RDW High: What does it Mean in Blood Test

What is RDW on Blood Test and Normal Range?

Red blood cells (RBC) play a very essential role in the human body. These cells basically bring oxygen throughout our bodies (from the major organs even to the tiniest capillaries of the body). Also known as “erythrocytes”, RBC’s contain “hemoglobin”, a protein which carries oxygen. RBC’s also eliminate carbon dioxide from our bodies with the help of the lungs by releasing it through exhalation. Erythrocytes are produced in the bone marrow and their life span is approximately 120 days.

By means of design, a normal red blood cell is round and flat in the center, like a donut but has no hole in the middle. An erythrocyte’s structure is significant with regards to detecting various illnesses related to the blood. Essentially, RBC’s size and volume are measured for any inconsistencies. This method is called red blood cell distribution width (RDW) blood test.

In a regular complete blood count (CBC) blood test, RDW is also included. It evaluates the red blood cells count together with size or volume that is in the blood. Apparently, there are two ways to measure RDW:

  1. Coefficient of variation (CV) or RDW-CV, which is measured as percentage. It is identified from standard deviation of the RDW-SD and the mean corpuscular volume (MCV) number.
  2. Standard deviation (SD) or RDW-SD, which is measured as fluid Liters (fL) and is the actual size measurement. It is calculated by getting the width at the 20% elevation plane of the distribution histogram.

Like any other laboratory test, RDW blood test result may differ depending on the machine used. However, these are the average range RDW:

  1. For RDW-CV : 10.2 – 14.5%
  2. For RDW-SD : 29-46 Fl

Indication of RDW Result

In analyzing RDW results, both the RDW value and the MCV are reviewed. It is actually calculates the normal red blood cell volume. Meaning, the MCV is the standard amount of space used in by every RBC. Again, please take note that RDW and MCV are indicated in a typical CBC blood test.

If the RDW result is above 14.5%, the RDW value is considered high (which is above the normal range). And if the result is below 10.2%, it is read as low RDW value (which is below the standard range). RDW and MCV values are read and evaluated together before obtaining a diagnosis. Here are some important things to consider when reading and interpreting RDW and MCV result:

  1. Low RDW

As mentioned earlier, the low RDW value is obtained if the result is below 10.2%, in which the RBC count is decreased. This indicates macrocytic anemia, a type of anemia in which the blood does not have enough hemoglobin concentration.

  1. High RDW, Normal MCV

When RDW value is high, but the MCV remains normal, it is indicative of Iron deficiency anemia (IDA) on its early stage. Classified as one of the most common type of anemia, IDA arises when the blood has insufficient mature red blood cells due to inadequate iron ion. Iron is a very important element found in the hemoglobin. Both of them work together and are responsible for oxygen transportation. If there is insufficient iron in the blood, the amount of hemoglobin is also inadequate. Remember that hemoglobin is essential for RBC maturation. Immature RBC’s size is increased, making the RDW value high.

Also, vitamin B12 and folate deficiencies are also considered. For Vitamin B 12 and folate are essential elements in the RBC production. Lacking of (any) these factors may result to larger RBC in size.

Other indications include sickle cell disease, chronic liver diseases, myelodysplastic syndrome, and dimorphic anemia (indicator of iron and folate insufficiency).

  1. High RDW, low MCV

Obtaining these results is indicative of serious diseases. This includes sickle cell anemia a disease which involves altered hemoglobin, causing the RBC to deform into a crescent shape when oxygen levels are low. This condition is inherited and causes severe pain when tissues lack oxygen. The attacks are sudden and patients are often hospitalized to receive proper and efficient management. A normal RBC lives for up to 120 days, but it lasts for only 10 to 20 days for people with sickle cell anemia.

Another condition is Thalassemia, another hemolytic illness in which hemoglobin is distorted but causes excessive destruction of RBC. Likewise to sickle cell anemia, thalassemia can be treated by regular blood transfusion, medications, etc. A more thorough testing is required for the mentioned diseases.

  1. High RDW, high MCV

These results show various health problems. First, hepatic diseases more likely exist since the liver’s main task is to filter the blood. If the liver is not functioning properly, toxic elements stay in the blood and would not be excreted. These toxic materials would then alter the normal blood function, making the size of RBC to increase.

Other indications include hemolytic anemia, myelodysplastic syndrome, vitamin B 12 deficiencies, and cytotoxic chemotherapy.

  1. Normal RDW, high MCV

Aplastic anemia can result into normal RDW but high MCV value. This rare disease is develop when there is a damage to the bone marrow, a part of our body responsible for generating red blood cells, white blood cells, and platelets.

Chronic liver disease and alcohol, antiviral, and chemotherapy usage may also demonstrate these results.

  1. Normal RDW, low MCV

A low MCV with normal RDW may be due to anemia that is caused by a chronic illness. It may also be caused by heterogyzous thalassemia (or thalassemia minor).

  1. Normal RDW, normal MCV

Occasionally, RDW and MCV with normal values may also indicate a health problem. Diseases with these results include anemia caused by chronic disease and by renal disease. Apparently, kidneys produce a hormone called erythropoietin which helps in the RBC formation by the bone marrow. Hence, if kidneys are damage, the RBC is also affected. Other indications include acute blood loss or hemolysis.


Indeed, there are significant and numerous diseases that can be detected by means of CBC blood test. A more detailed blood examination is recommended and results vary depending on the age of the patient. Whatever the outcome of the blood test demonstrate, it is important to reassess the condition and other related aspects as there are several factors affecting the RDW and MCV values.