With a test for hemoglobin or as part of a complete blood count (CBC) during a routine health exam or when you have signs and symptoms of anemia (weakness, fatigue) or polycythemia (dizziness, headache); at regular intervals to monitor a disorder that affects RBCs and to evaluate the effectiveness of treatment
A blood sample drawn from a vein or by a fingerstick (children and adults) or heelstick (newborns)
A hematocrit is a test that measures the proportion of your blood that is made up of red blood cells (RBCs). Blood consists of RBCs, white blood cells (WBCs), and platelets suspended in a fluid portion called plasma. The hematocrit is a ratio of the volume of red blood cells to the volume of all these components together, called whole blood. The value is expressed as a percentage or fraction. For example, a hematocrit value of 40% means that there are 40 milliliters of red blood cells in 100 milliliters of whole blood.
The hematocrit is a fairly quick and simple way of evaluating your red blood cells and checking for conditions such as anemia. It is often performed in conjunction with a hemoglobin level and is also one component of the complete blood count (CBC), a group of tests that are often used in the general evaluation of your health.
The hematocrit reflects both the number of red blood cells and their volume (mean corpuscular volume or MCV). If the number and/or size of RBCs decreases, so will the hematocrit and vice versa. In general, the hematocrit will rise when the number of red blood cells increases and the hematocrit will fall to less than normal when there is a decrease in RBCs.
RBCs are produced in the bone marrow and are released into the bloodstream when they are, or nearly are, mature. They typically make up roughly 37% to 49% of the volume of blood. RBCs contain hemoglobin, a protein that binds to oxygen. The primary function of RBCs is to carry oxygen from the lungs to the tissues and organs of the body. They also transport a small portion of carbon dioxide, a byproduct of cell metabolism, from tissues and organs back to the lungs, where it is expelled.
The typical lifespan of an RBC is 120 days and the bone marrow must continually produce new RBCs to replace those that age and degrade or are lost through bleeding. A number of conditions can affect the production of new RBCs by the bone marrow. 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.
In anemia, the body does not have the capacity to deliver enough oxygen to tissues and organs, causing fatigue and weakness. In polycythemia, too many RBCs are produced (resulting in increased hematocrit) and the blood can become thickened, causing sluggish blood flow and related problems.
How is the test used?
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 red blood cells (RBCs).
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
When is it ordered?
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 exam. These tests are often ordered when you have 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
- Paleness (pallor)
- Shortness of breath
- Fast or irregular heartbeat
- Cold hands or feet
Some signs and symptoms of polycythemia include:
- Disturbed vision
- Enlarged spleen
A hematocrit may sometimes be ordered when you have 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 you have been diagnosed with ongoing bleeding problems, anemia, or polycythemia to determine the effectiveness of treatment. It may also be ordered routinely if you are undergoing treatment for cancer that is known to affect the bone marrow.
What does the test result mean?
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 examples of 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 examples of 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 you are 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
- 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
Is there anything else I should know?
Can my hematocrit be tested at home?
No. This test requires instrumentation and trained laboratory personnel. A hematocrit is typically indirectly measured (i.e., calculated from RBC and MCV) by automated hematology analyzers. It can also be directly measured by spinning a blood-filled capillary tube in a centrifuge (so-called spun hematocrit), but this manual method is less commonly used.
What other tests may be done in addition to a hematocrit?
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.
Is anyone more at risk for abnormal hematocrit values?
Women of childbearing age tend to have lower hematocrit levels than men due to loss of iron and blood during menstrual periods and increased need for iron during pregnancy. People who have a chronic illness such as kidney disease, cancer, HIV/AIDS, chronic infection or autoimmune disorder (e.g., rheumatoid arthritis) are at risk for abnormally low hematocrit. Others who are at greater risk of a low hematocrit (anemia) include people with poor nutrition and diets low in iron or vitamins, people who have undergone surgery or people who have been severely injured. Someone who has family members with a genetic cause of anemia such as sickle cell or thalassemia also have a higher risk of having the condition and a higher risk of anemia.