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RDW Blood Test


What is RDW in blood test?

The RDW (Red blood cell Distribution Width) blood test is part of the complete blood count testing, measuring the variation of the red blood cell (RBC) volume. The test is performed with the blood collected by venipuncture. It seems that the RDW blood test is often used to determine the risk for premature death, especially in patients who have been diagnosed with different heart conditions or various forms of cancer. The high levels of RDW are associated with far more health risks than the low or normal RDW levels.

Normal Range

The health red blood cells have a standard size, with a diameter that varies between 6 and 8 μm. The normal range for the RWD blood test is between 11.5 and 14.5%. Even though elevated levels of RDW are believed to be associated with all forms of anemia, not all types of anemia actually come with such changes. In fact, there are many forms of anemia, in which the RDW is within the normal range, such as: anemia of chronic disease, aplastic anemia and thalassemia minor. The RDW levels might also be maintained within normal range in patients suffering from acute blood loss, depending on how much blood was actually lost.

In some patients, the RDW blood test is extremely useful for making the differential diagnosis between iron deficiency anemia (high levels of RDW and normal or reduced MCV levels) and thalassemia (normal RDW levels and reduced MCV levels). However, further investigations are going to be necessary in order to confirm the diagnosis. The levels of RDW might remain within the normal range in patients who are suffering from anemia of renal disease as well or hemolysis caused by other medical problems.


The low levels of RDW are encountered in the following medical conditions: anemia of chronic disease, heterozygous thalassemia and hemoglobin E trait. However, there may be patients presenting normal RDW levels, despite suffering from the above-mentioned medical problems. Microcytic anemia might also lead to low RDW levels, as all the blood cells are small in size and there is no significant variation to cause the RDW levels to become elevated.

High RDW

There are a wide range of medical conditions that lead to a significant difference whereas the cell size is concerned. In general, a high RDW means that the red blood cells present a high level of variety in regard to their size.

The high RDW is often encountered in persons who suffer from different forms of anemia (not in call cases). The doctor might decide to perform additional tests, such as the mean corpuscular volume, in order to identify the causes that have led to the appearance of anemia in the first place. The RDW blood test, performed at the same time with the MCV, allows the doctor to make the differential diagnosis between single-cause and multiple-cause anemia.

Among the types of anemia in which the RDW levels are elevated, there are macrocytic anemia (caused by in turn by vitamin B12 or folate deficiency) and iron deficiency anemia. The analysis of the red blood cells will show that some of these cells are increased in size (hence the name of the anemia), while others remain smaller in size. The variety in size leads to the elevated RDW levels. Also, when the red blood cells have different sizes, this modification is known as anisocytosis.

In case of patients who suffer from iron deficiency anemia, the levels of RDW are elevated, while the MCV is reduced. The levels of both RDW and MCV are elevated in patients who suffer from anemia caused by vitamin B12 and folate deficiency. If the patient suffers from both vitamin B12 and folate deficiency, the levels of RDW are going to be elevated. As for the MCV levels, these can be elevated, reduced or even within the normal range. In case of acute blood loss or a hemorrhage that has occurred recently, the levels of RDW are high, while the MCV levels are maintained within normal range.

Other conditions in which the RDW levels are elevated include: sickle cell anemia, chronic liver disease, chemotherapy (for different types of cancer), prolonged treatment with antiviral medication and chronic alcohol consumption. Patients diagnosed with myelodysplastic syndromes can also present high RDW levels.

How to calculate?

The RDW can be calculated with the help of a mathematical formula, as it follows:

RDW = (Standard deviation of MCV/mean MCV) x 100

Note: MCV stands for mean corpuscular volume (average volume of red blood cells)

So, the RDW is calculated by dividing the standard deviation of MCV to the mean MCV and then multiplying the result by 100. In case you are wondering what the standard deviation represents, you should know that this is actually the erythrocyte volume or the number of red blood cells that are found in the blood smear.

In conclusion, the RDW blood test is a faithful investigation that can help one determine the exact cause of anemia. It is often performed with the complete blood count and it is directly influenced by the mean corpuscular volume or the MCV, as it is abbreviated. It can be used for the differential diagnosis between different types of anemia, especially when the results from other tests are inconclusive. As you have seen for yourself, the RDW levels can be increased, reduced or within the normal range, each type signifying the potential presence of different medical problems.

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