Potential causes of a high hematocrit level include dehydration, smoking, living at high altitudes, lung disease, congenital heart disease, polycythemia vera or a erythropoetin-producing tumor in the kidney. Dehydration is the most common cause, and when there is enough fluid intake, the hematocrit returns to normal levels.
Heart and lung disease, as well as high-altitude living, can be causes of insufficient oxygen, and the body tries to compensate by producing more red blood cells. This results in an increase in the hematocrit level. Genetic defects that cause altered oxygen sensing or abnormal release of oxygen from the hemoglobin in red blood cells can also cause high hematocrit levels.
Low hematocrit levels may also be a sign of illness. The hematocrit level is evaluated as part of the complete blood count test to screen for and diagnose anemia. Symptoms of anemia include weakness, lack of energy, fainting and paleness. Other causes of a low hematocrit level include thalassemia, bleeding, chronic kidney disease, bone marrow disorders, chronic inflammatory disease, and nutritional deficiencies, such as iron, folate or vitamin B-12 deficiency. Pregnancy can also decrease hematocrit levels due to the presence of extra fluid in the blood. A recent blood transfusion can affect hematocrit blood test results.