RDW-SD (Red Cell Distribution Width) Size variation of red blood cells

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What is it?

Red cell distribution width (RDW) is a red blood cell parameter that measures variability of red cell volume/size (anisocytosis). Depending on the types of hematology analyzer instruments, RDW can be reported statistically as coefficient of variation (CV) and/or standard deviation (SD), RDW-CV and/or RDW-SD, respectively.

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.

RDW-CV is expressed in percentage and is calculated from MCV and standard deviation.

The reference range for RDW is as follows:


Optimal range

39 - 46 fl

What are low values associated with?

A low RDW (below 10.2%) means that the red blood cells vary very little in size. One reason for a low RDW level is macrocytic anemia. Macrocytic anemia is a blood disorder in which not enough red blood cells are produced, but the ones that are present are large. Another cause of a low RDW level is microcytic anemia. Microcytic anemia is a condition in which abnormally small red blood cells are present. In these two disorders the red blood cells do not vary much in size because they are either all small or all large. This is what causes the RDW level to be low.

What are high values associated with?

A high RDW (over 14.5%) means that the red blood cells vary a lot in size. There are many possible reasons why the RDW level can be too high. To determine what the possible cause of a high RDW level is, a comparison is made to the mean corpuscular volume (abbreviated MCV).

The MCV is the average amount of space occupied by each red blood cell.

If both the RDW and MCV levels are increased, there are several possible causes. One possible cause is liver disease. The liver is the largest organ in the body and is responsible for filtering (removing) harmful chemical substances, producing important chemicals for the body, and other important functions. Another cause of high RDW & MCV levels is hemolytic anemia. Hemolytic anemia is a condition in which the red blood cells are destroyed earlier than they should be. The RDW and MCV levels can both be increased if there is too little Vitamin B12 or folic acid in the body.

Another scenario is that the RDW level can be high, but the MCV level can be low. This can happen because of iron deficiency anemia. Iron deficiency anemia is a decrease in hemoglobin in the blood that is caused by an inadequate supply of iron. Hemoglobin is a substance present in red blood cells that help carry oxygen to cells in the body. Iron is needed to make hemoglobin, which is why a decreased amount of iron leads to a decreased amount of hemoglobin.

Another cause of a high RDW level and a low MCV level is thalessemia intermedia. Thalessemia intermedia is another type of blood disorder in which there is impaired production of one or more of the elements that make up hemoglobin. If the red blood cells are fragmented (broken) into smaller parts, this can cause the RDW to be high and the MCV to be low. In this situation, the red blood cells vary in size when they are broke up (which is why the RDW level is high) but the cells do not take up much space (which is why the MCV level is low).

A final possibility is that the RDW level is increased and the MCV level is normal. This can be caused by the beginning stages of a decrease in vitamin B12 or folic acid in the body. It can also be caused by the beginning stages of iron deficiency anemia (see above).

Biomarker category

Blood Health

Other biomarkers in this category

Ammonia, C-Reactive Protein (CRP), Ferritin, HAEMOGLOBIN (g/L), Haemoglobin A1c, HAEMOGLOBINe, Hematocrit (HCT), Hemoglobin, Iron, IRON (Serum), Lactate Dehydrogenase (LDH), Large Unstained Cells (LUC), Large Unstained Cells (Percent), Magnesium, Mean Cell Volume, Mean Corpuscular Haemoglobin (MCH), Mean Corpuscular Haemoglobin Concentration (MCHC), Mean Corpuscular Volume (MCV), Mean Platelet Volume (MPV), Platelet count, Plateletcrit (PCT), RDW-CV (Red Cell Distribution Width), RDW-SD (Red Cell Distribution Width), Red Blood Cells (Erythrocytes), Thrombocytes, TIBC, Transferrin saturation, UIBC

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