You may already know that it is possible to count the number of red blood cells in your body. What you may not know is that you can go to a lab to countthe varying sizes of red blood cell volume. The doctors refer to this process as RDW or "red blood cell distribution width". It is sometimes important to check the red cell distribution width, which is normally between 11 and 15. The test is usually a part of a Complete Blood Count (CBC) and indicates greater variation in size when the numbers are high. Keep reading to learn more about high RDW and what may be the reason behind your RDW high count.

What does RDW Mean in Blood Test?

RDW is a red blood cell parameter that basically counts or measures the difference in volume or size of red cells in a sample. Different types of hematology analyzer instruments are sometimes used to measure red cell distribution width – it is called RDW-CV when reported as coefficient of variation (CV) and is called RDW-SD when reported as standard deviation (SD).

RDW-SD takes measurements in "fL" and basically measures the width of red cells size distribution histogram – it calculates the width at the 20% height level of the histogram. The average RBC size therefore has no effect on this parameter and you get MCV or mean corpuscular volume.

On the other hand, RDW-CV is expressed in percentage and is calculated from MCV and standard deviation as follows:

RDW-CV (in percentage) = 1 SD of RBC volume/MCV x 100%

What Does High RDW Indicate?

You can have high RDW or low RDW for many different reasons. If you have high RDW, this may indicate different things. Your RDW high, which is above 14.5%, means that your red cells vary a lot in size. This may happen due to many different reasons, and to identify the issue, it is important to make a comparison to the MCV or mean corpuscular volume, which is the average amount of space each red cell occupies in your blood sample.

Increased RDW and Increased MCV

When the MCV and RDW levels are increased, it could be due to a liver disease. Your liver plays a big role in filtering harmful chemical substances and then helping it flush out of your system. When it fails to eliminate harmful substances, this may leave an effect on the size of your red blood cells. You may also notice increased levels of MCV and RDW due to hemolytic anemia, a condition characterized by early destruction of your red blood cells. Sometimes, you get your RDW blood test high due to vitamin B-12 deficiency in your body. Your body needs these essential vitamins to carry out specific chemical processes and bodily functions.

High RDW and Low MCV

In some cases, you may see RDW blood test high with lower levels of MCV. Again, you may see this scenario due to many different reasons, with one of the most common being iron deficiency in your body. The inadequate supply of iron in your body will cause iron deficiency anemia that causes a decrease in hemoglobin in your blood. Hemoglobin in your blood is essential for transferring oxygen on a cellular level. When there isn't enough iron to make hemoglobin, it will result in anemia, which will decrease your MCV levels while keeping RDW high.

You may also notice the same scenario due to thalassemia intermedia, a blood disorder causing impaired production of certain elements that play a role in the production of hemoglobin. This will lead to fragmented blood cells that in turn will cause the MCV to be low and the RDW to be high. It usually means your blood cells vary a lot in size, but those cells don't take a lot of space, resulting in low MCV levels.

Increased RDW and Normal MCV

There can be another possibility when you see your RDW high. You sometimes see your RDW levels on the higher side with your MCV levels completely normal. This usually means that vitamin-B12 is not enough, though it has not fallen below the desired level. It may also indicate the beginning stage of iron deficiency anemia.