Red Blood Cell (RBC) Count Test: Results and What They Mean

Medically Reviewed by Jabeen Begum, MD on April 29, 2024
Written by Jeffrey Weishaupt, Kendall K. Morgan
6 min read

A red blood cell count test measures how many red blood cells (RBCs) are in your blood. The red blood cells, or erythrocytes, have hemoglobin – a protein that transports oxygen to all parts of your body. The amount of oxygen transported to your body parts depends on how many red blood cells you have, or your RBC count. 

Other names for the red blood cell count test are RBC count and erythrocyte count. The test is usually done as a part of a complete blood count (CBC) to look for several health conditions.

Your doctor or another health care professional will usually do RBC testing during a complete blood count (CBC) test.

The CBC test provides essential information about the types and amounts of cells in the blood, including red blood cells, white blood cells, and platelets. The RBC test will find out if you have a low number of RBCs or a type of anemia. It also can tell if your RBC counts are higher than normal.

Men usually have a higher red blood cell count than women. The level of red blood cells in your blood will go down as you get older. Any number lower or higher than the normal RBC count could mean you have a medical condition.

Your doctor may order an RBC test when they see signs of weakness or fatigue during a routine checkup. You may also need this test to examine specific health conditions that are usually not obvious. It may just be done as part of a regular checkup. 

Health conditions that can change red cell counts include internal bleeding, kidney diseases, anemia, and others. 

Your doctor usually asks you to do this test to find out if you've had any change in your RBC count that may suggest you have a health condition. You may also get it regularly when you have certain health conditions, such as:

  • Bone marrow disorders
  • A disease that causes damage to blood vessels in your kidneys
  • A disorder that makes your blood cells break down faster or more than normal
  • A blood cancer

Normal range values may vary a little from one lab to the next. Ask your doctor or other care provider what they consider a normal result and what it means if your results fall outside of that range.

Normal RBC for adults

If you are a man or were assigned male at birth, a normal RBC test result is usually 4.7 million to 6.1 million RBCs per microliter of blood.

If you are a woman or were assigned female at birth, a normal RBC test result is usually 4.2 million to 5.4 million RBCs per microliter of blood.

Normal RBC for kids

Kids can have an RBC count that's a little lower and it's still normal. A normal result for kids is usually 4.0 million to 5.5 million RBCs per microliter of blood.

Signs of a high RBC count

You could have a high RBC count without any signs. But you may notice symptoms including:

  • Fatigue
  • Being short of breath
  • Headaches
  • Blurred vision
  • Trouble sleeping
  • Joint pain
  • Itchiness
  • Feeling numb or tingly
  • Nosebleeds

High RBC count causes

Many health conditions can increase your body’s red blood cell count. High RBC count causes may include: 

  • Heart failure, causing low oxygen levels in the blood
  • Congenital heart disease, a natural heart condition
  • Polycythemia vera, a condition in which the bone marrow produces high levels of red blood cells
  • Kidney tumors
  • Lung diseases, such as emphysema and pulmonary fibrosis 
  • Carbon monoxide exposure that's usually due to excessive smoking
  • Dehydration

You may also have high red cell counts if you:

  • Smoke cigarettes
  • Live at a high altitude
  • Take steroids or other performance-enhancing drugs
  • Are under stress

Most cancers that affect blood cell counts will make your red cell levels go down. But there are cancers that can make it go up, including:

  • Polycythemia vera
  • Renal cell carcinoma, the most common kidney cancer in adults
  • Hepatocellular carcinoma, the most common liver cancer in adults

Signs of a low RBC count

When you don't have enough red blood cells, it's a condition called anemia. You can have many kinds of anemia for different reasons. Your anemia could be mild or it could be more serious. Some of the signs of low RBC counts are similar to signs of high RBC counts. Symptoms of anemia or low RBC counts include:

  • Fatigue
  • Feeling weak
  • Being short of breath
  • Having pale or yellow skin, but this could be hard to see if your skin is darker
  • Irregular heartbeat
  • Feeling dizzy or lightheaded
  • Pain in your chest
  • Cold hands or feet
  • Headaches

Low RBC count causes

Several health conditions can lead you to have fewer than normal numbers of red blood cells in your body. Some low RBC count causes include:

  • Certain medications, including antiretroviral drugs for HIV infection, chemotherapy drugs, and more
  • Blood loss
  • Anemia
  • Leukemia, a type of blood cancer
  • Chronic kidney diseases
  • Cirrhosis, or scarring of the liver
  • Vitamin B12 or folate deficiency
  • Stomach ulcers
  • Lupus, a condition in which your immune system attacks your body
  • Hypothyroidism, an underactive thyroid
  • Inflammatory bowel disease (IBD)
  • Hodgkin lymphoma, cancer caused by white blood cells
  • Multiple myeloma, another type of cancer that comes from white blood cells
  • Lead poisoning
  • Rheumatoid arthritis

A blood sample is required for this test. A health care professional uses a needle to draw blood from a vein in your hand or arm. 

There are no specific guidelines to follow before going for RBC testing. The doctor usually asks you to:

  • Avoid intense exercise.
  • Relax and reduce stress.
  • Drink plenty of water.
  • Avoid or delay taking certain medicines.

You must tell your doctor about all the medicines and supplements you are taking. 

You don't need to do anything special to prepare for the test, but let your doctor or other provider know if you have a fear of needles or blood. A blood draw might hurt a little, but it shouldn't usually hurt a lot. You might feel a prick or sting. And you might have minor pain or a bruise afterward, but it should go away quickly. 

Several things may affect your test results. They include your age, health history, gender, and the method used for the test. 

Your test results show the number of red blood cells in your blood. Test results don't always mean that you have a health issue. If your results come back outside the normal range, your doctor will help you understand what it means and what other tests you may need.

Several things can affect your RBC count, including:

  • Dehydration, or less intake of water
  • Overhydration, or a high intake of water
  • Stress
  • Altitude
  • Pregnancy
  • Certain medications
  • Your position during the test

The red blood cell count test is done with a needle, so it has some risks. These include bleeding, bruising, infections, and dizziness. The pain of needle pricking is usually mild, but the area can become sore afterward. There are no severe risks in RBC count testing.

A red blood cell (RBC) count test measures the number of RBCs in your blood. You'll usually have this test together with a complete blood cell count test. If your counts are high or low, it may mean you have one of many health conditions. Ask your doctor what it means if your RBC count is higher or lower than normal.

  • What level of RBC is concerning?

Anytime your RBC level is outside the normal range, it may be cause for some concern. But you don't necessarily need to worry a lot. Your doctor may need to run more tests to see why your RBC counts are abnormal if you don't already know. They can help you decide on treatments or other next steps, depending on the cause.

  • What is a normal nucleated red blood cell count?

Nucleated red blood cells are premature RBCs in your bone marrow. If you're a healthy adult, you usually won't have any of these in your blood. Doctors don't look at NRBCs as much as RBCs. But sometimes, a count of them might be done if you're very ill. An NRBC test can be used to predict if you're going to get worse when you have a serious illness such as sepsis, trauma, acute pancreatitis, or severe heart disease.