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RDW Blood Test: High & Low Levels + Normal Range

Written by Biljana Novkovic, PhD | Last updated:
Puya Yazdi
Medically reviewed by
Puya Yazdi, MD | Written by Biljana Novkovic, PhD | Last updated:

RDW or red blood cell distribution width is a measure of how equal your red blood cells are in size. It can help your doctor diagnose various blood-related disorders and diseases. In addition, it is also increased in seemingly unrelated disorders and diseases that nevertheless affect blood cell production and lifespan. These include inflammatory, autoimmune, liver, kidney, and heart disease. Keep reading to learn why having a high RDW is bad and how to improve your values.

What is Red Blood Cell Distribution Width (RDW)

A normal red blood cell is shaped like a disk with a depressed center. It is very flexible, which enables it to change shape – this is needed for a red blood cell to squeeze through the narrowest of blood vessels called capillaries [1].

Normally, red blood cells are relatively equal in shape and size. However, in some conditions and diseases, red blood cells can have a distorted shape or be smaller or larger than normal while still maintaining their core function (oxygen and carbon dioxide transport) [1].

Red blood cell distribution width (RDW) is the variation of the size/volume of your red blood cells. Basically, it tells you how equal or unequal your red blood cells are in size. It is a part of a complete blood count, which also measures your hemoglobin, hematocrit, and red and white blood cell counts [2, 3].

Low values mean that your red blood cells are roughly similar in size, which is normal and desirable. Higher values mean that your red blood cells are produced in different sizes. In other words, there is some issue with red blood cell production or survival [2, 3, 4].

Along with the MCV (mean corpuscular volume), MCH (mean corpuscular hemoglobin), and MCHC (mean corpuscular hemoglobin concentration), a high RDW can serve as a sign of several underlying diseases, such as anemia [2, 5, 6, 7]:

  • Anemias
  • Iron and vitamin B12/folate deficiency
  • Inflammation
  • Injuries and bleeding/hemorrhage
  • Liver disease
  • Kidney disease
  • Hereditary red blood cell disorders, such as thalassemia

Normal Range

RDW normally ranges from 11.5 – 15%. The range may slightly vary between laboratories.

However, a normal RDW doesn’t mean a condition such as anemia is absent. Your doctor will interpret your results in conjunction with your medical history and other test results.


A low value indicates that your red blood cells are uniform in size. This is desirable and not a cause for concern [3].

However, it is still possible to have a blood-related disease and low RDW levels [3].

High RDW

A high RDW means that your red blood cells are unequal in size. This condition is called anisocytosis [2, 3].

It happens when, for any reason, the body is having issues producing red blood cells.

Causes of High RDW

Causes listed below are commonly associated with high RDW. If your RDW is high, work with your doctor or another health care professional to get an accurate diagnosis.

1) Nutrient Deficiency

Various nutrient deficiencies can cause your RDW to increase, such as:

This happens because all of these nutrients are needed to produce healthy red blood cells. Any of these deficiencies can eventually lead to anemia.

2) Alcoholism

Alcoholics can have a high RDW without having liver disease. This is because alcohol can have toxic effects on red blood cells [13].

3) Liver Disease

RDW is increased in various liver diseases, including hepatitis, alcoholic liver cirrhosis, biliary cirrhosis, and liver cancer [14, 15, 16].

4) Inflammation

Studies show that higher RDW is associated with inflammation and higher levels of inflammatory cytokines, such as IL-6, IL-8, and TNF-alpha [6, 17, 18, 19, 20, 1, 21].

Inflammatory cytokines can hinder red blood cell production, and thereby increase RDW levels. In addition, oxidative stress that often accompanies chronic inflammation can decrease the lifespan of red blood cells and further increase RDW values [6, 7].

Higher RDW was found in studies of people with inflammation-associated diseases such as inflammatory bowel disease (IBD), celiac disease, polycystic ovary syndrome (PCOS), and major depressive disorder (MDD) [22, 23, 24, 22].

In addition, RDW increases with inflammation and increased disease activity in autoimmune diseases such as rheumatoid arthritis, lupus, psoriasis, Sjogren’s, systemic sclerosis, and ankylosing spondylitis [1, 25, 26, 27, 28, 29, 30].

5) Blood transfusions

If a person has many blood transfusions, their RDW can increase due to the differences in blood cells between the donor and the recipient [12, 31].

6) Bleeding (Hemorrhage)

Bleeding in Injuries increase RDW [32]. Bleeding may not necessarily be visible, as is the case with gut bleeding [33, 34].

7) Kidney Disease

Erythropoietin, a hormone produced by the kidneys, is needed for the production and maturation of red blood cells. Abnormal production of this hormone happens in kidney disease leading to increased RDW [1].

People with decreased kidney function have higher RDW levels [35].

8) Sleep Disturbances and Disorders

In a study of over 17.5k adults, those getting less or more than 7-8h of sleep per night were more likely to have higher RDW. This was especially the case for people sleeping over 10h per night — their chances of having elevated RDW were increased by almost 70% [36].

RDW is also affected by shift work. In a study of 7k women, those on rotating shifts had almost 50% increased odds of having elevated RDW compared with women working day shifts [37].

