Hematocrit is the percentage of red blood cells in a person’s blood. Low red blood cell levels indicate conditions such as anemia. High red blood cell levels could signal polycythemia, which can increase 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 a normal range or not.

Both high and low hematocrit levels can be detrimental to a person’s health, and can result from a variety of conditions and lifestyle factors.

Read on to learn more about what this measure of red blood cell volume means, symptoms of abnormal levels, and what low and high levels might indicate.

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Hematocrit is a measure of the percentage of red blood cells in the body. For example: if a person has 50 milliliters (ml) of red blood cells in 100 ml of blood, their hematocrit level is 50%.

Red blood cells transport oxygen around the body and give blood its characteristic red color. In addition to oxygen, 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.

Having an adequate amount of red blood cells is essential to keep the body’s processes running smoothly.

However, the percentage of red blood cells in someone’s blood can change depending on a variety of lifestyle factors and even environmental changes. According to research, red blood cell counts tend to increase at high altitude.

Exercise, particularly strength training, may also affect hematocrit levels. A 2018 study found that females who participated in 16 weeks of strength exercise had lower levels at the end compared when they started. However, the study had a small sample size of 26 middle-aged, sedentary Turkish women. This means that these findings aren’t necessarily representative of a wider population.

There are many conditions that can affect red blood cell production or their life cycle. This can make it difficult for a medical professional to diagnose these conditions. A doctor will use a hematocrit test to confirm whether a person’s red blood cell count is affecting a health condition.

Doctors usually test hematocrit levels as part of a complete blood count (CBC).

A CBC is composed of a range of tests, and may include:

  • red blood cell count
  • reticulocyte count (young red blood cells)
  • an analysis of hemoglobin levels
  • an analysis of red blood cells, including size and shape
  • white blood cell tests
  • platelet tests

A doctor will also take into consideration a person’s sex, race, and age. It is important to note that certain blood-related conditions, such as sickle-cell anemia, affect particular demographic groups at higher rates.

Learn more about sickle cell anemia in African Americans here.

Dehydration can raise hematocrit levels, so this test is useful if a doctor suspects severe dehydration is the cause of a person’s symptoms.

A doctor may request frequent hematocrit tests to monitor the effect of chemotherapy on person’s bone marrow.

Normal hematocrit levels are:

Newborn babies have high hematocrit levels that gradually decrease as they get older.

If a person has recently received a blood transfusion, it may affect their results. Additionally, pregnant individuals may have lower levels than usual because the body increases its blood volume during pregnancy.

Other factors may push levels into a higher range, such as smoking and chronic obstructive pulmonary disease (COPD).

When a person has low hematocrit levels they tend to present with the following symptoms:

  • pale complexion
  • weakness
  • fatigue
  • low energy
  • trouble breathing
  • irregular heartbeat
  • cold hands or feet

These symptoms also indicate anemia, a condition where hemoglobin levels are lower than normal. Hemoglobin is a protein found in red blood cells that carries oxygen around the body.

Mild anemia is treatable, and it is particularly common in women. Severe anemia could signal a more serious underlying health condition that requires more extensive treatment.

Doctors associate anemia with several health conditions that include:

Nutrient deficiency

A person may lack B12, folate, or iron in their diet.

Learn more about how nutrient deficiency anemia is diagnosed and treated.

Chronic bleeding

This commonly occurs due to digestive tract ulcers, which are sores caused by the bacteria H.pylori or chronic use of anti-inflammatory medications such as ibuprofen, other non-steroidal anti-inflammatory drugs (NSAIDs), and aspirin. Many women also experience excessive blood loss from heavy menstrual bleeding.

Bone marrow disorder

This includes aplastic anemia, which damages stem cells in bone marrow.

Cancer

These are cancers that spread to bone marrow, such as leukemia and lymphoma.

Learn more about bone marrow cancers here.

Kidney failure

Kidney disease can lower the production of red blood cells, reducing hematocrit levels.

Thalassemia

When a person has this condition, their body does not produce enough hemoglobin.

Learn more about thalassemia here.

Sickle cell anemia

This condition changes the shape of red blood cells. These cells die earlier than normal, and they also clump together, which impairs blood flow.

Learn more about sickle cell anemia here.

Autoimmune disease

Conditions like rheumatoid arthritis and lupus may reduce red blood cell count.

Learn more about autoimmune diseases here.

When a person has high hematocrit levels they tend to present with these symptoms:

  • flushed skin
  • dizziness
  • vision problems
  • headaches
  • enlarged spleen

These symptoms 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 tends to focus 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.

In some cases, dehydration causes polycythemia. When a person does not drink enough, their plasma levels drop, and this increases the proportion of red blood cells in their blood volume. A person can lower their red blood cell count by rehydrating.

Some conditions that can cause high hematocrit levels include:

Lung or pulmonary disease

When the lungs cannot absorb oxygen effectively and oxygen levels drop, the body compensates by making more red blood cells. One common pulmonary disease causing this is COPD.

Learn more about COPD here.

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 try and overcome the oxygen deficit the body produces more red blood cells.

Learn more about heart disease here.

Kidney cancer

Sometimes kidney cancer cells create more erythropoietin. Erythropoietin is a hormone that tells the bone marrow to create more red blood cells.

Learn more about kidney cancer here.

Genetic disease

The JAK2 gene, which controls the number of blood cells made in the bone marrow, can affect certain conditions. When someone has a mutated JAK2 gene, the body could make a protein that signals the bone marrow to create more red blood cells than it needs.

Learn more about genetic disorders here.

A person should speak with a doctor if they are experiencing any of the symptoms listed above that could be a sign of high or low hematocrit levels, including fatigue, weakness, vision problems, and dizziness.

These symptoms can also indicate an underlying condition, so it is important that a person contact a doctor in a timely manner to prevent future complications.

If a person is receiving chemotherapy treatment, a doctor should perform regular hematocrit tests to monitor bone marrow health.

Hematocrit is the percentage of blood cells in a person’s blood volume. A doctor may choose to test an individual’s hematocrit level due to certain symptoms.

A low hematocrit level means the are too few red blood cells in the body. In these cases, a person may experience symptoms that signal anemia. Common symptoms include fatigue, weakness, and low energy.

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. For example, males tend to have higher levels than females. Pregnant individuals can experience a decrease in hematocrit levels, and strength training may also reduce levels.

There are also a number of health conditions that can cause hematocrit levels outside the normal range. Excessive bleeding, thalassemia, and kidney disease are causes of low levels. COPD and sickle cell anemia can cause high levels.