RBC Distribution Width (RDW)

What Is RBC Distribution Width?

Red blood cell distribution width is a test which is part of the complete blood count and measures the variation in red blood cell size. Normal red blood cells are similar in size but variations can be indicative of some causes of anaemia.[1]

Which tests include this marker?

What Role Does It Play In The Body?

Red blood cells are the most common type of blood cell in circulation. Their main function is to distribute oxygen around the body via the protein, haemoglobin.

Red blood cells have no nucleus which means they have a greater surface area and this enhances their ability to transport oxygen. They are a biconcave disc shape with a depressed centre. The normal size of a red blood cell is between 6 and 8 µm (micrometres) and they have a thickness of 2 µm.

Under some physiological conditions, red blood cells may undergo an increase or decrease in size. The makeup of the cell allows these changes to take place. Usually, an RBC distribution width which is lower than the conventional parameters is deemed clinically meaningless. However, an increase above the normal can indicate that an individual’s red blood cells are not equally sized (anisocytosis).

How Does Red Blood Cell Distribution Width Affect My Wellbeing?

The condition where red blood cells are not equal in size is called anisocytosis. There are some physiological conditions which can cause red blood cells to be different sizes, including: 

  • Erythropoietin deficiency
  • Ageing
  • Physical exercise
  • Pregnancy
  • Black ethnicity 

Several disorders can result in anisocytosis, including:

  • Nutritional deficiencies e.g. iron, folate or vitamin B12
  • Cardiovascular disease
  • Cancer
  • Diabetes
  • Kidney disease
  • Liver disease
  • Chronic obstructive pulmonary disease

RBC distribution width can help to give a good indication of the health status of individuals.[2] It is often used in conjunction with mean corpuscular volume, the average size of red blood cells, to diagnose different anaemias.  Anaemias can be classified according to the size of the cell i.e.:

  • Normocytic anaemia - normal MCV
  • Macrocytic anaemia - increased MCV
  • Microcytic anaemia - decreased MCV2

How Can I Improve My Result?

Your diet can help to keep anaemia at bay. Eating a healthy and balanced diet and ensuring you get all the nutrients you need.


Red blood cells need iron to function efficiently.  Good sources of dietary iron include:

  • Red meat – beef, lamb and pork
  • Fish
  • Poultry
  • Dark green leafy vegetables – spinach, cabbage and broccoli
  • Nuts
  • Seeds
  • Pulses[3]

Plant-based sources of iron such as dark green leafy vegetables, nuts, seed, pulses and fortified cereals may be absorbed better in the presence of vitamin C.[4] Try eating fortified breakfast cereals alongside a glass of orange juice or swap chips for a jacket potato with a side of green leafy veg – be sure to eat the skin of the potato. You should also refrain from drinking tea with your meals as this can affect iron absorption. Instead, you should drink it between meals to ensure you absorb iron effectively.[5]

You also need to make sure your vitamin B12 levels are adequate too. However, you should avoid eating too much as this can have adverse effects and could contribute to a high mean corpuscular volume (MCV). There is a strong association between MCV and RBC distribution width. Early vitamin B12 deficiency can present with a normal MCV but increased distribution width, however, increased MCV and width can cause full blown deficiency. 

Good sources of vitamin B12 include:

  • Meat
  • Fish
  • Dairy products

As vitamin B12 is found only in animal products, vegetarians and vegans can find themselves at risk of becoming deficient. Therefore, these groups may need to consider supplementation.


Exercise is an important aspect of a healthy lifestyle. Regular exercise can help to increase blood flow to muscles which enables oxygen to reach the tissues. Exercise also helps to increase the oxygen carrying capacity of red blood cells.[6]

Both too much and too little iron can influence energy levels and could affect your ability to exercise. Low iron can also influence how well and quickly your muscles repair themselves after exercise.[7] Iron deficiency anaemia may also be more common in athletic groups and may need iron supplementation.


Tests that include this marker


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[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] 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.

[3] British Dietetic Association. (2017). Food Fact Sheet: Iron. Available at: https://www.bda.uk.com/foodfacts/iron_food_fact_sheet.pdf

[4] 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.

[5] 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.

[6] Mairbäurl, H. (2013). Red Blood ells in Sports: Effects of Exercise and Training on Oxygen Supply by Red Blood Cells. Frontiers in Physiology: 4.

[7] Beard, J and Tobin, B. (2000). Iron Status and Exercise. The American Journal of Clinical Nutrition: 72(2), pp 594S-597S. 

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