RDW is higher in people with sleep apnea [38, 39].

9) Hereditary Red Blood Cell Disorders

Hereditary red blood cell disorders, such as sickle cell anemia, thalassemia, and spherocytosis can increase RDW [12, 40, 12]. In all of these disorders red blood cells become misshapen.

However, thalassemia patients may also have normal RDW levels [40, 12, 41].

10) Cancer

RDW is higher in various cancers, including gastric, liver, colon, and kidney cancer [42, 43, 44, 45].

There are many factors in cancer that can interfere with red blood cell production, such as chronic inflammation and poor nutritional status.

In cancer, RDW often increases with disease severity and metastasis [46].

High RDW & Health

1) RDW and Metabolic Syndrome

Metabolic syndrome is a group of health issues that together increase the risk of heart attack, stroke, and diabetes. These issues are:

  • too much fat around the waist
  • elevated blood pressure
  • high triglycerides
  • elevated blood sugar
  • low HDL cholesterol

People who have three of the five issues are considered as having metabolic syndrome.

A large study of over 217k workers found an association between higher RDW (>14%) and metabolic syndrome [47].

Similarly, another smaller study with 3.5k people found that people with higher RDW (>14%) were more likely to have metabolic syndrome [48].

2) RDW and Heart Disease

In a study of over 25.5k people, every 1% increase in RDW was associated with a 13% higher risk of having a heart attack over the next 15 years [49].

In another study of 156 people with high blood pressure, higher RDW was associated with an increased risk of plaque buildup in the arteries (atherosclerosis) [50].

3) RDW and Diabetes

In a study that monitored 2.6k people with normal blood sugar over 4 years, those with the highest RDW values had almost 2 times higher risk of developing diabetes compared to people with the lowest RDW values [51].

4) RDW and Cancer

In a study of over 25k people, men with the highest RDW had a 30% higher risk of getting cancer over the next 15 years than men with the lowest values. In postmenopausal women, highest RDW values were associated with a 22% higher risk. However, no link between RDW and cancer was found in premenopausal women [52].

5) RDW and Mortality

In a study of over 8k adults and a 6-12 year follow up, higher initial RDW values were associated with a higher risk of death. For every 1% increase in RDW, all-cause mortality risk increased by 22% [53].

In another study of over 51k people who were hospitalized due to critical illness and followed over one year, a higher initial RDW measurement was associated with higher all-cause mortality [54].

Finally, a meta-analysis looked at 13 trials with over 10k people who experienced acute coronary syndrome, which is decreased blood flow to the heart such that part of the heart muscle is unable to function properly or dies. A low RDW during the syndrome was associated with a significantly lower heart disease and all-cause mortality and a lower risk of subsequent risk of having a major adverse cardiovascular event (such as heart disease or stroke) [55].

Decreasing RDW

If you have an elevated RDW, the most important thing is to work with your doctor to find out what’s causing your high RDW and to treat any underlying conditions. The additional lifestyle changes listed below are other things you may want to discuss with your doctor. None of these strategies should ever be done in place of what your doctor recommends or prescribes!

1) Eat a Balanced Diet

Eat a healthy and nutritious diet in order to prevent nutrient deficiencies. It is important that your diet includes enough iron, folate, and vitamin B12 [8, 11, 12]. Correcting nutritional deficiencies can help improve red blood cell production and decrease RDW levels.

However, remember that nutrient deficiencies can also have non-dietary causes, such as gut issues (malabsorption), in which case they can’t be corrected by simple dietary adjustments.

2) Stop Smoking

Smokers have higher RDW, and it increases with the number of cigarettes smoked per day and the duration of smoking [56].

3) Avoid Alcohol

Excessive alcohol can cause red blood cell damage [13]. In addition, alcohol also decreases the absorption of nutrients such as vitamin B12 and folate, which are necessary for red blood cell production [57, 58].

Avoid alcohol until your RDW improves [13].

4) Get Enough Sleep

Make sure you are getting enough sleep, but don’t overdo it. Studies suggest that 7-8h of sleep per night may be ideal [36].

In a study with over 17.5k adults, those getting 7 – 8h of sleep had the lowest RDW levels. Getting 5, 9, and ≥ 10 hours/night was associated with 23%, 29%, and 67% higher odds of having high RDW, respectively [36].

5) Exercise Regularly

People who engage in little or no physical activity have higher RDW levels [59, 60].

Research suggests that exercise, including light-intensity physical activity, can improve RDW [61, 62, 63].

In a study with over 8k people, for every increase in the number of workout sessions per week the odds of having an elevated RDW were 11% lower [64].

About the Author

Biljana Novkovic

Biljana Novkovic

Biljana received her PhD from Hokkaido University.
Before joining SelfHacked, she was a research scientist with extensive field and laboratory experience. She spent 4 years reviewing the scientific literature on supplements, lab tests and other areas of health sciences. She is passionate about releasing the most accurate science and health information available on topics, and she's meticulous when writing and reviewing articles to make sure the science is sound. She believes that SelfHacked has the best science that is also layperson-friendly on the web.


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