Hematocrit Levels: Definition, Low Levels, High Levels, And More

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Medical News TodaySubscribeWhat is a normal hematocrit level?

Medically reviewed by Angelica Balingit, MD — Written by Lauren Martin — Updated on December 21, 2023

What is it?
Testing
Test results
Low hematocrit
High hematocrit
Contacting a doctor
FAQ
Summary

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. Learn more.

What is hematocrit?

blood samples in test tubes to check hematocrit level — WLADIMIR BULGAR/SCIENCE PHOTO LIBRARY/Getty Images

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.

Why do doctors test hematocrit levels?

A doctor can use a hematocrit test to diagnose certain health conditions or monitor aspects of treatment. Doctors may test hematocrit levels as part of a complete blood count (CBC).

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.

Understanding test results

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 typical for certain ages:

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 can signify chronic anemia.

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.

Low hematocrit levels

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 low. Hemoglobin is a protein found in red blood cells that carries oxygen around the body.

Causes

Mild anemia is more common in females, pregnant people, and older adults. Severe anemia could signal a more serious underlying health condition that requires more extensive treatment.

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.

High hematocrit levels

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 too many red blood cells. This means blood is thicker and clots more easily.

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.

Contacting a doctor

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.

Frequently asked questions

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

Summary

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.

Read this article in Spanish.

Genetics
Heart Disease
Blood / Hematology
Urology / Nephrology

How we reviewed this article:

SourcesMedical News Today has strict sourcing guidelines and relies on peer-reviewed studies, academic research institutions, and medical journals and associations. We only use quality, credible sources to ensure content accuracy and integrity. You can learn more about how we ensure our content is accurate and current by reading our editorial policy.

Anemia of inflammation or chronic disease. (2018).https://www.niddk.nih.gov/health-information/blood-diseases/anemia-inflammation-chronic-disease
Badireddy M, et al. (2023). Chronic anemia.https://www.ncbi.nlm.nih.gov/books/NBK534803/
Cicek G. (2018). The effects of different exercise types on hematological parameters in sedentary women.https://www.researchgate.net/publication/325856293_The_Effects_of_Different_Exercise_Types_on_Hematological_Parameters_in_Sedentary_Women
Guo Y, et al. (2022). Blood routine reference value range should be adjusted according to regional and ethnic characteristics.https://www.frontiersin.org/articles/10.3389/fpubh.2022.934101/full
Hematocrit. (n.d.).https://www.stanfordchildrens.org/en/topic/default?id=hematocrit-167-hematocrit
Mondal H, et al. (2023). Hematocrit.https://www.ncbi.nlm.nih.gov/books/NBK542276/
Pillai AA, et al. (2023). Polycythemia.https://www.ncbi.nlm.nih.gov/books/NBK526081/
Polycythemia vera. (2022).https://www.nhlbi.nih.gov/health/polycythemia-vera
Tests for kidney cancer. (2020).https://www.cancer.org/cancer/types/kidney-cancer/detection-diagnosis-staging/how-diagnosed.html
Turner J, et al. (2023). Anemia.https://www.ncbi.nlm.nih.gov/books/NBK499994/
What is thalassemia? (2023).https://www.cdc.gov/ncbddd/thalassemia/facts.html

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Medically reviewed by Angelica Balingit, MD — Written by Lauren Martin — Updated on December 21, 2023

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A note about sex and gender

What is hematocrit?

Why do doctors test hematocrit levels?

Understanding test results

What is a normal hematocrit level?

Low levels

High levels

Accuracy of results

Low hematocrit levels

Causes

High hematocrit levels

Causes

Contacting a doctor

Frequently asked questions

What does a hematocrit level tell a person?

What happens if hematocrit is low?

What does high hematocrit and hemoglobin mean?

Summary

How we reviewed this article:

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