RDW stands for Red Blood Cell Distribution Width. It is usually ordered as a part of the routine lab test to check for the variation in volume and size of red blood cells.

Erythrocytes or popularly known as red blood cells carry oxygen from the lungs to different parts of the body. If there is an abnormality in the width and volume of red blood cells, it could indicate an abnormality in the body.

RDW measures the distribution width of red blood cells

Image 1: RDW measures the distribution width of red blood cells.

Picture Source: selfhacked.com

What is the normal size of red blood cells?

The standard size of the red blood cell is around six to eight micrometers in diameter. If the size is larger than the usual, then it indicates an elevated RDW. However, it is important to note that even if the RBC is small, but it is plenty in numbers, it would still indicate an elevated RDW.

The same thing goes for numerous large red blood cells. It is a must to note that the RDW alone is not enough parameter to interpret a complete blood count. It is only used as supplementing data for hemoglobin and MCV (mean corpuscular value). (1, 2, 3, and 4)

Why is the RDW test important?

By checking the red blood cell’s distribution width, some types of diseases will be diagnosed such as the following:

  • Certain types of anemia like thalassemias/inherited blood disorders
  • Heart-related diseases
  • Diabetes mellitus
  • Liver-related diseases
  • Cancer

In a routine complete blood count procedure, the doctor will look at RDW if he observes the following:

  • Symptoms indicative of anemia such as pale skin, dizziness, and numbness
  • Chronic illness like HIV and AIDS
  • Vitamin and mineral deficiency
  • Blood-related disorder history such as sickle cell anemia
  • Blood loss secondary to trauma and/or surgery
  • Diseases that could affect red blood cells (3, 4, and 5)

RDW from a laboratory standpoint

  • Small RDW – A single uniform cell population could lead to a small RDW because RBC volumes cluster tightly around the mean.
  • Increased RDW – A large range of cell size causes the histogram to look like a broad peak and with the presence of two unique populations such as two narrow peaks, the RDW of the non-uniform populations would increase significantly. (2, 5, and 6)
blood is extracted and sent to the laboratory

Image 2: A blood is extracted and sent to the laboratory to check for blood compositions including RDW.

Picture Source: healthline.com

What to do during the RDW Blood Test?

  1. There is no special preparation for the test. However, there are instances when the doctor will ask you to fast before the procedure if there are other blood tests that need to be done.
  2. The test will only take a few minutes. A sample of your blood is withdrawn from your vein and placed in a sterile tube.
  3. A sterile bandage and pressure are put on the injection site to stop the bleeding.
  4. The tube containing the blood sample will be immediately sent to the laboratory for testing. (1, 3, and 6)

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RDW is a part of the routine Complete Blood Count procedure

Image 3: RDW is a part of the routine Complete Blood Count procedure.

Picture Source: i0.wp.com

Is measuring RDW a must?

In routine blood check-up such as CBC/Complete Blood Count, the doctor might include RDW in testing patients that exhibit signs and symptoms of anemia such as weakness, headache, fatigue, pale skin, and cold and clammy hands/feet.

The measurement of RDW is also emphasized if the patient has a family history of blood disorders such as sickle cell anemia and thalassemia. It is also one of the parameters to look at in patients with chronic medical conditions like Chron’s disease, diabetes and HIV/AIDS.

Obtaining RDW measurement is not that difficult. The lab technician will draw a blood sample and the sample is sent to the laboratory for thorough analysis. It is a quick procedure and usually does not require any special preparations and downtime.

There might be some special instructions if a battery of tests has to be performed such as in the case of a patient with complex medical issues. (6, 7, and 8)

Measuring red blood cell distribution width

There are different ways to measure the distribution width of red blood cells. One is the use of an electronic supplement that can analyze the sample of blood and detect pulses produced by red blood cells. If the pulse is strong, the red blood cell size is huge. However, if the pulse is weak, it indicates that the red blood cells are small.

Hematology analyzer is used to measure the size of red blood cells and to calculate the red blood cell distribution width according to the coefficient of variation in, and the red blood cell population’s standard deviation.

The formula to calculate RDW-CV is RDW-CV=1SD x 100/MCV.

Another method of finding out red blood cell distribution width is by taking a look at the standard deviation of the red blood cell distribution, which is the actual measurement of the width of the red blood cell distribution curve.

It is the perfect method in reflecting red cell anisocytosis because it is directly measured and has nothing to do with MVC. (3, 7, 8, and 9)

Result Interpretation

A result that is not within the normal range simply means that you could be suffering from infections, nutrient deficiency, and other medical conditions.

