RDW or red blood cell distribution width is a measure of how equal your red blood cells are in size. It can help 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 [2, 5, 6, 7]:

  • Iron or vitamin deficiency
  • Anemias (different types, including sickle cell anemia)
  • Inflammation
  • Sleep disorders
  • Blood loss due to bleeding (hemorrhage)
  • Thalassemia
  • Liver disease
  • Kidney disease
  • Cancer

However, RDW can still be at a normal level in patients with leukemia, or certain types of anemia (such as aplastic anemia). Therefore, it is still important to keep an eye on your other blood-cell related test results to fully rule out these possibilities [3].

Normal Range

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

Your red blood cells will become more unequal with age [8].


A low value indicates that your red blood cells are uniform in size. This is desirable [3].

However, it is still possible for you to have a blood-related disease and low RDW [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, your body is having issues producing red blood cells.

Causes of High RDW

1) Nutrient Deficiency

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

This happens because you need these nutrients to produce healthy red blood cells. Any of these deficiencies can eventually lead to anemia.

2) 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  [14, 15, 16, 17, 18, 1, 19].

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 [14, 20].

Higher RDW was found in people with inflammation-associated diseases such as inflammatory bowel disease (IBD), celiac disease, polycystic ovary syndrome (PCOS), and major depressive disorder (MDD) [21, 22, 23, 21].

3) 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% [24].

RDW is higher in people with sleep apnea [25, 26].

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 [27].

4) Bleeding (Hemorrhage)

Injuries increase RDW [28].

Bleeding may not necessarily be visible, as is the case with gut bleeding [7, 29].

5) Blood transfusions

If a person has many blood transfusions, their RDW can increase due to the differences in blood between the donor and the recipient [13, 30].

6) Liver Disease

RDW is increased in various liver disease, including hepatitis, alcoholic liver cirrhosis, biliary cirrhosis, and liver cancer [31].

In an observational study, RDW of 423 liver disease patients was significantly higher than the RDW of healthy people [32].

In 446 patients with hepatitis B, higher RDW corresponded to increased liver damage and inflammation [33].

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 [6].

8) Alcoholism

Alcoholics can have a high RDW without having liver disease. This is because alcohol can have toxic effects on red blood cells [34].

9) Thalassemia

Thalassemia can cause RDW to increase. However, thalassemia patients may also have normal RDW levels [35, 13, 36].

8) Sickle Cell Anemia and Hereditary Spherocytosis

Sickle cell anemia is an inherited condition. People with this condition have higher RDW values because many of their red blood cells become misshapen [35].

Hereditary spherocytosis, another condition in which red blood cells get deformed, is also a cause of high RDW levels [37, 38].

9) Cancer

RDW is higher in various cancers, including gastric, liver, colon, and kidney cancer [39, 40, 41, 42].

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 [43].

Health Effects of High RDW

1) RDW is Associated with Autoimmune Disease

There is a link between high RDW and increased disease activity in autoimmune diseases such as rheumatoid arthritis, lupus, psoriasis, Sjogren’s, systemic sclerosis, and ankylosing spondylitis [1, 44, 45, 46, 47, 48, 49].

2) RDW is Associated with Metabolic Syndrome

People with higher RDW have higher rates of metabolic syndrome. This was shown in over 217k people [50, 51].

3) High RDW increases the risk of Heart Disease

Elevated RDW has been associated with different types of heart disease [52, 53, 54].

In a study of over 25.5k people, every 1% increase in RDW increased the risk of heart attack by 13%. Those with the lowest RDW levels had a 71% lower risk of heart attack (myocardial infarction) compared to people with the highest levels [55].

High RDW is associated with an increased risk of plaque buildup in the arteries (atherosclerosis) in patients with high blood pressure (hypertension) [56].

4) High RDW Increases the Risk of 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 [57].

5) High RDW Increases Dementia Risk

In a study of over 2,500 seniors, people with higher RDW had a higher risk of dementia. The risk was more significant in people without anemia [58].

6) High RDW Increases Cancer Risk

In a study with over 25k people, the risk of cancer was 30% higher in men with the highest compared to those with the lowest RDW levels. Postmenopausal women with highest RDW levels had a 22% increased risk of cancer. No link between RDW and cancer was found in premenopausal women [59].

7) High RDW is Associated with Mortality

High RDW increases inflammation and oxidative stress, which contributes to mortality (death) risk. In various studies, hospital patients and adults 45+ with high RDW values had a higher risk of heart disease-associated, infection-associated, and all-cause mortality [60, 61, 62].

Additionally, in a review of 13 trials (with 10,410 patients), low RDW was associated with a lower risk of mortality [63].

Ways to Decrease RDW

1) Eat a Balanced Diet

Eat a healthy, balanced, and nutritious diet. To prevent nutrient deficiencies, it is important that your diet consists of the recommended amounts of iron, folate, and vitamin B12 [9, 12, 13].

Correcting nutritional deficiencies will improve red blood cell production and help decrease RDW levels.

2) Decrease Alcohol

Reduce your alcohol intake [34]. Excessive alcohol can cause red blood cell damage [34].

In addition, alcohol also decreases the absorption of nutrients such as vitamin B12 and folate, which are necessary for red blood cell production [64, 65].

3) Exercise More

People who engage in little or no physical activity have higher RDW levels [66, 67].

Exercise, including light-intensity physical activity, has been shown to improve RDW [68, 69, 70].

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 decreased by 11% [71].

4) Stop Smoking

Smoking increases oxidative stress. In smokers, higher RDW was linked to more cigarettes smoked per day as well as longer duration of smoking [72].

5) Get Enough Sleep

Make sure you are getting enough sleep, but don’t overdo it.

In a study with over 17.5k adults, those getting 7 – 8h of sleep had the lowest RDW levels. People getting 5, 9, and ≥ 10 hours/night increased their odds of having high RDW by 23%, 29%, and 67%, respectively [24].

Irregular RDW Levels?

Check with Lab Test Analyzer!

LabTestAnalyzer helps you make sense of your lab results. It informs you which labs are not in the optimal range and gives you guidance about how to get them to optimal. It also allows you to track your labs over time. No need to do thousands of hours of research on what to make of your lab tests.

LabTestAnalyzer is a sister company of SelfHacked. The proceeds from your purchase of this product are reinvested into our research and development, in order to serve you better. Thank you for your support.

Click here to subscribe


1 Star2 Stars3 Stars4 Stars5 Stars
(12 votes, average: 4.42 out of 5)

FDA Compliance

The information on this website has not been evaluated by the Food & Drug Administration or any other medical body. We do not aim to diagnose, treat, cure or prevent any illness or disease. Information is shared for educational purposes only. You must consult your doctor before acting on any content on this website, especially if you are pregnant, nursing, taking medication, or have a medical condition.