Erythrocyte volume fraction (EVF), also known as hematocrit, is a measure of the fraction or portion of red blood cells in your blood. EVF is presented as a percentage of the total blood volume.
A normal EVF indicates that you have a balanced level of red blood cells in your blood, high enough to provide enough oxygen to all tissues in your body, and low enough to avoid blood clots.
Measuring EVF can give you valuable information on the blood composition. This can help diagnose conditions that change the amount of red blood cells.
EVF is also measured to monitor effects of medical treatments that might affect the production or survival of red blood cells.
A low level of hematocrit means that you have a lower than normal percentage of red blood cells in your blood. This condition is also known as anemia and can be caused by a variety of factors. Nutritional deficiencies of iron, folate or vitamin B12 are common causes of anemia.
Other underlying causes include bone marrow disorders or kidney disease resulting in low red blood cell production.
A high level of hematocrit, also known as polycythemia, means that you have a higher than normal percentage of red blood cells in your blood. Having high levels of hematocrit increases the risk of blood clots which may cause severe events like stroke and heart attacks.
The underlying causes behind polycythemia can either derive from an increase in red blood cells - found in conditions like polycythemia vera or living at high altitudes, or a decrease in blood volume - common when you get dehydrated due to a more concentrated blood. Smoking and testosterone use have also been shown to have the ability to increase hematocrit levels.