The hematocrit test is often used to check for anemia, usually along with a hemoglobin test or as part of a complete blood count (CBC). The test may be used to screen for, diagnose, or monitor a number of conditions and diseases that affect the proportion of the blood made up of red blood cells (RBCs). RBCs circulate in the blood and carry oxygen throughout the body. (For more, see the "What is being tested?" section.)

A hematocrit may be used to:

• Identify and evaluate the severity of anemia (low RBCs, low hemoglobin, low hematocrit) or polycythemia (high RBCs, high hemoglobin, high hematocrit)
• Monitor the response to treatment of anemia or polycythemia and other disorders that affect RBC production or lifespan
• Help make decisions about blood transfusions or other treatments if anemia is severe
• Evaluate dehydration

Some conditions affect RBC production in the bone marrow and may cause an increase or decrease in the number of mature RBCs released into the blood circulation. Other conditions may affect the lifespan of RBCs in the circulation. If there is increased destruction of RBCs (hemolysis) or loss of RBCs (bleeding) and/or the bone marrow is not able to produce new ones fast enough, then the overall number of RBCs and hematocrit will drop, resulting in anemia.

The hematocrit can indicate if there is a problem with RBCs, but it cannot determine the underlying cause. In addition to the full CBC, some other tests that may be performed at the same time or as follow up to establish a cause include a blood smear, reticulocyte count, iron studies, vitamin B12 and folate levels, and in more severe conditions, a bone marrow examination.

The hematocrit is routinely ordered as a part of the complete blood count (CBC). It may also be ordered by itself or with a hemoglobin level as part of a general health examination. These tests are often ordered when a person has signs and symptoms of a condition affecting RBCs, such as anemia and polycythemia.

Some signs and symptoms of anemia include:

• Weakness or fatigue
• Lack of energy
• Fainting
• Paleness (pallor)
• Shortness of breath

Some signs and symptoms of polycythemia include:

• Disturbed vision
• Dizziness
• Headache
• Flushing
• Enlarged spleen

A hematocrit may sometimes be ordered when someone has signs and symptoms of serious dehydration, such as extreme thirst, dry mouth or mucous membranes, and lack of sweating or urination.

This test may be performed several times or on a regular basis when someone has been diagnosed with ongoing bleeding problems, anemia, or polycythemia to determine the effectiveness of treatment. It may also be ordered routinely for people undergoing treatment for cancer that is known to affect the bone marrow.

Red blood cells (RBCs) typically make up roughly 37% to 49% of the volume of blood.

Since a hematocrit is often performed as part of a complete blood count (CBC), results from other components, such as RBC count, hemoglobin, reticulocyte count, and/or red blood cell indices, are taken into consideration. Age, sex, and race are other factors to be considered. In general, the hematocrit mirrors the results of the RBC count and hemoglobin.

A low hematocrit with low RBC count and low hemoglobin indicates anemia. Some causes include:

• Excessive loss of blood from, for example, severe trauma, or chronic bleeding from sites such as the digestive tract (e.g., ulcers, polyps, colon cancer), the bladder or uterus (in women, heavy menstrual bleeding, for example)
• Nutritional deficiencies such as iron, folate or B12 deficiency
• Damage to the bone marrow from, for example, a toxin, radiation or chemotherapy, infection or drugs
• Bone marrow disorders such as aplastic anemia, myelodysplastic syndrome, or cancers such as leukemia, lymphoma, multiple myeloma, or other cancers that spread to the marrow
• Kidney failure—severe and chronic kidney diseases lead to decreased production of erythropoietin, a hormone produced by the kidneys that stimulates RBC production by the bone marrow.
• Chronic inflammatory diseases or conditions
• Decreased production of hemoglobin (e.g., thalassemia)
• Excessive destruction of red blood cells, for example, hemolytic anemia caused by autoimmunity or defects in the red blood cell itself; the defects could be hemoglobinopathy (e.g., sickle cell anemia), abnormalities in the RBC membrane (e.g., hereditary spherocytosis) or RBC enzyme (e.g., G6PD deficiency)

A high hematocrit with a high RBC count and high hemoglobin indicates polycythemia. Some causes of a high hematocrit include:

• Dehydration—this is the most common cause of a high hematocrit. As the volume of fluid in the blood drops, the RBCs per volume of fluid artificially rises; with adequate fluid intake, the hematocrit returns to normal.
• Lung (pulmonary) disease—if someone is unable to breathe in and absorb sufficient oxygen, the body tries to compensate by producing more red blood cells.
• Congenital heart disease—in some forms, there is an abnormal connection between the two sides of the heart, leading to reduced oxygen levels in the blood. The body tries to compensate by producing more red blood cells.
• Kidney tumor that produces excess erythropoietin
• Smoking
• Living at high altitudes (a compensation for decreased oxygen in the air)
• Genetic causes (altered oxygen sensing, abnormality in hemoglobin oxygen release)
• Polycythemia vera—a rare disease in which the body produces excess RBCs inappropriately

A recent blood transfusion will affect hematocrit results.

Pregnancy usually causes slightly decreased hematocrit values due to extra fluid in the blood.