RBC Distribution Width (RDW)

July 21, 2020

What is RBC distribution width (RDW)?

The red blood cell distribution width measures the variation in red cell size. Normal red blood cells should be similar in size, but variations may indicate the presence of anaemia.[1]

Why take a red blood cell distribution blood test?

A red blood cell distribution width is usually taken as part of a full blood count, alongside other red cell indices including mean cell volume, mean cell haemoglobin and mean cell haemoglobin concentration. All these measurements relate to the size and volume of circulating red blood cells. They can be used for the diagnosis and monitoring of blood and/or bone marrow diseases.[2]

You can check your level of red blood cell distribution together with other red blood cells within Forth’s Vitality and Ultimate blood tests.

What function do red blood cells have in the body?

The main function of red blood cells, also known as erythrocytes, is to transport the gases, oxygen and carbon dioxide, from the lungs to the tissues and vice versa. Removing carbon dioxide from the body also helps to maintain the pH of the blood.[3]

Up to 3 million red blood cells are produced in the bone marrow every second and released into circulation. They are the most abundant blood cell, but they are small at about 6 micrometres in diameter. Red blood cells have a lifespan of approximately 120 days, at which point they are removed from circulation by special cells called macrophages in the liver and spleen.[4]

Unlike most cells, red blood cells do not contain a nucleus and are biconcave in shape which means they have a bigger surface area, giving them more room to store haemoglobin. Haemoglobin is a vital protein in the human body and has an important function. The main role of haemoglobin is to transport oxygen from the lungs to all cells and tissues in the body. Approximately 96% of the dry weight of red blood cells is haemoglobin. The structure of haemoglobin consists of four heme molecules each of which contains an iron ion. Each of the iron ions can bind to one molecule of oxygen forming oxyhaemoglobin. When oxyhaemoglobin reaches a cell which requires oxygen, the oxygen dissociates from the haemoglobin and enters the cell.[5]

How do changes in red blood cell distribution affect health and wellbeing?

The name for different sized red blood cells is anisocytosis. The condition can arise in several disorders, including:

  • nutritional deficiencies e.g. iron, folate or vitamin b12
  • cardiovascular disease
  • cancer
  • diabetes
  • kidney disease
  • liver disease
  • chronic obstructive pulmonary disease

Red blood cell distribution width gives a good indication of the health status of an individual.[6] It is often used alongside mean corpuscular volume to diagnose different types of anaemia.

If you are worried about your red blood cell distribution level or just want to check where you fall on the range, you can test your level with a simple at-home blood test.

What can cause red blood cell distribution to change?

Anaemia can cause changes to red cell distribution width. It can be used to diagnose different types of anaemia in conjunction with mean corpuscular volume. Anaemia is classified according to the size of red blood cells, for example:

  • Normocytic anaemia – normal MCV
  • Macrocytic anaemia – increased MCV
  • Microcytic anaemia – decreased MCV

What are the most common symptoms of anaemia?

The symptoms of most types of anaemia are relatively similar and include:

  • Pale skin
  • Feeling tired and weak most of the time
  • Dizziness
  • Not able to complete as much exercise as you usually would
  • Feeling like you are short of breath after exercise
  • Experience heart palpitations[5]

How to keep red blood cell distribution in the healthy range

A nutrient which is essential for red blood cell health is iron. Good sources of iron are red meat and liver. It is recommended that red meat is consumed once per week to help keep iron levels within normal parameters. Women who are of menstruating age need to keep their iron intake increased, particularly around the time of their periods. Individuals who follow a vegan or vegetarian diet will need to get their iron from plant-based sources.

Good food sources of iron are:

  • red meat
  • liver
  • green leafy vegetables e.g. spinach
  • dried fruit
  • pulses
  • nuts
  • seeds

The absorption of iron from plant-based sources is better when vitamin C is present.[7] For example, fortified breakfast cereals paired with a glass of orange juice. Tea can affect the absorption of iron, so you should refrain from drinking tea with your meals and switch to drinking it between meals instead. [8]

Iron levels can be influenced by too much or too little exercise. Not having enough iron can affect your muscles ability to recover following exercise.

A high mean corpuscular volume level can be caused by macrocytic anaemia, a major cause of which is vitamin B12 deficiency.  Therefore, it is important to ensure you are getting enough vitamin B12 in your diet. People who find it more difficult to get a good amount of B12 in their diet are those who follow a vegan diet. The reason for this is because much of the vitamin B12 comes from animal products. Vegans and possibly vegetarians may need to supplement their diet.

Good sources of vitamin B12 are:

  • meat
  • fish like salmon or cod
  • eggs
  • milk and dairy products
  • vegan-friendly products
    • fortified breakfast cereals[9]
    • yeast extracts like marmite


[1] Patel, K, V., Ferrucci, L., Ershler, W, B., Longo, D, L and Guralnik, J, M. (2009). Red Cell Distribution Width and the Risk of Death in Middle-Aged and Older Adults. Arch Intern Med: 9(169), pp 515-523

[2] Lab Tests Online UK. (2015). Red Cell Indices. Available at: https://labtestsonline.org.uk/tests/red-cell-indices

[3] Kuhn, V et al. (2017). Red Blood Cell Function and Dysfunction: Redox Regulation, Nitric Oxide Metabolism, Anemia. Antioxidants and Redox Signaling: 26(13).

[4] Dean L. Blood Groups and Red Cell Antigens [Internet]. Bethesda (MD): National Center for Biotechnology Information (US); 2005. Chapter 1, Blood and the cells it contains. Available from: https://www.ncbi.nlm.nih.gov/books/NBK2263/

[5] Panawala, L. (2017). What is the Function of Hemoglobin in the Human Body? Pedia. Available at: https://www.researchgate.net/publication/313841668_What_is_the_Function_of_Hemoglobin_in_the_Human_Body

[6]Salvagno, G, L., Sanchis-Gomar, F., Picanza, A and Lippi, G. (2014). Red Blood Cell Distribution Width: A Simple Parameter with Multiple Clinical Applications. Crit Rev Clin Lab Sci: Early Online.

[7] Lane, D, J, R and Richardson, D, R. (2014). The Active Role of Vitamin C in Mammalian Iron Metabolism: Much More than Just Enhanced Iron Absorption. Free Radical Biology and Medicine: 75, pp 69-83.

[8] Zijp, I, M., Korver, O and Tijburg, L, B, M. (2000). Effect of Tea and Other Dietary Factors on Iron Absorption. Critical Reviews in Food Science and Nutrition: 40(5), pp 371-398.

[9] NHS. (2019). Treatment Vitamin B12 or Folate Deficiency Anaemia. Available at: https://www.nhs.uk/conditions/vitamin-b12-or-folate-deficiency-anaemia/treatment/

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This information has been medically reviewed by Dr Nicky Keay

Nicola has extensive clinical and research experience in the fields of endocrinology and sport and exercise medicine. Nicky is a member of the Royal College of Physicians, Honorary Fellow in the Department of Sport and Exercise Sciences at Durham University and former Research Fellow at St. Thomas' Hospital.

Dr Nicky Keay

Dr Nicky Keay

BA, MA (Cantab), MB, BChir, MRCP.