The hematocrit, also known as the "Hct", "crit" or PCV (packed cell volume) determines the percentage of red blood cells in the plasma. The term hematocrit means "to separate blood." When the patient's blood sample is spun in a centrifuge, the white blood cells and platelets rise to the top in what is known as the "buffy coat." The heavier red blood cells sink to the bottom, where they can be calculated as a percentage of the total blood sample.
Normal hematocrit values are:
If the RBC and the hemoglobin are both normal, it is possible to estimate the hematocrit as being approximately three times the hemoglobin. For example, a person whose hematocrit is 30% would have a hemoglobin of approximately 10 gm.
Because the hematocrit is a percentage of red blood cells, as compared to the total volume of blood, any increase or decrease in plasma volume affects the hematocrit. A patient with a severe burn loses huge amounts of plasma from damaged capillaries. As a result, the vascular space loses fluid in relation to cellular elements and the patient's hematocrit will be significantly increased. A decreased hematocrit occurs due to overhydration, which lowers the percentage of red blood cells in relation to the liquid plasma portion of blood.
A hematocrit is frequently done to assess the extent of significant blood loss. A hematocrit that is done immediately after a hemorrhage usually does not show the extent of RBC loss because at the time of the hemorrhage, plasma and red blood cells are lost in equal proportions. However, within several hours after hemorrhage, plasma volume increases due to a shift of interstitial fluid into the vascular space. Red blood cells, however, cannot be replaced quickly, as the bone marrow takes approximately ten days to produce mature red blood cells. As a result, a hematocrit done several hours after a bleeding episode will show a more accurate picture - the hematocrit will be decreased because the plasma volume has compensated for fluid loss while the red blood cells that have been lost cannot be replaced for days. It is important for the nurse to remember that a hematocrit value must always be interpreted in relation to the patient's hydration status and to the time the sample was drawn. When packed red blood cells are given to correct an anemia, the hematocrit should rise approximately 3% for each unit transfused.
How a patient responds to a low hematocrit depends on whether the blood loss is acute or chronic. If a person with a normal blood cell volume loses blood suddenly through a massive hemorrhage, the person may develop signs and symptoms of circulatory shock; the blood pressure will fall and the patient will show signs of tissue hypoxia and shock. However, a person with chronically low hemoglobin, such as a patient on renal dialysis, may tolerate abnormally low hemoglobin. A major reason for anemia in patients with renal failure is lack of erythropoetin to stimulate bone marrow production.
Hematocrit: critical low and high values: