Hematocrit is the percentage of red blood cells in a person‘s blood. A typical hematocrit range in adults is 36–54%. However, age, sex, and health conditions can influence hematocrit levels.
Low red blood cell levels can indicate conditions such as anemia. High red blood cell levels often signals dehydration, but could indicate other conditions, such as polycythemia, increasing a person’s chance of developing a blood clot.
If a person feels tired, dizzy, or short of breath, a doctor may want to test their hematocrit levels to see if those levels fall into the typical range.
This article explains hematocrit levels, including symptoms of abnormal levels and what low or high levels might indicate.
A note about sex and gender
Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.
Hematocrit measures the percentage of red blood cells in the body, relative to the whole blood. For example, if a person has 50 milliliters (ml) of red blood cells in 100 ml of blood, their hematocrit level is 50%.
Whole blood includes plasma, proteins, and other cells, including white blood cells and platelets.
Red blood cells transport oxygen around the body and give blood its characteristic red color.
They also contain hemoglobin, a protein that binds to oxygen molecules. This allows red blood cells to pick up oxygen from the lungs and deliver it throughout the body.
An adequate amount of red blood cells is essential to keep the body’s processes running smoothly.
A doctor
A CBC involves a range of tests and may include:
- red blood cell count
- reticulocyte count, which are young red blood cells
- an analysis of hemoglobin levels
- the mean corpuscular volume — the average size and volume of a red blood cell
- white blood cell tests
- platelet tests
A doctor may request a hematocrit test to monitor a person with suspected blood loss, or for someone undergoing treatment for anemia or polycythemia.
A doctor will also consider a person’s sex, race, and age because certain blood-related conditions, such as sickle-cell anemia, affect particular demographic groups at higher rates.
According to a 2022 research article investigating routine blood test results of adult men in various regions of China, ethnic factors can affect hematocrit levels and require further research and adjustment.
High and low hematocrit levels can be detrimental to a person’s health and result from various conditions and lifestyle factors.
What is a normal hematocrit level?
Different institutions will define typical hematocrit levels differently. However, the following ranges are
| Age and sex | Hematocrit level |
|---|---|
| adult males | 40–54% |
| adult females | 36–48% |
| children | 30–44% |
Newborn babies have high hematocrit levels that slowly decrease over the neonatal period.
If a person has recently received a blood transfusion, it may affect their results. Additionally, pregnant people may have lower levels than usual because the body has more fluid, reducing the percentage of red blood cells.
Other factors, such as smoking and chronic obstructive pulmonary disease (COPD), may push levels into a higher range.
Low levels
Healthcare professionals may consider the following hematocrit levels as low:
- below 40% for adult males
- below 36% for adult females
- below 30% for children
A level under this value
High levels
Healthcare professionals may consider the following hematocrit levels as high:
- above 54% for adult males
- above 48% for adult females
- above 44% for children
Hematocrit levels above the typical ranges can have adverse effects or indicate underlying health conditions.
Accuracy of results
Results from hematocrit tests are typically accurate and provide a clear reading of a person’s red blood cell levels.
However, lifestyle factors and environmental changes can affect the percentage of red blood cells in someone’s blood. For example, red blood cell counts tend to increase at high altitudes.
Dehydration may also raise hematocrit levels, so this test is useful if a doctor suspects severe dehydration is causing a person’s symptoms.
A person with low hematocrit levels may have the following symptoms:
- pale complexion
- weakness
- fatigue or low energy
- trouble breathing
- irregular heartbeat
- cold hands or feet
These symptoms can indicate anemia, a condition where hemoglobin levels are
Causes
Mild anemia is more
Doctors associate anemia with several health conditions, including:
- Nutrient deficiency: A person may lack adequate amounts of vitamin B12, folate, or iron.
- Chronic bleeding: Heavy menstrual bleeding or blood loss due to causes such as digestive tract ulcers can lead to anemia.
- Bone marrow disorder: This includes aplastic anemia, which damages stem cells in bone marrow.
- Cancer: Cancer can develop in or spread to the bone marrow, such as leukemia and lymphoma.
- Kidney failure: Kidney disease can lower the production of red blood cells, reducing hematocrit levels.
- Thalassemia: When a person has thalassemia, their body
does not produce enough hemoglobin. - Sickle cell anemia: This condition changes the shape of red blood cells. These cells die earlier than usual and clump together, impairing blood flow.
- Autoimmune conditions: Conditions such as rheumatoid arthritis and lupus
may reduce red blood cell count.
A person with high hematocrit levels may experience the following symptoms:
- flushed skin
- dizziness
- vision problems
- headaches
- abdominal fullness or bloating, which may indicate an enlarged spleen
These symptoms can signal polycythemia, a condition where the body produces
Doctors cannot cure polycythemia, so treatment focuses on symptom management. The main goal is to avoid stroke and deep vein thrombosis (DVT), which is a blood clot usually occurring in a deep vein in the leg.
Causes
Dehydration increases hematocrit levels, since the liquid portion of the blood is less. When a person does not drink enough, their plasma levels drop, increasing the proportion of red blood cells in their blood volume.
A person can lower their red blood cell count by rehydrating.
Increased hematocrit does not always indicate polycythemia. However, in polycythemia, there is a higher number of red blood cells due to the following conditions:
- Lung or pulmonary disease: When the lungs cannot absorb oxygen effectively, and oxygen levels drop, the body compensates by making more red blood cells. This could occur due to COPD.
- Heart disease: If the structure of a person’s heart reduces its ability to pump blood around the body, it can no longer sustain vital organs with oxygen. To overcome the oxygen deficit, the body may produce more red blood cells.
- Kidney cancer: Sometimes, kidney cancer cells create
more erythropoietin. Erythropoietin is a hormone that tells the bone marrow to create more red blood cells. - Genetic disease: When someone has a mutated JAK2 gene, which controls the number of blood cells made in the bone marrow, the body could make a protein that signals the bone marrow to create more red blood cells than it needs.
A person should speak with a doctor if they experience symptoms of high or low hematocrit levels, including fatigue, weakness, vision problems, and dizziness.
These symptoms can also indicate an underlying condition, so a person must contact a doctor promptly to prevent future complications.
If a person is receiving treatment that affects their red blood cell count, a doctor should perform regular hematocrit tests. This may include chemotherapy and treatment for polycythemia and all types of anemia.
Below are some common questions about hematocrit levels.
What does a hematocrit level tell a person?
A person’s hematocrit level explains the percentage of red blood cells in their blood. High or low levels can indicate underlying health conditions, such as anemia, dehydration, or polycythemia.
What happens if hematocrit is low?
Low hematocrit levels could indicate anemia. This may cause symptoms such as:
- weakness
- fatigue
- cold hands and feet
- difficulty breathing
What does high hematocrit and hemoglobin mean?
High levels of hematocrit and hemoglobin could indicate polycythemia, which could cause symptoms such as:
- dizziness
- flushed skin
- abdominal fullness or bloating
- vision problems
Hematocrit is the percentage of blood cells in a person’s blood volume. A low hematocrit level means there are too few red blood cells in the body. In these cases, a person may experience symptoms that signal anemia.
If a person has too many red blood cells, they have a high hematocrit level. A person may experience dizziness and headaches, which can be a sign of the condition polycythemia.
Demographic and lifestyle factors can influence a person’s hematocrit levels. Certain health conditions can also affect hematocrit levels. Excessive bleeding, thalassemia, and kidney disease may cause low levels, while COPD and sickle cell anemia can cause high levels.