Red Blood Cells (Erythrocytes / RBC)

Red blood cells carry oxygen from the lungs to the rest of the body. They also carry carbon dioxide back to the lungs so it can be exhaled. Red blood cells (RBC) are made in the bone marrow and contain hemoglobin, a protein that carries oxygen to the tissues in the body. These cells are also known as erythrocytes.

Anemia is a condition that results from a decrease in the number of RBCs. Elevated RBC counts are seen in other conditions where there is low oxygen levels, certain drugs, kidney disease, or bone marrow overproduction. If your RBC count results are abnormal, additional tests are usually done to diagnose the cause of the high or low level of red blood cells.

A CBC measures two other components of your red blood cells:

- hemoglobin: oxygen-carrying protein

- hematocrit: percentage of red blood cells in your blood

Abnormal levels of red blood cells, hemoglobin, or hematocrit may be a sign of anemia, heart disease, or too little iron in your body.

What is a red blood cell (RBC) count?

Red blood cell count is a blood test that measures the number of erythrocytes circulating in your blood. This test is almost always done as part of a complete blood count (CBC) that also measures other types of blood cells.

Red blood cells carry oxygen from your lungs to every cell in your body. Your cells need oxygen to grow, reproduce, and stay healthy. An RBC count that is higher or lower than normal is often the first sign of an illness. So the test may allow you to get treatment even before you have symptoms.

What is the red blood cell (RBC) test used for?

A red blood cell (RBC) count is almost always part of a complete blood count, a group of tests that measure many different parts and features of your blood. The RBC measurement is used to help diagnose red blood cell disorders, such as anemia, a condition in which your body does not make enough healthy red blood cells.

Why do I need a red blood cell count?

You may get this test as part of a complete blood count, which is often included in a routine checkup. You may also need this test if you have symptoms of a low or high red blood cell count.

Red blood cell count, hemoglobin and hematocrit: 

The results of these three markers are related because they each measure a feature of red blood cells. Lower than usual measures in these three areas are a sign of anemia. Anemia has many causes. They include low levels of certain vitamins or iron, blood loss, or another medical condition. People with anemia might feel weak or tired. These symptoms may be due to the anemia itself or the cause of anemia. A red blood cell count that's higher than usual is known as erythrocytosis. A high red blood cell count or high hemoglobin or hematocrit levels could point to a medical condition such as blood cancer or heart disease.

When should I get a red blood cell test?

RBC count is usually tested as part of a complete blood count, which is a common lab test that can be used to detect or monitor many different health conditions.

Your health care provider may order this test:

- As part of a routine check-up

- If you are having symptoms of anemia, such as fatigue, headache, loss of appetite, or problems concentrating

- If you are having other symptoms of blood cell changes, such as fever, infection, or weakness

- When you are receiving treatment that can affect your blood cell counts, such as chemotherapy

- To monitor a long-term health problem that may change your blood count results, such as chronic kidney disease

Interpreting red blood cell test results:

RBC count is written as a number value in cells per microliter (number of RBCs x106/µL). The normal range for red blood cells depends on factors like age and sex and can vary slightly among different laboratories due to the equipment used in different laboratories. In general, reference ranges for adults typically fall between 4 and 6 million cells per microliter of blood. Males typically have a higher number of RBCs than females.

The number of RBCs can also be affected by elevation in the area where testing is conducted. At higher altitudes, less oxygen is available thus the need for your body to increase the number of RBCs that can deliver the required amount of oxygen.

The expected RBC count in babies, children, and adolescents can vary significantly based on their age. RBC counts are often highest in newborns and decrease through infancy before reaching levels similar to adults during childhood and adolescence. Because multiple factors can affect your RBC count, it’s important to talk to your doctor to understand what your test results mean.

The typical lifespan of an RBC is 120 days, so they have to be replaced constantly. Erythropoietin is a hormone that is made in the kidneys and is released to stimulate the bone marrow to make more red blood cells. Health conditions that disrupt the normal creation and function of RBCs can cause abnormal RBC count.

References:

Barbalato L, Pillarisetty LS. Histology, Red Blood Cell. 2022 Nov 14. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan–. PMID: 30969524.

Peter Klinken S. Red blood cells. Int J Biochem Cell Biol. 2002 Dec;34(12):1513-8. doi: 10.1016/s1357-2725(02)00087-0. PMID: 12379271.

Kuhn V, Diederich L, Keller TCS 4th, Kramer CM, Lückstädt W, Panknin C, Suvorava T, Isakson BE, Kelm M, Cortese-Krott MM. Red Blood Cell Function and Dysfunction: Redox Regulation, Nitric Oxide Metabolism, Anemia. Antioxid Redox Signal. 2017 May 1;26(13):718-742. doi: 10.1089/ars.2016.6954. Epub 2017 Jan 18. PMID: 27889956; PMCID: PMC5421513.

Hamasaki N, Yamamoto M. Red blood cell function and blood storage. Vox Sang. 2000;79(4):191-7. doi: 10.1159/000056729. PMID: 11155068.

Cortese-Krott MM, Rodriguez-Mateos A, Sansone R, Kuhnle GG, Thasian-Sivarajah S, Krenz T, Horn P, Krisp C, Wolters D, Heiß C, Kröncke KD, Hogg N, Feelisch M, Kelm M. Human red blood cells at work: identification and visualization of erythrocytic eNOS activity in health and disease. Blood. 2012 Nov 15;120(20):4229-37. doi: 10.1182/blood-2012-07-442277. Epub 2012 Sep 24. PMID: 23007404.