Hematocrit & red blood cells
HEMATOCRIT (Hct)
The hematocrit is the ratio of the volume of red blood cells to the total volume of blood. The hematocrit increases when red blood cells or plasma volume decreases. The hematocrit value is useful when assessing the body's fluid balance, infections, nutritional deficiencies or anemia.
The following formula is used to determine the hematocrit:
The volume of blood = Volume of plasma / 1 - hematocrit
Reference values:
- Men 38–50 %
- Women 34–46 %
RED BLOOD CELLS (RBC)
Red blood cells (erythrocytes) contain hemoglobin and use it to carry oxygen from the lungs to various tissues. The number of red blood cells may increase (a condition known as erythrocytosis) due to dehydration, stress, smoking or living at high altitudes (for example, in the mountains). It may decrease (erythrocytopenia) due to anemia or another illness. Red blood cells make up almost one-half (40–45 %) of the total blood volume. Usually, the total volume (mass) of red blood cells in the body is stable. In contrast, the hemoglobin volume (Hb), red blood cell count (RBC) and hematocrit level (Hct) in the blood fluctuate depending on the time of day or due to several other factors. Fluctuation causing an increase in the hemoglobin volume occurs especially in acute situations such as dehydration, burns or long-term physical strain.
The increase in red blood cells (erythrocytosis) may be relative or absolute. Relative erythrocytosis does not increase the body's total volume of red blood cells. Absolute erythrocytosis does. Erythrocytosis can thus be primary or secondary. Primary erythrocytosis involves the production of red blood cells independently of the hormone erythropoietin (a hormone that stimulates the production of red blood cells). Secondary erythrocytosis, however, involves increased erythropoietin production or an altered response to it.
Relative erythrocytosis is often associated with significant obesity, hypertension, smoking or excessive alcohol use.
Investigating erythrocytosis usually requires further testing and assessment by a hematologist (a physician specializing in blood diseases).
Reference values:
- Men 4.2–5.7 xE12/l
- Women 3.85–5.2 xE12/l