Overview

What is a hematocrit test?

A hematocrit test (Hct) is a simple blood test that measures how much of your blood consists of red blood cells. Blood is made of red blood cells, white blood cells, platelets and plasma (the fluid part). Having a healthy number of red blood cells is important because they have a very important job — carrying oxygen throughout your body.

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Hematocrit (pronounced “hi-MAT-uh-krit”) tests are part of a larger test called a complete blood count (CBC). You may have this test:

As part of routine blood work during an annual exam
If you’re having symptoms of a condition that affects your red blood cells
If you have a blood disorder and your provider needs to monitor your red blood cells

Test Details

How does the test work?

In adults, healthcare providers often draw blood from a vein in your arm. In infants, they usually draw blood from their heel. They collect the blood in one or more collecting tubes.

Afterward, the sample goes to a lab. A machine spins the blood around, separating it into parts. The machine calculates the percentage of red blood cells.

How do I prepare for the test?

There’s nothing special you need to do to prepare. You can eat, drink and continue taking medicines as you normally would.

If you have a fear of needles, let the healthcare providers know so they can suggest ways for you to relax.

What to expect during this blood test?

The blood draw should only take a few minutes. A healthcare provider will:

Find a vein to draw blood (usually a vein on the inside of your elbow).
Place a band of rubber (tourniquet) on your upper arm.
Clean the skin where the needle will go in.
Insert a needle into your vein (it may sting a little at first, but the pain passes fast).
Collect the blood flowing through the needle in a collecting tube or syringe.
Remove the needle and tourniquet and bandage the area.

Shortly after, you should be able to go home. Before you go, ask when you can expect your Hct results. Usually, it takes anywhere from 24 hours to a few days.

Are there risks of side effects?

Most people feel fine after their blood test. But others feel faint when they get blood drawn. It’s normal if you feel this way. But you should still let your provider know how you’re doing. They’ll take steps to help you feel better.

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Results and Follow-Up

What type of results do you get and what do the results mean?

Your hematocrit test result will be a number that represents a percentage. For example, 45 means 45% of your blood is red blood cells. Here are normal hematocrit levels by sex and age:

Males: 41% to 50%
Females: 36% to 44%
Infants: 32% to 42%
Newborns: 45% to 61%

The range for what counts as normal can vary by lab. This is why you should compare your results to the reference range in your results. Look for the letters H or L next to the number to see if your results are (H)igh or (L)ow.

High hematocrit

A high hematocrit level is called erythrocytosis. It may mean you have a condition called polycythemia vera. But often, it’s a sign that your body is making lots of red blood cells because more oxygen is needed to reach your cells. Causes of high hematocrit levels include:

Living in a place with a high altitude can also cause high hematocrit. But that doesn’t mean there’s a concern. Your healthcare provider will consider factors like these when they interpret your test results.

Low hematocrit

Low hematocrit may be a sign that you don’t have enough healthy red blood cells (anemia). Suspected anemia — which causes symptoms like fatigue and weakness — is the most common reason for hematocrit testing. Other causes of low hematocrit include:

Hematocrit may also be low during pregnancy because your body is carrying more fluid. This means your blood plasma will be higher than usual in relation to your red blood cells.

When should I call my doctor?

Ask your provider if you don’t understand your results. Keep in mind that abnormal hematocrit levels aren’t always concerning. Your provider will consider these values and your other test results to see if there’s an issue. They can tell you what the numbers mean. And they can explain what needs to happen next if your hematocrit is high or low.

Your provider may have ordered testing because you have symptoms of conditions that affect hematocrit levels. If that’s the case, let your provider know if your symptoms get worse.

Additional Common Questions

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

A hematocrit test measures the percentage of red blood cells in your blood. A hemoglobin test measures the amount of hemoglobin in your red blood cells. Hemoglobin is a key protein that helps your red blood cells carry oxygen throughout your body.

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.

A note from Cleveland Clinic

It can be stressful when you have a blood test showing hematocrit (Hct) levels that are higher or lower than normal. But there are lots of reasons this happens. Sometimes, higher or lower hematocrit levels are signs of serious medical conditions. But sometimes, abnormal levels mean less serious conditions. Other times, the results are temporary. Don’t jump to the worst conclusions. Ask your healthcare provider to explain what the numbers mean for your health.

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