However, it is important to keep in mind that even if your RDW is within normal limits, you could still be suffering from an underlying medical condition, especially if other blood parameters are deviated from normal such as MCV or mean corpuscular volume. (3, 4)

What does a high RDW mean?

If the RDW reading is more than the normal limits, it could indicate any of the following:

  1. Iron deficiency
  2. Folate deficiency
  3. Vitamin B12 deficiency
  4. Macrocytic anemia (the body does not produce enough normal red blood cells/the cells produced are larger than the usual)
  5. Microcytic anemia (deficiency of normal red blood cells/red blood cells appear smaller than the usual)
  6. Alcoholism because alcohol can be extremely toxic to red blood cells.
  7. Possible liver-related diseases such as alcoholic liver cirrhosis, hepatitis, biliary cirrhosis, and liver cancer.
  8. Inflammatory conditions like celiac disease, inflammatory bowel disease, major depressive disorder, and polycystic ovary syndrome.
  9. Increased autoimmune diseases such as lupus, rheumatoid arthritis, psoriasis, ankylosing spondylitis, and Sjogren’s systemic sclerosis.
  10. Blood transfusions, especially when done multiple times can significantly increase RDW secondary to the differences in blood cells between the recipient and the donor.
  11. Kidney-related diseases secondary to abnormal production of erythropoietin, a hormone produced by the kidneys.
  12. Sleep-related disorders
  13. Hereditary red blood cell disorders such as sickle cell anemia, thalassemia, and spherocytosis.
  14. Cancer like liver cancer, gastric cancer, colon cancer, and kidney cancer. (2, 8, 9, and 10)

To confirm the above-mentioned diagnosis, the doctor will order a complete blood count and compare the level of RDW and MCV.

An elevated RDW can be corrected by working with your doctor to find out the root cause of high RDW. Finding the cause is a must to effectively treat the underlying medical conditions.

What does a low RDW mean?

If the RDW level is low, it means that the red blood cells are equal in size and there is really no cause for concern. However, the possibility of blood-related conditions should not be taken for granted.

What should you do to keep RDW and other blood components in healthy levels?

  1. Make sure you eat a balanced meal to prevent the possibility of experiencing nutritional deficiencies. Your regular diet should include folate, iron, and vitamin B complex.
  2. Stop all vices such as smoking and alcohol drinking. Heavy smoking elevates RDW and excessive drinking of alcoholic beverages can cause damage to red blood cells. Heavy alcohol use can also interfere with the absorption of essential nutrients such as folate and vitamin B12, which are vital in the production of red blood cells.
  3. Get enough rest and sleep (about 7 to 8 hours of sleep) per night.
  4. Make it a habit to exercise regularly to improve RDW. (5, 9, and 10)

What to keep in mind?

  • By measuring RDW, you will be able to differentiate the underlying cause of anemia. A high RDW indicates iron-deficiency anemia while a normal RDW indicates thalassemia.
  • A high RDW is associated with an increased in mortality rate as it causes a poor prognosis in some types of cardiac-related diseases.
  • RDW has a prognostic value in non-hematologic conditions.
  • RDW Blood Test has some limitations. If the level of RDW is checked after a blood transfusion, it won’t lead to an accurate result. It usually leads to a false high result, especially if the lab uses EDTA anti-coagulated blood.
  • To calculate RDW, MCV should be first determined. So, an error in MCV calculation could lead to an error in RDW.
  • The reference range for RDW varies from one laboratory to another. The average value for RDW is around 11.8 to 15.6 percent. However, the number increases with age. (2, 6, and 10)


  1. https://medlineplus.gov/lab-tests/rdw-red-cell-distribution-width/
  2. https://www.healthline.com/health/rdw-blood-test
  3. https://www.medicalnewstoday.com/articles/321568.php
  4. https://emedicine.medscape.com/article/2098635-overview
  5. https://www.mdedge.com/ccjm/article/195281/hematology/three-neglected-numbers-cbc-rdw-mpv-and-nrbc-count
  6. https://care.diabetesjournals.org/content/33/3/e40
  7. https://www.medicinenet.com/rdw_test/article.htm
  8. http://eclinpath.com/hematology/tests/red-blood-cell-distribution-width/
  9. https://www.revespcardiol.org/en-red-blood-cell-distribution-width-articulo-S1885585712000175?redirect=true
  10. https://www.verywellhealth.com/red-cell-distribution-width-4583796

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