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The RDW value in a blood test describes more than just the size of the red blood cells. It gives doctors important clues about your health, including whether you might have anemia, nutrient deficiency, or early signs of chronic disease.
When you go for a blood test, particularly a complete blood count (CBC), many parameters are listed on the results, such as hemoglobin, white blood cells, and platelets. One of the least known but fundamental parameters of CBC is the RDW (Red cell distribution width).
Understanding RDW can help you understand your health before problems become serious. In this article, we shall break down the meaning of RDW, the reasons for its measurement, and how it helps doctors go beyond just looking at red blood cell size.
What is RDW?
RDW is short for Red cell distribution width, a measurement of the variation in size of red blood cells. Red blood cells are about the same size to carry oxygen smoothly throughout the body. However, a deficiency of vitamin B12 or bone marrow issues can cause the red cells to differ in size.
If the red blood cells are the same size, RDW is normal, but if they vary in size, RDW is high. This variation in cell size is known as anisocytosis.
How is RDW measured?
An RDW blood test is done using a simple blood draw. The collected sample is sent to a laboratory where a lab specialist examines the red blood cells under a microscope to assess differences in their sizes. RDW is shown as a percentage; it compares the sizes of the smallest red cells to the largest red cells.
- A normal RDW is usually between 11.5% and 14.5%
- A high RDW value means a significant variation in red blood cell sizes.
- A low RDW value is rare and not usually a concern.
Labs use automated machines to calculate the RDW percentage, which is then indicated in the CBC results as RDW-CV or sometimes RDW-SD, depending on the method used.
Why is the RDW value important?
- Evaluation of red blood cell health: Red blood cells are responsible for carrying oxygen to every part of the body. To perform this task, they need to be the right size and uniformly shaped. If the red blood cells vary in size, it means something is interfering with their normal development or survival.
- Used in combination with other indices to detect disorders: RDW is useful when looked alongside other red blood cell measures in the CBC, especially MCV (Mean corpuscular volume), the average size of the red blood cells, and Hemoglobin, which shows how much oxygen the blood carries.
| RDW | MCV | Cause |
| High | Low | Iron deficiency anemia |
| High | High | Vitamin B12 |
| Normal | Low | thalassemia |
| Normal | normal | Healthy or early-stage chronic disease |
- For early detection of health issues, underlying conditions might develop, like slow-forming anemia or mild vitamin deficiency. In such cases, the hemoglobin level might still be normal, but RDW could already be high. Therefore, RDW can detect a problem before it gets serious.
- To guide treatment, the level of RDW can increase first during the treatment of anemia or nutrient deficiency. This is because new red blood cells are being made and entering the bloodstream, usually a different size from the older ones. Therefore, a higher RDW can mean that the body is responding to treatment.
High RDW with normal hemoglobin
High RDW with normal hemoglobin could mean:
- Early stages of nutrient deficiency
Despite normal hemoglobin levels, a rising RDW may be one of the first signs that the body is running low on nutrients like iron, vitamin B12, and folate. This is because the bone marrow may produce red cells of uneven sizes as it struggles to make them efficiently with limited nutrients.
- Recovering from anemia or blood loss
When patients are being treated for anemia or recovering from blood loss, their bone marrow begins producing new red blood cells. The cells may be different sizes from the older ones that are still circulating. When the RDW rises temporarily, it indicates recovery.
- Chronic inflammation
Some studies suggest that even with normal hemoglobin, a high RDW could be linked to chronic inflammation, heart disease, liver or kidney dysfunction, autoimmune conditions, and cancer.
Precautions when interpreting RDW
RDW is not a diagnosis by itself. It simply shows variation in red blood cell size.
It must be interpreted alongside other blood tests like MCV, hemoglobin, and clinical symptoms.
A normal RDW doesn’t always mean everything is fine, and a high RDW doesn’t always mean something is wrong.
Take away
RDW is recorded as a percentage on your CBC test result. It carries important information about how red blood cells function. It informs doctors whether the red blood cells are uniform or varied in size. It also gives early detection of nutrient deficiencies, different types of anemia, or even chronic health problems.
When combined with other values, RDW becomes a powerful tool for detecting trouble. If you ever see your RDW marked as high or low, don’t ignore it; talk to your doctor to understand what it could mean for your health.
