other forms of anemia, where the body lacks sufficient healthy red cells to carry oxygen.12
High levels: An abnormally thick layer of red blood cells is a hallmark of polycythemia, a condition characterized by an excess of red cells.1

By quantifying these components, the hematocrit—often part of a complete blood count—allows clinicians to detect these imbalances before more severe symptoms manifest.12