What Is RDW-SD in a CBC Blood Test

Blood tests are a key tool in modern medicine and provide valuable insight into a patient’s health. 

One of the parameters measured in a complete blood count (CBC) is less common but equally important component RDW-SD. 

RDW-SD stands for Red Cell Distribution Width – Standard Deviation and plays a key role in the diagnosis of various diseases. 

This article explores what RDW-SD is, what it means and how it can help healthcare professionals care for patients.

Understanding the Complete Blood Count (CBC)

A complete blood count (CBC) is a common blood test that provides essential information about the quantity and quality of blood cells in a person’s bloodstream. 

rdw sd high

This test is useful for diagnosing and monitoring various medical conditions, including anemia, infections, and inflammation. 

Among the parameters evaluated in a CBC, RDW-SD is a relatively less understood metric.

What Is RDW-SD

RDW-SD is the abbreviation of Red Cell Distribution width – Standard Deviation. 

It is a measure of the change in the size of red blood cells (erythrocytes) in a blood sample. 

Red blood cells are important for carrying oxygen from the lungs to body tissues and for removing carbon dioxide from the body. 

RDW-SD, as part of the CBC, measures the standard deviation of the width of these red blood cells and reflects their variation in size. 

Information is generally given in fL (femtoliters).

Significance of RDW-SD

Anemia Diagnosis – RDW-SD plays a crucial role in the diagnosis of anemia. 

Anemia is a condition characterized by a lack of red blood cells or a decrease in the amount of hemoglobin in the blood, leading to a reduction in oxygen-carrying capacity. 

In anemia, the size of red blood cells can vary and RDW-SD helps detect this variation.

Iron Deficiency Anemia – RDW-SD may also be particularly useful for differentiating different types of anemia. 

For example, in iron deficiency anemia, red blood cells tend to be smaller (microcytic) and have a high degree of variation in size. 

RDW-SD can help identify this condition.

Other Blood Disorders – In addition to anemia, changes in RDW-SD can also indicate other blood disorders, such as thalassemia, a genetic disorder that affects the production of hemoglobin, and vitamin B12 deficiency anemia.

Inflammation and Chronic Diseases – RDW-SD can sometimes increase in response to inflammation and some chronic diseases. 

Elevated RDW-SD values ​​may indicate underlying health problems, requiring further investigation.

Prognostic Value – Recent studies have suggested that RDW-SD may serve as a prognostic indicator for several medical conditions. 

It has been linked to adverse outcomes in heart disease, cancer, and other chronic diseases, making it a valuable tool in patient management.

High RDW-SD Levels

Indication of Anemia – Elevated RDW-SD values ​​often indicate anemia. 

Anemia is a condition characterized by a reduced number of red blood cells or a decrease in the amount of hemoglobin in the blood. 

In these cases, the size of the red blood cells may vary, and a high RDW-SD indicates a significant difference in cell size.

Iron Deficiency Anemia – Elevated RDW-SD levels may also be an indicator of iron deficiency anemia. 

In this condition, red blood cells are generally smaller (microcytic) and have a high degree of variation in size. 

Elevated RDW-SD values ​​may be an indication for healthcare professionals to investigate possible iron deficiency.

Vitamin B12 and Folic Acid Deficiency – Conditions such as pernicious anemia, which result from vitamin B12 deficiency, can also cause high levels of RDW-SD. 

Anemia due to folic acid deficiency can lead to similar results. 

Changes in the size of red blood cells are indicators of these deficiencies.

Chronic Disease and Inflammation – In some cases, elevated RDW-SD levels may be associated with chronic disease and inflammation. 

These diseases can affect the bone marrow’s ability to produce normal red blood cells and cause fluctuations in size.

Low RDW-SD Levels

Uniform RBC Indication – Low RDW-SD values ​​indicate that the RBCs in the sample are relatively uniform in size. 

Although this may seem like a positive sign, sometimes it can also be a sign of certain health problems.

Certain Hematologic Diseases – In some cases, low RDW-SD values ​​may be associated with hematologic diseases such as thalassemia, a genetic disorder that affects hemoglobin production, or sickle cell disease, which causes deformation of red blood cells.

Bone Marrow Diseases – Diseases that affect the bone marrow, such as: Other conditions, such as aplastic anemia, can also lead to low RDW-SD values. 

These disorders often result in a reduced ability to produce red blood cells of different sizes.

Interpreting RDW-SD Values

RDW-SD values ​​are usually reported in a standard CBC report and are expressed in femtoliters (fL). 

A normal RDW-SD value falls within a certain range, usually between 35.0 fL and 47.0 fL. 

Values ​​outside this range may indicate an underlying medical condition that requires further evaluation.

It is important to note that although RDW-SD can provide valuable information, it is only one part of a complete diagnostic process. 

In addition to RDW-SD, other blood parameters and clinical findings are often considered to determine the exact cause of any abnormalities.


In the field of diagnostic medicine, the RDW-SD is a lesser known but essential tool. 

Its role in assessing variation in red blood cell size can help healthcare professionals detect anemia, differentiate between different types of anemia, and even provide prognostic information for various chronic diseases. 

While it is important to understand the meaning of RDW-SD, it is equally important to recognize that the interpretation of these values ​​requires a holistic approach that takes into account other clinical data and patient history. 

For people undergoing blood tests, measuring RDW-SD provides insight into the health of their blood and can be an important piece of the diagnostic puzzle for healthcare providers.


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