What Is the Hematocrit Test?

Medically Reviewed by Jabeen Begum, MD on November 09, 2022
5 min read

You need blood to carry oxygen through your body, remove carbon dioxide, and fight infection. To do this, your blood has different types of blood cells that do different jobs. If the levels of any of these blood cells are off, it can cause a host of problems for your body. A hematocrit test tells your doctor if your levels of red blood cells are off.

A hematocrit test (HCT test), sometimes called a packed cell volume test (PCV test), is a test that measures your hematocrit. Hematocrit is the level of red blood cells within your blood.

Your blood is made up of plasma and blood cells. Plasma is the liquid part of your blood and is made of protein, salts, and water. Over half your blood composition is plasma.

Suspended in that plasma are three types of blood cells:

  • Platelets, also called thrombocytes, are small fragments that help your blood form clots so you stop bleeding.
  • Red blood cells are the most abundant type of blood cell in your blood. They’re round with a slightly indented center and contain a protein called hemoglobin. Hemoglobin carries oxygen and removes carbon dioxide.
  • White blood cells are part of your immune system. These cells are round with a distinct center. Their job is to fight off unknown invaders by producing antibodies.

Hematocrit tests measure how much of your blood is red blood cells and determine if that amount is normal.

Hematocrit tests are sometimes done as part of routine blood tests, but your provider may order them if they suspect you have a red blood cell disorder. Your red blood cell levels may also be checked before you have surgery. 

A red blood cell count outside the normal range can mean you have a disease or disorder. A low number of red blood cells is called anemia, while having too many red blood cells is called polycythemia.

Anemia. When you’re anemic, you don’t have enough red blood cells to carry oxygen to your tissues. 

There are several different types of anemia. The most common type is iron deficiency anemia. Your bone marrow produces blood cells, and it needs iron to make hemoglobin. If you’re lacking iron, your body can’t produce enough hemoglobin for your red blood cells. This type of anemia is common in pregnant women and people who have experienced blood loss.

Other types of anemia include:

  • Anemias associated with bone marrow disease. Diseases, such as cancers like leukemia and myelofibrosis, can affect your bone marrow and may cause your bone marrow to underproduce blood cells. 
  • Anemia of inflammation. Some types of inflammatory diseases, including cancer, Crohn’s disease, HIV/AIDS, kidney disease, and rheumatoid arthritis, can interfere with your body’s production of red blood cells.
  • Aplastic anemia.Aplastic anemia is a rare but life-threatening type of anemia in which your body doesn’t produce enough red blood cells. It can be caused by autoimmune diseases, infections, medicines, and toxins.
  • Hemolytic anemias. Hemolytic anemias happen when your red blood cells are destroyed faster than your body can replace them. You can be born with inherited hemolytic anemia, or you can acquire hemolytic anemia from some autoimmune disorders, cancers, infections, medicines, tumors, and viruses. Hemolytic anemia can also be acquired from an overactive spleen, a reaction to a blood transfusion, or a mechanical heart valve.
  • Sickle cell anemia.Sickle cell anemia is a genetic condition in which the red blood cells form a crescent, or sickle, shape due to defective hemoglobin. These oddly shaped red blood cells die quickly, causing your body to always be lacking enough red blood cells.
  • Vitamin deficiency anemia (pernicious anemia). Your body also needs vitamins, especially vitamin B12 and folate (B9), to produce healthy red blood cells. A lack of these vitamins, or inability to absorb B12, can cause your body to be unable to produce the red blood cells you need.

Symptoms of anemia may include:

  • Chest pain
  • Cold hands and feet
  • Dizziness or lightheadedness
  • Fatigue
  • Headaches
  • Irregular heartbeat
  • Pale or yellowish skin
  • Shortness of breath
  • Weakness

Polycythaemia. Polycythaemia, also called erythrocytosis, is caused by too many red blood cells, which can cause your blood to become thicker and travel slower through your blood vessels and organs. There are two types of absolute polycythemia, which means polycythemia caused by the overproduction of red blood cells: polycythemia vera (PV) and secondary polycythemia. 

Polycythemia vera is a rare, slow-progressing bone marrow cancer. Most of the time it’s caused by a mutation on the JAK2 gene, a gene within the stem cells of bone marrow. This mutated gene tells your stem cells to reproduce constantly, The new cells also reproduce and, because they have this damaged gene, continue to reproduce until the abnormal cells overwhelm the normal cells. Most people are not born with this mutation; it happens sometime during your life, although doctors don’t know what causes it.

Secondary polycythemia happens when your body produces too much erythropoietin, a hormone made by the kidneys that tells your bone marrow to produce red blood cells. This can be caused by a problem with the kidneys, such as a tumor. It can also be caused by conditions that prevent your body from getting oxygen, like chronic obstructive pulmonary disease (COPD) and sleep apnea.

There are also conditions that cause your hematocrit level to rise because of too little plasma. These include apparent polycythemia and relative polycythemia. Apparent polycythemia is a lack of plasma that can be caused by drinking too much alcohol, smoking, and taking certain medications. Relative polycythemia happens when dehydration causes a lack of plasma.

Symptoms of polycythemia may include:

  • Bleeding problems like excessive bruising or bloody noses
  • Blurry vision
  • Confusion
  • Dizziness
  • Gout
  • Headaches
  • High blood pressure
  • Itchy skin
  • Red skin
  • Stomach discomfort
  • Tiredness

Both anemia and polycythemia can become life-threatening.

The hematocrit test is done like any other blood test. The lab will likely draw blood from a vein in your arm or hand. To determine your hematocrit, the lab spins your blood sample at a high speed. This causes the plasma and blood cells to separate, and the heavier red blood cells sink to the bottom of the tube. The amount is measured to calculate what percentage of your blood is red blood cells.

Normal ranges of hematocrit can vary depending on your age, race, and sex. Generally, normal ranges are considered to be:

  • For adult men, 38.3%-48.6%
  • For adult women, 35.5%-44.9%
  • For newborns, 45%-61%
  • For infants, 32%-42%
  • For kids, 30%-44%

Some factors can cause abnormal hematocrit results despite no underlying disease. These include: 

  • Living at a high altitude
  • Pregnancy
  • Recent blood transfusion
  • Severe dehydration
  • Severe recent blood loss

If your results come back and your hematocrit is abnormal, your doctor may order further testing to find the cause.

Show Sources

American Red Cross Blood Services: “Platelets and Thrombocytopenia.”
Cleveland Clinic: “Function of White Blood Cells,” “Hematocrit,” “Polycythemia Vera.”
Johns Hopkins Medicine: “Hemolytic Anemia.”
Mayo Clinic: “Anemia,” “Hematocrit test.”
MedlinePlus: “Blood.”
National Health Services: “Polycythaemia.”
OneBlood: “The science behind separating blood and platelets.”
Stanford Health: “Hematocrit.”
University of Rochester Medical Center: “What Are Red Blood Cells?”

View privacy policy, copyright and trust info