Overview

What is a hematocrit test?

A hematocrit test (Hct) is a simple blood test that measures the percentage of red blood cells in your blood. Red blood cells are important because they carry oxygen throughout your body. Test results showing low or high hematocrit levels may be signs of blood disorders or other medical conditions.

When do healthcare providers order hematocrit tests?

Hematocrits are parts of complete blood counts (CBC). You may have a CBC during an annual physical or if you have symptoms of conditions linked to low or high hematocrit levels.

What’s the difference between a hematocrit and a hemoglobin test?

A hematocrit is the percentage of red blood cells in your blood. Hemoglobin is part of your red blood cell. Hemoglobin helps your red blood cells carry oxygen throughout your body. Hemoglobin also gives your red blood cells their color. A hemoglobin test measures the amount of hemoglobin in your red blood cells.

Test Details

How do healthcare providers obtain blood for hematocrit levels?

Blood tests to check hematocrit levels are simple, safe procedures with very few complications. Sometimes, providers obtain blood from a vein, using a needle to fill one or more sample tubes. Other times, providers use a needle prick to obtain a drop of blood. Here are blood test details:

Your provider looks for a vein from which to draw blood. They typically pick a vein on the inside surface of your elbow.
They place a tourniquet on your upper arm. Pressure from the tourniquet helps fill veins below the tourniquet, making it easier for your provider to see or feel your veins.
Your provider cleans the area around the vein selected for the blood draw.
Next, they insert a needle into your vein. This may sting at first.
Your blood moves through the needle and into a collecting tube or syringe.
When your provider has collected enough blood to be tested, they’ll release the tourniquet and remove the needle.
Your provider will put gentle pressure on the site where they placed the needle. They do that to prevent bleeding.
They’ll place a small bandage on the spot where they placed the needle.

Do I need to prepare for this blood test?

No, this test doesn’t require special preparation such as fasting.

Are there any side effects or complications?

Most people feel fine after their blood test. Some people may feel faint. If that happens, tell your healthcare provider. They’ll take steps to help you to feel better.

Results and Follow-Up

What’s the normal range for hematocrit levels?

Here are normal hematocrit levels by sex and age:

Adult males: 41% to 50%
Adult females: 36% to 44%.
Infants: 32% to 42%
Newborns: 45% to 61%

What does it mean if your hematocrit is high?

A high hematocrit level may be a sign of several conditions, including polycythemia vera. Your healthcare provider is your best resource for information about your test result. Some conditions that may cause high hematocrit levels include:

Heart disease: Studies show high hematocrit levels increase your risk of developing cardiovascular disease.
Dehydration: Dehydration lowers all water content in your body, including plasma in your blood. Lower plasma levels increase the ratio of red blood cells to blood volume. That increases hematocrit levels.
Scarring or thickening of your lungs: Scarring in your lungs makes it difficult for your red blood cells to carry oxygen throughout your body.
Obstructive sleep apnea: Studies show links between obstructive sleep apnea and high hematocrit levels.
Smoking: Smoking affects hematocrit levels in two ways. Smoking makes your body produce more erythropoietin, the kidney hormone that stimulates red blood cell production. More red blood cells mean higher hematocrit levels.
Carbon monoxide poisoning: Carbon monoxide affects your capillaries, decreasing the amount of plasma in your blood and increasing the number of red blood cells. More red blood cells mean higher hematocrit levels.
Testosterone use: Studies show testosterone use increases red blood cell numbers, which increases hematocrit levels.

What does it mean if your hematocrit is low?

Suspected anemia is the most common reason for hematocrit testing. Your healthcare provider is your best resource for information about your test result. Other conditions include:

Blood loss due to injury or illness: Blood loss affects the number red blood cells. Fewer red blood cells mean lower hematocrit levels.
Leukemia: You have fewer red blood cells than normal because leukemia cells in your bone marrow are displacing healthy red blood cells.
Hemolytic anemia: Your red blood cells are breaking down or dying faster than your body can replace them.
Hyponatremia: This condition means you have too much water in your body, which literally causes your blood to be watered down.
Kidney disease: Low hematocrit levels are a sign of anemia. Anemia is a common complication of kidney disease.
Thyroid disease: Thyroid hormones support red blood cell production. Fewer red blood cells mean lower hematocrit levels.

What hematocrit level is considered anemic?

If your hematocrit level is below the normal range for your sex, your provider may do more tests to check on your red blood cell levels. Anemia happens when you don’t have enough red blood cells.

What does it mean when hematocrit and hemoglobin are low?

Three factors may cause both hematocrit and hemoglobin levels to drop:

You’re losing blood from injury or illness.
Your bone marrow isn’t making enough red blood cells.
Your red blood cells are dying earlier than usual.

When should I call my healthcare provider?

Your provider may have ordered a blood test because you have symptoms of conditions that affect hematocrit levels. If that’s your situation, you should contact your provider if your symptoms get worse.

A note from Cleveland Clinic

You may have had a blood test showing your hematocrit levels are higher or lower than normal. There are lots of reasons why this happens. Sometimes, higher or lower hematocrit levels are signs of serious medical conditions. Unusual hematocrit levels may simply happen because you have less serious conditions like obstructive sleep apnea. Ask your healthcare provider to interpret your individual test results. They’ll explain your results, and if you have an underlying condition, they’ll determine treatments for your condition.

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Hematocrit